Chapter VI: Insect Management

James Robinson

Insects are a major limiting factor in commercial vegetable production. Minor insect damage lowers the crop’s value because the market demands clean, unblemished produce. Growers need to quickly recognize insect problems and practice early control to prevent a buildup and keep insect pests from getting out of control.

Insect Life Cycle

Insects either have a complete or incomplete life cycle. Insects in the complete life cycle group have four distinct stages, the egg, larvae, pupae and adult. Examples of these insects are beetles and moths. Beetles lay their eggs either singly or in groups, and they hatch into either grubs or larvae that move about freely on the plant feeding on roots, tubers, leaves, or fruits. After reaching maturity, they then pupate (the resting stage) and develop into adults. Adult beetles also may damage plant parts, so two damaging stages may exist. Figure VI-1 depicts growth stages for insects having a complete life cycle.

figure VI-1 shows life cycle of a lady beetle: eggs, larva, pupa, adult

Figure VI-1. Complete Life Cycle

Moths and butterflies also have a complete life cycle similar to beetles except that the damaging stage is the larvae or worm stage which usually feeds on the stems, leaves or fruits. The adult stage, moths and butterflies, feed on nectar or may not feed at all. Insects with a complete life cycle almost always have a chewing mouthpart.

Insects with incomplete life cycles include grasshoppers and true bugs (stink bug and squash bugs). Many insects in this category have piercing, sucking mouthparts and suck juice from plants. Some, such as the grasshopper, chew on leaves and stems. Regardless, insects with an incomplete life cycle are unique in that they hatch from eggs into tiny nymphs that resemble the adult stage. They stay in the nymphal stage for several weeks, while growing and molting into larger insects until they reach adulthood. Adults have fully developed wings and can fly great distances. Nymphs either do not have wings or have wings that cannot be used for flight.

Insects with an incomplete life cycle can be controlled at any stage, but are easier to control in the nymphal stage just after they hatch from the eggs. Figure VI-2 depicts the developmental stages of insects with incomplete life cycles.

figure VI-2 shows insect with incomplete life cycle

Figure VI-2. Incomplete Life Cycle. Image courtesy of North Dakota State University

Insect Injury

Insects injure plants by chewing leaves, stems and roots, sucking juices, egg laying or transmitting diseases.

Injury by Chewing Insects

Insects take their food in a variety of ways. One method is by chewing off external plant parts. Such insects are called chewing insects. It is easy to see examples of this injury. Perhaps the best way to gain an idea of the prevalence of this type of insect damage is to try to find leaves of plants with no sign of insect chewing injury. Cabbageworms, armyworms, grasshoppers, the Colorado potato beetle and the fall webworm are common examples of insects that cause chewing injury.

Injury by Piercing-Sucking Insects

Another important method which insects use to feed on plants is piercing the epidermis (skin) and sucking sap from cells. In this case, only internal and liquid portions of the plant are swallowed, while the insect feeds externally on the plant. These insects have a slender and sharp pointed part of the mouthpart which is thrust into the plant and through which sap is sucked. This results in a very different but nonetheless severe injury. The hole made in this way is so small that it cannot be seen with the unaided eye, but the withdrawal of the sap results in either minute white, brown or red spotting on leaves, fruits and/or twigs; leaf curling; deformed fruit; or a general wilting, browning and dying of the entire plant. Aphids, scale insects, squash bugs, leafhoppers and plant bugs are examples of piercing-sucking insects.

Injury by Internal Feeders

Many insects feed within plant tissue during a part or all of their destructive stages. They gain entrance to plants either in the egg stage when the female thrust into the tissues with sharp ovipositors and deposit the eggs there, or by eating their way in after they hatch from the eggs. In either case, the hole by which they enter is almost always minute and often invisible. A large hole in a fruit, seed, nut, twig or trunk generally indicates where the insect has come out, and not the point where it entered.

The chief groups of internal feeders are indicated by their common group names: borers; worms or weevils in fruits, nuts or seeds; leaf miners; and gall insects. Each group, except the third, contains some of the foremost insect pests of the world. In nearly all of them, the insect lives inside the plant during only a part of its life and emerging sooner or later as an adult. Control measures for internal feeding insects are most effective if aimed at adults or the immature stages prior to their entrance into the plant.

A number of internal feeders are small enough to find comfortable quarters and an abundance of food between the upper and lower epidermis of a leaf. These are known as leaf miners.

Gall insects sting plants and cause them to produce a structure of deformed tissue. The insect then finds shelter and abundant food inside this plant growth. Although the gall is entirely plant tissue, the insect controls and directs the form and shape it takes as it grows.

Injury by Subterranean Insects

Subterranean insects are those insects that attack plants below the surface of the soil. They include chewers, sap suckers, root borers and gall insects. The attacks differ from the above ground forms only in their position with reference to the soil surface. Some subterranean insects spend their entire life cycle below ground. In other subterranean insects, there is at least one life stage that occurs above the soil surface; these include wireworm, root maggot, pillbug, strawberry root weevil, and corn rootworm. The larvae are root feeders while the adults live above ground.

Injury by Laying Eggs

Probably 95% or more of insect injury to plants is caused by feeding in the various ways just described. In addition, insects may damage plants by laying eggs in critical plant tissues. As soon as the young hatch, they desert the plant causing no further injury.

Use of Plants for Nest Materials

In addition to laying eggs in plants, insects sometimes remove parts of plants for the construction of nests or for provisioning nests.

Insects as Disseminators of Plant Diseases

In 1892, a plant disease (fireblight of fruit trees) was discovered to be spread by an insect (the honeybee). At present, there is evidence that more than 200 plant diseases are disseminated by insects. The majority of them, about 150, belong to the group known as viruses; 25 or more are due to parasitic fungi; 15 or more are bacterial diseases; and a few are caused by protozoa.

