Basics of Bug Identification

This article by Rick Foster appeared in "American Vegetable Grower," March 1991.

Proper identification is the first and most essential step in making appropriate pest-management decisions. Trying to manage insects without identifying them properly would be like asking a doctor to treat an illness without describing your symptoms. If you don’t identify the pest correctly, you may apply an insecticide that is not necessary, apply the wrong insecticide, or apply it at the wrong time. One way to increase your skill in identifying insect pests is to collect as much reference material as possible. Most extension entomologists have numerous publications that will assist you. Many are available free of charge or for a nominal fee. There are also a number of good reference books that have excellent pictures of many insects.

Most insects have one of two distinctly different types of life cycles. The first is called incomplete metamorphosis. With this type of life cycle, the immature insect, called a nymph, looks very much like the adult, except that it is smaller and lacks wings. Insects with incomplete metamorphosis usually feed in much the same manner and on the same food in the immature and the adult stages. Some examples of pest insects that have this type of life cycle include the true bugs, leafhoppers, and aphids.

The other type of life cycle is complete metamorphosis. These insects have an immature stage, called a larva, that looks nothing at all like the adult. The larval stage often feeds in a completely different manner than the adult. There is also a pupal stage which occurs between the larval and adult stages. It is during this stage that the remarkable transition from caterpillar-to-butterfly or maggot-to-fly takes place.

Some examples of insects with this type of life cycle include caterpillars, maggots, and beetles. An important point is that once these insects become adults, they do not grow any larger.

Common Vegetable Pests

• Mites.
  Mites are not insects. Therefore, the best way to identify them is by counting their legs. They have eight, while insects have six. However, mites are very tiny; counting their legs requires the use of a hand lens. Mites have sucking mouth-parts and generally feed on the underside of leaves. Leaves that have had considerable mite feeding will appear off-color, and may show symptoms of wilting. There may also be some very fine silken webbing associated with the mites on the underside of the leaves. Mites tend to be more of a problem during hot, dry weather, so if these conditions occur, be prepared to look for mites. To do this, shake a leaf over a sheet of white paper. If the dust spots walk, you probably have mites.

• Aphids.
  Aphids are soft-bodied insects that occur in colonies on the underside of leaves. They also have sucking mouth-parts with which they remove plant juices from the leaves of most vegetable crops. Leaves that have been fed upon by aphids may become curled. Aphids are rather small insects, usually reaching on a length of 1/16-inch. They may have wings, but most will be wingless. Aphids are somewhat unique insects in that they can reproduce without mating, and give birth to live young. Scout for aphids by looking on the underside of leaves.

• True Bugs.