Insects may spread plant diseases in the following ways:

  • By feeding, laying eggs or boring into plants, they create an entrance point for a disease that is not actually transported by them.
  • They carry and disseminate the causative agents of the disease on or in their bodies from one plant to a susceptible surface of another plant.
  • They carry pathogens on the outside or inside of their bodies and inject plants hypodermically as they feed.
  • The insect may serve as an essential host for some part of the pathogens life cycle, and the disease could not complete its life cycle without the insect host.

Examples of insect vectored plant diseases are shown below.

Disease Vector
 Fireblight (bacterial)  Pollinating Insects
 Tomato curly top (virus)  Beet leafhopper
 Cucumber mosaic (virus)  Aphids

Benefits and Value of Insects

Insects must be studied carefully to distinguish the beneficial from the harmful. Producers have often gone to great trouble and expense to destroy insects, only to learn later that the insect destroyed was not only harmless, but it was actually engaged in saving their crops by eating destructive insects.

Insects are beneficial to the vegetable grower in several ways:

  • Insects aid in the production of vegetables by pollinating the blossoms. Melons, squash and many other vegetables require insects to carry their pollen before fruit set.
  • Parasitic insects destroy other injurious insects by living on or in their bodies and their eggs. Insects also act as predators, capturing and devouring other insects.
  • Insects destroy various weeds in the same ways that they injure crop plants.
  • Insects improve the physical condition of the soil and promote its fertility by burrowing throughout the surface layer. Also, the dead bodies and droppings of insects serve as fertilizer.
  • Insects perform a valuable service as scavengers by devouring the bodies of dead animals and plants and by burying carcasses and dung.

Many of the benefits from insects enumerated above, although genuine, are insignificant compared with the good that insects do fighting among themselves. There is no doubt that the greatest single factor in keeping plant-feeding insects from overwhelming the rest of the world is that they are fed upon by other insects.

Insects that eat other insects are considered in two groups known as predators and parasites. Predators are insects (or other animals) that catch and devour other creatures (called the prey), usually killing and consuming them in a single meal. The prey generally is smaller and weaker than the predator. Parasites are forms of living organisms that live on or in the bodies of living organisms (called the hosts) from which they get their food, during at least one stage of their existence. The hosts usually are larger and stronger than the parasites and are not killed promptly but continue to live during a period of close association with the parasite. Predators are typically very active and have long life cycles; parasites are typically sluggish and tend to have very short life cycles.

Insect Control

Insect control is also important to keep the pests from spreading to other crops, and it may help reduce the incidence of disease by killing insect vectors. Insects attacking vegetables can be divided into three categories:

  • Soil Insects
  • Chewing Insects
  • Sucking Insects

Soil Insects

Soil insects include wireworms, white grubs, fire ants, cutworms, seed maggots and the sweet potato weevil. These insects can be damaging because they feed on the roots, stems and tubers of plants. Often soil insects, especially cutworms, are common in uncultivated soil sites that have had grass and weeds growing the previous season. These undisturbed areas often harbor high populations of soil insects. Once seeds or transplants are planted, soil insects are difficult to control and may begin feeding immediately on the crop. There is a real need for producers to inspect fields for soil insects prior to planting. One or two soil insects per square foot of soil can cause serious damage.

Oftentimes soil insects are clumped in a field, that is, they may be in one area and not in another. Low areas or those areas with the most vegetation often hold the most insects. Controlling soil insects is much easier if done prior to planting. Most insecticides for the control of soil insects should be applied 6 weeks before planting and incorporated into the top 6 inches of the soil. Liquid or granular materials may be used. These can either be broadcast or banded in the row. Sometimes producers will apply insecticides at planting. Make sure to read the label for proper rates and application techniques. Some insecticides may interfere with seed germination and should not be placed in the furrow in contact with the seed.

Chewing Insects

Many chewing insects have a complete life cycle. Therefore, depending on species, there may be one or two damaging stages. Grasshoppers have a chewing-type mouthpart but have an incomplete life cycle. Chewing insects include all species of beetles, grasshoppers and moths and butterfly larvae (most often called worms).

Chewing insects damage foliage, stems and fruit. They may become as numerous as to completely defoliate plants. Eggs of most insects are laid on the plant, and the larvae upon hatching begin to feed. Others may invade the crop by “marching in” or by flying into the field.

Control of chewing insects is basically twofold. One, the grower must watch for eggs and small larvae that begin to feed; two, he must watch for the adults and control them when necessary. Control of these insects is important in the early infestation of the plant. Often, the insect after hatching may bore into the fruit or stem and be hidden from pesticide applications.

These insects often become numerous because producers do not begin treatment early enough. It is vital that fields be watched and these insects controlled at the earliest possible moment.

Sometimes a single application timed properly will control a generation. However, repeated applications are needed to control others like the corn earworm in sweet corn.

Sucking Insects

Sucking insects include aphids (“plant lice”), stink bugs, squash bugs, leafhoppers and spider mites. Spider mites are not insects but are just as damaging and numerous as are some insects. Sucking insects have an incomplete life cycle. After hatching from the egg, they may begin to feed and move about on the plant.

They are usually attracted to the most succulent part of the plant. Aphids usually are found in the terminal or on flowers. Stink bugs and squash bugs readily feed on the tender fruit. These insects damage the plant by reducing the vigor or by injecting a toxin or disease-causing organisms into the plant. Heavy feeding may cause flowers to abort or the leaves to turn yellow and fall off. Feeding on the fruit may cause catfacing injury, hard spots or twisted and misshapen fruit.

Control is easiest to obtain soon after the insects hatch from eggs. This is when the insects are the smallest and most vulnerable to the pesticide. Look for egg clusters, so that timing of the insecticide can be more accurate. Most true bugs have large eggs that can be seen without the aid of a magnifying glass. They are often on the undersurface of leaves and laid in tight groups and glued together, or in the case of squash bugs, they may be laid singly but in a loose fitting group and not glued together.

Pest Control with a Minimum of Chemicals

The “if a little is good, more will be better” attitude leads to a serious misuse of pesticides.

Overuse of pesticides has a number of adverse effects:

  • Food products may contain unsafe pesticide residues if improperly treated with pesticide.
  • Beneficial insects, earthworms and birds may be harmed or killed along with harmful insects if pesticides are carelessly used.
  • Each time producers spray they expose themselves to the possibility of inhalation or absorption of the toxin.
  • Careless use of pesticides near water may contaminate water supplies.
  • Misuse of pesticides can lead to the development of chemical resistance in the target pest.
  • The use of pesticides can lead to outbreaks of secondary pest species.

A growing public concern over the use and misuse of pesticides has led increasing numbers of vegetable producers to seek means of natural pest control. Although some people do not have the time or knowledge to practice all available alternative methods for controlling pests, there are many cultural practices which will help reduce losses. Proper soil preparation, careful plant selection and good cultural practices can be combined with biological and mechanical controls to reduce the need for chemical pesticides.

Temperature, humidity, precipitation and natural enemies all influence insect populations. In some years, troublesome insects may not be numerous enough to significantly damage plants. In other years, large insect populations may cause serious damage or completely destroy host plants.

Effective control of specific insects must be preceded by proper identification of these insects. Once an insect’s identity is known, you can learn about its life cycle, seasonal cycle, habits and host plants, and thus exercise more effective control measures.

Several control methods are often combined in order to minimize damage by insect pests. Since insect control methods vary in their effectiveness, you may wish to select alternative methods to correspond with differences in plant growth and productivity, insect damage, weather conditions and cultural practices. Various control methods will now be considered.

Resistant Plant Varieties

Use available plant species or varieties which are resistant to, or at least tolerant of, insect activity. Insect resistance in plants frequently is interpreted as meaning “immune to insect damage.” Actually, it is a term for distinguishing plant varieties which exhibit less insect damage when compared to other varieties under similar growing and pest population conditions. Some varieties may be less “tasty” to insect pests, or may possess certain physical or chemical properties which discourage insect feeding or egg-laying, or may be able to support large insect populations without suffering appreciable damage.

Before buying seeds or plants, check with your local county Extension agent for information on resistant varieties which will grow well in your area. Examples of vegetable varieties that have shown resistance to specific insect pests are listed in Table VI-1. Some varieties may be resistant to insect attack, but may be subject to certain restrictions such as soil pH, drainage or temperature. Your experience with different varieties will indicate the ones best suited for your operation.

Table VI-1. Vegetable Varieties that have shown Some Resistance to Specific Insect Pests

Vegetable Variety Insect resistance
Bean (snap) Wade Striped Flea Beetle
Broccoli De Cicco Striped Flea Beetle
Cabbage Early Globe
Red Acre
Round Dutch
Cabbage Looper, Imported Cabbageworm
Cabbage Looper, Imported Cabbageworm
Cabbage Looper, Imported Cabbageworm
Cabbage (Chinese) Michihili Diamondback Moth
Collard Georgia Striped Flea Beetle, Harlequin Bug
Corn (sweet) Golden Security Corn Earworm
Cucumber Ashley
Piccadilly
Poinsett
Pickleworm, Spotted Cucumber Beetle
Pickleworm
Spotted Cucumber Beetle
Kale Vates Diamondback Moth
Mustard Florida Broadleaf Diamondback Moth, Striped Flea Beetle
Radish Cherry Belle
White Icicle
Diamondback Moth, Harlequin Bug
Harlequin Bug
Squash Early Prolific
Straightneck
White Bush
Scallop
Zucchini
Pickleworm, Striped
Cucumber Beetle
Pickleworm, Striped
Cucumber Beetle
Striped Cucumber Beetle
Sweetpotato Centennial
Jewel
Sweetpotato Flea Beetle, Potato Wireworm
Sweetpotato Flea Beetle, Potato Wireworm
Turnip Seven Top Diamondback Moth, Striped Flea Beetle
Rutabaga American Purple Top Diamondback Moth, Striped Flea Beetle

Cultural Controls

Many cultural practices can be used to reduce the potential for, or actual damage of plants caused by insects:

  • Plowing and cultivating exposes soil insects to adverse weather conditions, birds and other predators. In addition, deep plowing will bury some insects and prevent their emergence.
  • Crop rotation can be effective against insects that develop on a narrow range of food plants and also against insects with short migration ranges. Movement of crops to different sites will isolate such pests from their food source. If an alternate site is not available, then change the sequence of plants grown in the field. Do not plant members of the same plant family in the same location in consecutive seasons. For example, do not follow melons with cucumbers or squash.
  • Proper use of fertilizers and water will induce healthy plant growth and increase the capability of plants to tolerate insect damage. However, excessive amounts of organic matter or manure can encourage millipedes, pillbugs, white grubs and certain other pests.
  • Changes in planting or harvesting time often will reduce plant damage or keep insect pests separated for susceptible stages of the host plant. Delayed planting, until the soil is warm enough for corn and bean seeds to germinate quickly, reduces seed maggot damage. Hot caps or row covers placed over plants used during the early season not only will preserve heat, but also will protect plants from damaging wind, hail and insects. In some situations, a healthy transplant will overcome insect damage more easily than a small plant developing from seed in the field.
  • Removing soil residues and disposing of weeds and other volunteer plants eliminates food and shelter for many insect pests such as cutworms, webworms, aphids, white grubs, millipedes and spider mites. When plants stop producing, till them into the soil or take them to the compost pile.
  • Companion planting (an orderly mixing of crop plants) is a cultural practice aimed at diversifying insect populations. Numerous claims have been made about the ability of certain plants to protect certain other plants from insect damage. However, no data from scientific studies are available to prove the value of companion plantings.

Mechanical Control Methods

Preventive devices often are easy to use, although their effectiveness varies. Here are examples of such devices:

  • Cheesecloth or spun bound polyester row covers for plant beds, hot beds and cold frames to prevent insect egg laying.
  • Mesh covers for tomatoes and other plants to keep out large insects and birds.
  • Aluminum foil mulch to repel aphids.

Black light traps are effective tools for monitoring insect species in a given area, but usually provide little protection for the crop. Light traps attract both harmful and beneficial insects that ordinarily would not be found in the area. Attracted insects may not be caught in the traps, but may remain in the area, and the harmful ones may cause damage later. Also, some species such as wingless insects and those insects only active in the daytime are not caught in the traps. Consequently, the value of black light traps is questionable. Where black lights are used, it is recommended that they be placed 50 to 75 feet away from the area which is to be protected.

Biological Control Methods

Generally, biological control can be defined as the direct or indirect use of parasites, predators or pathogens (bacteria, viruses, fungi protozoans) to hold pest insect populations at low levels to avoid economic losses. Biological control methods fall into three categories:

  • Introduction of natural enemies which are not native to the area (these enemies must then establish and perpetuate themselves).
  • Enlarging existing populations of natural enemies by collecting, rearing and then releasing them back into the environment.
  • Conservation of beneficial organisms by such means as the judicious use of pesticides and the maintenance of alternate host insects, so parasites and predators can continue to develop.

Many beneficial organisms occur naturally around crops, but often they are not numerous enough to control a pest before it inflicts severe damage. In fact, parasites and predators appear to be most effective when a pest population has stabilized or is relatively low. Their influence on an increasing pest population usually is minimal since any increase in parasite and predator numbers depends on an even greater increase in pest numbers. Pathogens, however, seem to be most effective when pest populations are large. Consequently, the nature of the host insect-natural enemy relationship makes it impossible to have an insect-free environment and at the same time maintain sizable populations of beneficial insects.

The following is a list of some of the more popular biocontrol agents:

  • Bacillus thuringiensis: (Dipel, Thuricide, Biological Worm Killer). This bacterial insecticide provides effective control of the larvae of several moths and butterflies. The bacterial spores are harmless to warm blooded animals and beneficial insects.
  • Bacillus popilliae: (Milky Spore, Doom, Japedimic). This bacterial insecticide controls grubs of Japanese beetles in the eastern U.S., and some testing has been done for control of white grubs (Phyllophaga spp. and Cotinis spp.) in Texas. It has not been effective against the principal white grub species in Texas.
  • Nosema locustae: A spore (Protozoan) used to control grasshoppers. The material is sprayed on the plants which grasshoppers ingest. The spores germinate inside the grasshopper, causing death. Control is extremely slow and growers may not be satisfied with results. Baits have proven more effective.
  • Trichogramma wasp: Adult wasps are available from several sources. The tiny wasps attack the eggs of more than 200 pest species, including cutworms, armyworms, fruit worms and many moth and butterfly eggs deposited in orchards and field crops. Wasps should be released when the moths are first seen, but a sequence of releases throughout the season is preferable to a single, large release. Results will depend on the timing of the releases, selection of Trichogramma species and placement of wasps near host egg masses.
  • Green lacewings (Chrysopa): The larvae, known as aphid lions, prey on many vegetable pests including aphids, spider mites, leafhoppers, thrips, moth eggs and small larvae. Adult lacewings feed on honeydew, nectar and pollen. Introduced lacewings must have a readily available supply of food, or they will leave. Eggs are sometimes available.
  • Praying Mantis: Egg cases, containing about 200 individual eggs, are available from a number of sources. The mantis is a voracious predator. In addition, it is cannibalistic immediately after hatching, so few nymphs survive the first week of life. However, the mantis is a poor searcher for food and usually waits for prey to come to it. This greatly influences the kinds of insects it captures and kills. Food preferences include grasshoppers, crickets, bees, wasps and flies.
  • Lady beetles: Adult beetles are available from several sources. Aphids are the preferred hosts, but lady beetles will eat mealy bugs, spider mites and certain other soft bodied pests and eggs. They do not, however, kill grubs, caterpillars and other beetles. Unless an ample supply of live aphids or other hosts are available at the release point, lady beetles will disperse and leave the area. In some cases, most of the beetles will leave the area regardless of the availability of food. Lady beetles can be encouraged to remain on a plant by using small meshed screen cages (remove cages before they devour all of their food supply). In hot dry weather these beetles enter a nonactive (aestivation) state in which feeding and reproduction cease. Little control from lady beetles can be expected during this time.

Natural Insecticides

Despite all efforts, noninsecticidal methods at times will fail to prevent excessive insect damage. At such times, the use of insecticides may be the only alternative left. Insecticides chosen should have only low toxicity for humans and other warm blooded animals. They should be used only when needed and according to label directions. A better understanding of insecticides will enable you to use these materials more effectively and to realize that they can be an aid without harming you or the environment.

  • Pyrethrum: Botanical Insecticide. This slightly toxic insecticide is derived from the flowers of a species of Chrysanthemum imported mainly from Kenya and Ecuador. The material causes rapid paralysis of most insects, but the insects usually recover unless the pyrethrum is used in combination with a synergist or other poison. Pyrethrum, mixed with synergists such as piperonyl butoxide or piperonyl cyclonene to increase toxicity and produce longer residual action, is used extensively in crop sprays and dusts. This chemical is registered for use on most vegetables at any time during the growing season.
  • Nicotine: Botanical Insecticide. Pure nicotine is a tobacco extract highly toxic to warm blooded animals. The insecticide usually is marketed as a 40% liquid concentrate of nicotine sulfate, which is diluted in water and applied as a spray. Dusts can irritate the skin. Nicotine is used primarily for piercing-sucking insects such as aphids, whiteflies, leafhoppers and thrips. Nicotine is more effective when applied during warm weather. It degrades quickly, so it can be used on many food plants nearing harvest. It is registered for use on a wide range of vegetable and fruit crops.
  • Sabadilla Botanical Insecticide: Sabadilla is obtained from the seeds of a lily-like plant and acts as both a contact and stomach poison for insects. It is not particularly toxic to mammals, but does cause irritation of the eyes and respiratory tract. A mask should be worn when working with this insecticide. This material deteriorates rapidly upon exposure to light and can be used safely on food crops shortly before harvest. Sabadilla generally is used as a 5 to 20% dust or as a spray.
  • Rotenone: Botanical Insecticide. Rotenone is extracted from the roots of Derris plants in Asia and cube plants in South America. This general insecticide is harmless to plants, highly toxic to fish and many insects, moderately toxic to mammals, and it leaves no harmful residues on vegetable crops. It acts as both a contact and stomach poison to insects. It is slow acting, and in the presence of sun and air, its effectiveness is lost within a week after application. Wear a mask during application because rotenone can irritate the respiratory tract. Rotenone dusts and sprays have been used for years to control aphids, certain beetles and caterpillars on plants.

Organic Controls

Growers have been using soap to control insects since the early 1800′s. Researchers have not yet determined exactly how soaps work. Some soap simply wash off the outer waxy coating of the insect cuticle, destroying its watertight nature and causing the insect to dry up and die. Other soaps have additional insecticidal properties which may affect the nervous system. These soaps appear to have toxic activity only against plant eating insects, and thus may spare beneficial insects such as lady beetles, honeybees, lacewings and predatory mites. Although a number of soaps tested have insecticidal properties, only Safer’s Insecticidal Soap is currently registered for use on edible crops. It controls such pests as spider mites, aphids, mealybugs, whiteflies, harlequin bugs, stink bugs and thrips.

Organic growers have been using a spray mixture containing onions, garlic and pepper mixed together to control insects for many years. Research indicates that of combination of these materials have been erratic and in many cases ineffective for insect control. Sprays of food-derived substances do not appear to be good choices as a pesticide. Some success may be achieved with them, but it is likely to be sporadic. Spraying several times a week might help to bring infestations under control. Control with one application should not be expected.

Controlling Insects with Pesticides

Insects often pass the winter in a life stage that can damage crops early the next season. They may pass the winter in the field in grass, trash or in weedy fence rows. Squash bugs spend the winter in the adult stage and often invade fields soon after planting and begin feeding. Good squash bug control can be achieved if producers realize this and treat them before the plants actually begin to bloom. Deep plowing of fields in the winter can eliminate some overwintering stages.

Fall grown vegetables are usually heavily attacked by insects. More insecticides may have to be used on fall vegetables.

Some cultural control measures can be used to suppress insect pressure. Examples of these measures are deep plowing, controlling weeds and grass around the field and destroying crop residue soon after harvest.

Carbaryl (Sevin) insecticide is an excellent material, and vegetables sprayed with Carbaryl can be harvested soon after spraying. However, repeated use of Carbaryl often results in a spider mite outbreak.

Use alternate classes of insecticides in the spray program. Do not rely on one product for the entire season. Be prepared to switch when insect pressure or different species are found. For specific insect control recommendations, refer to the Texas AgriLife Extension publication B-1305, “Texas Guide for Controlling Insects on Commercial Vegetable Crops.”

Various reference materials are available in the form of USDA publications and books. A good reference book is ‘Destructive and Useful Insects’ by Metcalf, Flint, and Metcalf. The Peterson Field Guide series are also excellent references. ‘The Field Guide to the Insects of America North of Mexico’ by D.J. Borror and R.E. White is an excellent reference for the identification of the various insects.

Safe Use of Pesticides

When it is necessary to use insecticides, use them wisely and safely. The following tips will help you make better use of insecticides:

  • Inspect plants and monitor insect numbers and activity on a regular basis. Pay particular attention to underside of leaves where insects and their eggs frequently occur. If treatments are applied when an infestation first starts, insect numbers can be maintained at lower levels much more easily and with smaller amounts of chemicals.
  • When applying insecticides to plants, treat all plant surfaces unless otherwise stated on the product’s label. This ensures that an insect anywhere on the plant will be exposed to a lethal amount of the chemical. Do not apply insecticides to wilted plants or during the hottest part of the day. Apply dusts only when the wind is calm and plants are dry. Sprays should be applied when the wind is no more than 5 to 10 mph. Retreatment may be necessary after rainfall.

Common Insects Attacking Vegetable Crops

Soil Insects

Wireworm

Plants Attacked: Corn, small grains, grasses, potatoes and other root crops such as sweet potatoes.

Description: Adults are usually hard shelled, brownish gray or nearly black, somewhat elongated with the body tapering toward each end. The larvae usually are hard, dark brown, smooth, wire like worms from ½ to 1½ inches long when grown.

Life History: Young adults remain in the soil until spring. The subsequent egg stage requires a few days to a few weeks to hatch. Larvae spend from two to six years in the soil feeding on roots of grasses and other plants. Pupation usually is completed in a few weeks.

Damage: Crops may fail to emerge, or stay thin and patchy. Wireworms feed on seed and underground plant parts. On potatoes they cause small holes or trail-like appearance in their feeding habits.

White Grubs

Plants Attacked: Corn, beans and potato tubers and other root vegetable crops.

Description: Adult beetles are from ½ to 1 inch long, vary from light to dark brown and are robust in form. The larvae are white, curved bodied grubs with brown heads and three pairs of legs. The hind part of the abdomen appears darker because soil particles inside show through the body wall. There are probably 100 species, and many cause damage.

Life History: The life cycle of the more abundant species extends over three years. In late spring pearly white eggs are deposited from 1 to 8 inches deep in the soil. Approximately 3 weeks later the eggs hatch, and the larvae feed on roots and decaying matter. In autumn when cooler temperatures prevail, the larvae emigrate downward and remain inactive until the following spring when they return to feed on plant roots near the soil surface. Greatest damage occurs at this time. At the next autumn, they begin to go deep into the soil, returning to the surface in the spring of the third year; they feed until June. Then oval, earthen cells are made and pupation follows. Adult beetles form in the pupae in a few weeks; they remain in the cells throughout the winter and emerge from the soil the following year to begin feeding, mating and egglaying. In Texas the period from egg to adult seems to be two years for most species.

Damage: Most severe damage by grubs occurs on crops which follow grass sod the next year. Grubs feed on the roots of crops attacked and destroy the root system.

Chewing Insects

Fruit Worms

There are many types of worms that feed on plants, the fruit or leaves and buds of the plant. These worms include armyworm, beet armyworm, fall armyworm, melon worms, pickle worms, tomato hornworm, tomato pinworm, yellow striped armyworm and the corn ear worm, or the tomato fruit worm.

Plants Attacked: Sweet corn, beans, peas, beets, peppers, melons, squash, cucumber, cauliflower, broccoli and similar crops. The description of these various insect larvae is completely different. Many have elongate bodies, may be striped with spots, and most of them are green to brown to reddish color. The stripes may be white or yellow. Most all of them have three legs behind the head and five sets of legs along the abdomen.

Life History: Many spend the winter as pupae 2 to 6 inches below the soil surface and emerge as moths during the spring and early summer to begin depositing eggs on their favorite crops or host plant. Fresh laid eggs usually are waxy white, but soon turn yellow and darker as the insect matures inside. They are about half the size of a pinhead and variously shaped. A female can deposit from 500 to 3,000 eggs singly on foliage and fruit of many plants. They prefer the tender growing point of the plant to deposit eggs, but eggs commonly are deposited in the curl of various plants and in the whirl of corn or grassy crops. Later in the season, they may deposit eggs directly on the fruit of the plant. The eggs usually hatch in 2 to 4 days, but it may take up to ten days in cold weather; the worm stage lasts 2 to 4 weeks. Full grown larvae crawl down the host plant or drop to the ground when they burrow forming a wall shell cell and pupate. The adult moths usually emerge in 10 to 25 days following pupation. Time from egg to adult varies from 1 to 2 months depending upon weather conditions. There may be 4 to 7 generations of these various worms throughout the year.

Damage: Newly hatched larvae begin feeding immediately on the part of the plant where the eggs were laid. They may then bore into the fruit such as tomatoes or the ears of corn or feed on pods of beans and peas. The worms cause considerable damage because they gnaw or eat out sections of the fruit, and may cause the fruit to rot.

Cabbage Looper

Plants attacked: Cabbage, cauliflower, broccoli, Brussels sprouts, lettuce and occasionally beans, tomatoes and other crops.

Description: Light, grayish brown moth with a small lighter colored spot near center of forewings. Moths have a wing spread of about 1½ inch. Larvae are light green caterpillars with a few white or pale yellow stripes. Larvae travel with a characteristic looping motion. They have three legs behind the head and three legs at the tip of the abdomen.

Life History: There are continuous generations in the Lower Rio Grande Valley with reproduction slowing down during cold periods. In colder areas the insects overwinter as pupae in flimsy silken cocoons attached to plant residue. A complete generation occurs in 3 to 6 weeks.

Damage: Cabbage loopers are voracious feeders which can strip foliage from infested plants in a short time. Often, when cabbage looper populations become crowded, a virus disease strikes causing high larval mortality.

Thrips

Plants Attacked: General feeders on vegetables, flowers and field crops.

Description: These are slender, spindle shaped, active insects varying from pale yellow to yellowish-brown. Adults average about 1/25 inch long. Four slender wings are present on females, fringed with long hairs and black margins. Males are wingless, and the larvae resemble adults but have no wings and are smaller.

Life History: The minute eggs are inserted into leaves or stems. These hatch in 2 to 10 days. The larval stage lasts from 5 to 30 days. Adult females can reproduce regularly without mating with the rarely found males. All stages can be found during warmer months, but during colder months, only adults and larvae can be found. It is probable that 5 to 8 generations occur per year, but more may occur in the warmer parts of the state.

Damage: Thrips puncture plants, rasp the surface and then suck the juice. This causes the formation of whitish blotches that first appear as dashes. Severely attacked plants develop a gray or silver appearance and may become distorted. Damage may be found first in the leaf sheaths and stems or in the undersides of a bent leaf where the insects always are most abundant.

Sucking Insects

Aphids

Aphids are small, sluggish soft-bodied insects often called plant lice. Most species give birth to living young and the young build up very rapidly.

Plants Attacked: All vegetables. Most common vegetables are peas, beans, tomatoes, lettuce, turnips, broccoli and corn.

Description: The most common aphids are the melon or cotton aphid, green peach aphid, cabbage aphid and the pea aphid. They are usually 1/16 inch long, soft-bodied and pear-shaped. They may be black, gray, green, red or yellow, depending on the species.

Life History: Most species give birth to living young; they do lay eggs, and some have wings. There can be 15 to 20 generations per year of certain species.

Damage: Aphids congregate in large numbers and, therefore, may infest vegetables such as mustard greens and leaf lettuce; they also may cause the plant to stunt. Disease transmission is probably more important than actual feeding damage.

Squash Bugs

Plants Attacked: All cucurbits with preference for squash.

Description: The adults are brownish gray to dark gray bugs about 5/8 inch long. The immature, or nymphs, when first hatched are green with black legs. Later, they become grayish-white with nearly black legs and antennae.

Life History: Adults overwinter, unmated in any type of shelter. They appear in the spring as plants begin to vine and mate. Yellowish to bronze-brown eggs are laid in clusters on the underside of leaves usually in vine angles. Eggs hatch in 1to 2 weeks, and nymphs feed in groups on the stems of the plant for 6 to 8 weeks before transforming to adults.

Damage: Leaves attacked by the squash bug will rapidly become black, crisp and dead. Attacked plant stems often are enlarged but later wither and die.

Stink Bugs

Plants Attacked: Seed beets, okra, squash, beans, peas, corn, cowpeas, and tomatoes.

Description: The adults are approximately ½ inch long, and each has a triangular shaped shield on the back that extends just back of the shoulders narrowing posterior to a point. Front wings are thickened and stiff about the base, but the distal half is much thinner and membranous. Crushed bugs often have an odor fitting their name. The nymphs are without wing covers and smaller but otherwise similar to adults.

Life History: Life history and habits of each of the stink bugs are similar. Generally barrel shaped eggs are deposited in clusters usually on the underside of foliage. Eggs often are beautifully colored and ornamental. Development from egg to adult occurs in 4 to 6 weeks. From 1 to 3 or perhaps four generations may occur annually. They overwinter as adults in places affording protection from cold weather.

Damage: Damage is caused by nymphs and adults sucking sap primarily from pods, buds, blossoms and seeds. In removing the liquid from contents of developing seeds causes them to become flattened and shriveled. If the pods are attacked at an early stage of development, catfacing or pitted holes will occur on bean pods and squash fruit.

Whiteflies

Plants Attacked: Potato, tomato, eggplant, pepper and sweet potato.

Description: Adults are 1/16 inch in length. They have four wings which along with the dorsal part of the body are covered with white, waxy powder. The nymphs are light green, oval, flattened and about the size of a pinhead. They are attached to the leaf surface until mature, with the last instar more elevated and slightly segmented. The bodies are covered with radiating long filamentous threads resembling young, soft scale insects.

Life History: Overlapping generations occur in the Lower Rio Grande Valley during spring, summer and fall. Adults emerge, mate and begin depositing elongated yellow eggs and attaching them to the host plant by short stalk. Before hatching, the eggs darken; nymphal period is one month.

Damage: Both nymphs and adults feed by sucking plant juices. Heavy feeding gives plants a mottled appearance, or it causes them to turn yellow and die. The sticky honeydew excreted by the insect often glazes the lower leaves and permits development of black sooty mold on plants, thus detracting from the plant’s beauty and cutting down on photosynthesis.

Spider Mites

Plants Attacked: Various plants are attacked including tomatoes, eggplants, beans, corn, peas and various cucurbits.

Description: The two-spotted spider mite has two forms, a green form with a dark spot on each side and the more common which is a reddish form. Some species of mites may be yellow. All mites are very tiny and almost microscopic. They are about 1/60 inch long. Life History: Adult mites lay eggs on leaf undersides and spin webs beneath which eggs hatch and mites feed. Spider mites reproduce rapidly during hot, dry weather.

Damage: Mites pierce leaf tissue and suck sap in the larval, nymphal and adult stages. Plants attacked begin to lose color, fading from green to yellow and eventually turn red. Heavy infestation may kill some plants, and heavy webbing may appear on certain plants.

The following table is courtesy of S.E. Webb and P.A. Stansly at University of Florida, IFAS Extension Publication ENY-419 titled ‘Insecticides Currently Used on Vegetables’.

Table 1.

Insecticides For Use On Vegetables

Insecticide

General Characteristics

Signal Word

MOA1

Typical Target Pests

Carbamates
*Furadan(carbofuran) systemic action Danger-Poison 1A beetles, some caterpillars
*Lannate(methomyl) very short residual Danger-Poison 1A caterpillars, leafhoppers
Larvin(thiocarb) larvacide & ovicide Warning 1A caterpillars
Sevin(carbaryl) use can result in aphid and mite outbreaks Caution – 4F, XLR, Bait; Warning – 80S 1A beetles, leafhoppers, caterpillars
*Temik(aldicarb) systemic action Danger-Poison 1A aphids, mites, some beetles
*Vydate(oxamyl) contact action, systemic if applied to soil Danger-Poison 1A aphids, thrips, some beetles
Organophosphates
*Counter(terbufos) systemic action Danger-Poison 1B soil pests
*Diazinon Caution 1B aphids, beetles, caterpillars, soil pests, thrips
Dibrom(naled) some short residual fumigant action Danger 1B caterpillars
Dimethoate local systemic Warning 1B aphids, leafhoppers, mites
*Di-Syston(disulfoton) systemic action Danger-Poison 1B aphids
Imidan(phosmet) Warning 1B caterpillars, sweetpotato weevil
Lorsban(chlorpyrifos) long residual Caution – 15GWarning -

75WG, *4E

Danger – 50W

1B caterpillars, soil pests
Malathion short residual Warning 1B broad spectrum
*MSR Spray Concentrate (oxydemetonmethyl) systemic; contact & stomach action Warning 1B aphids, thrips & other sucking insects
*Mocap(ethoprop) contact action Danger-Poison 1B aphids, caterpillars
*Monitor(methamidophos) long residual Danger-Poison 1B aphids, caterpillars & other pests
*Penncap-M(methyl parathion) contact & fumigant action; slow release formulation Warning 1B caterpillars, thrips
*Thimet(phorate) systemic action Danger-Poison 1B soil pests, thrips
Organochlorines
*Endosulfan(endosulfan) fairly long residual Danger-Poison 2A aphids, beetles, caterpillars, whiteflies
Pyrethroids
*Ambush(permethrin) Warning 3 beetles, caterpillars, leafhoppers, thrips
*Ammo(cypermethrin) Caution 3 beetles, caterpillars, leafhoppers, thrips
*Asana(esfenvalerate) Warning 3 beetles, caterpillars, leafhoppers
*Baythroid XL(beta-cyfluthrin) Warning 3 beetles, caterpillars, leafhoppers, thrips
*Brigade(bifenthrin) Warning 3 beetles, caterpillars, leafhoppers, thrips, whiteflies
*Danitol(fenpropathrin) Danger 3 caterpillars, leafhoppers, whiteflies
*Force(tefluthrin) Caution 3 soil pests
*Mustang Max (zeta-cypermethrin) Warning 3 beetles, caterpillars, leafhoppers, thrips
*Pounce(permethrin) Caution -(3.2EC, 1.5G)

Warning – (25WP, WSP)

3 beetles, caterpillars, leafhoppers, thrips
*Proaxis(gamma-cyhalothrin) Caution 3 beetles, caterpillars, leafhoppers, plant bugs, stink bugs
Pyronyl Crop Spray (Pyrethrins) contact, stomach, & fumigant action; extract from chyrsanthemums Caution 3 broad spectrum
*Warrior (lambda-cyhalothrin) Warning 3 beetles, caterpillars, leafhoppers, thrips
Neonicotinyls
Actara(thiamethoxam) local systemic Caution 4A aphids, potato leafhopper, some beetles, stinkbugs, whiteflies
Admire(imidacloprid) systemic, long residual Caution 4A aphids, leafhoppers, some beetles, whiteflies
Assail(acetamiprid) local systemic, ovicidal effects Caution 4A aphids, Colorado potato beetle, whiteflies
Belay(clothianidin) systemic, long residual Caution 4A Colorado potato beetle, aphids, leafhoppers
Platinum (thiamethoxam) systemic, long residual Caution 4A aphids, potato leafhopper, some beetles, stinkbugs, whiteflies
Provado(imidacloprid) local systemic Caution 4A aphids, leafhoppers, some beetles, whiteflies
Venom(dinotefuran) systemic or locally systemic, depending on application method, long residual Caution 4A aphids, Colorado potato beetle, leafhoppers, leafminers, thrips, whiteflies
Other insect nerve poisons
Acramite(bifenazate) contact, long residual, ovicidal activity against spider mites Caution un mites
*Agri-Mek(abamectin) active once ingested; some contact action; mostly stomach poison Warning 6 leafminers, mites, some beetles, tomato pinworm
Avaunt(indoxacarb) ingestion plus contact, slightly to moderately translaminar Caution 22 caterpillars
Beleaf(flonicamid) contact & ingestion, causes rapid cessation of feeding Caution 9C aphids
Coragen(rynaxypyr) long residual, causes rapid cessation of feeding None 28 caterpillars, Colorado potato beetle, leafminers
Fulfill(pymetrozine) feeding inhibitor Caution 9B aphids, whiteflies
*Proclaim(emamectin benzoate) ingestion & topical; translaminar, not systemic Caution 6 caterpillars
Radiant(spinetoram) ingestion & contact; enters leaf but does not translocate Caution 5 caterpillars, some beetles and thrips
SpinTor(spinosad) ingestion & contact; enters leaf but does not translocate Caution 5 caterpillars, some beetles and thrips
Insect Growth Regulators
Confirm(tebufenozide) slow acting Caution 18 caterpillars
Courier(buprofezin) disrupts egg hatch and molting; use in rotation iwth other insecticides Caution 16 whiteflies
*Dimilin(diflubenzuron) slow acting, disrupts molting process, reduces egg hatch of pepper weevil Caution 15 caterpillars, pepper weevil
Esteem Ant Bait(pyriproxyfen) breaks reproductive cycle of ants; slow acting but effective Caution 7C ants
Extinguish[(S)-methoprene] slow acting Caution 7A fire ants
Intrepid(methoxyfenozide) Caution 18 caterpillars
Knack(pyriproxyfen) use in combination or rotation with other insecticides Caution 7C whiteflies
Neemix(azadirachtin) slow acting, also acts as feeding repellent Caution – Azatin XL Plus; Warning – Neemix 4.5 un broad spectrum
Rimon(novaluron) disrupts cuticle formation and deposition at molting, resulting in death of larva; no effect on adult insect Warning 15 caterpillars
Trigard(cyromazine) most effective against small leafminer larvae Caution 17 dipterous leafminers, maggots, some beetles
Miscellaneous
Bacillus thuringiensis(B.t.) var. aizawai

(B.t.) var. kurstaki

pest must ingest; slow acting but feeding stops long before death Caution 11 caterpillars or beetles, depending on strain
Cryolite(Kryocide) pest must ingest; not rainfast; an inorganic fluorine compound Caution un beetles, caterpillars
M-Pede(potassium salts of fatty acids) contact activity; phytotoxic at high temperatures Warning aphids and other soft-bodied arthropods
Movento(spirotetramat) ingestion, fully systemic in plant after foliar application Caution 23 aphids, psyllids, whiteflies
Mycotrol(Beauveria) contact; slow acting aphids, leafhoppers, whiteflies
Oberon(spiromesifen) inhibitor of lipid synthesis; most effective on juvenile stages of mites and on nymphs and pupae of whiteflies and psyllids Caution 23 mites, psyllids, whiteflies
SunSpray Ultra Fine Spray Oil(mineral oil) contact activity Caution aphids, mites, whiteflies
M-Pede(potassium salts of fatty acids) contact activity; phytotoxic at high temperatures Warning aphids and other soft-bodied arthropods
*Vendex (fenbutatin-oxide) Danger-Poison 12B mites
*Restricted Use PesticideOriginally adapted from: Welty, Celeste. Insecticides for use on vegetables in Ohio. pp. 46-48, 2002 Ohio Vegetable production Guide, Ohio State University.

1Mode of Action codes for vegetable pest insecticides from the Insecticide Resistance Action Committee (IRAC) Mode of Action Classification v.3.3 October 2003.

1A. Acetylcholinesterase inhibitors, Carbamates (nerve action)

1B. Acetylcholinesterase inhibitors, Organophosphates (nerve action)

2A. GABA-gated chloride channel antagonists (nerve action)

3. Sodium channel modulators

4A. Nicotinic acetylcholine receptor agonists (nerve action)

5. Nicotinic acetylcholine receptor allosteric activators (nerve action)

6. Chloride channel activators (nerve and muscle action)

7A. Juvenile hormone mimics (growth regulation)

7C. Juvenile hormone mimics (growth regulation)

9B & 9C. Selective homopteran feeding blockers

10. Mite growth inhibitors (growth regulation)

11. Microbial disruptors of insect midgut membranes

12B. Inhibitors of mitochondrial ATP synthase (energy metabolism)

15. Inhibitors of chitin biosynthesis, type 0, lepidopteran (growth regulation)

16. Inhibitors of chitin biosynthesis, type 1, homopteran (growth regulation)

17. Molting disruptor, dipteran (growth regulation)

18. Ecdysone receptor agonists (growth regulation)

22. Voltage-dependent sodium channel blockers (nerve action)

23. Inhibitors of acetyl Co-A carboxylase (lipid synthesis, growth regulation)

28. Ryanodine receptor modulators (nerve and muscle action)

un. Compounds of unknown or uncertain mode of action

 

 

 

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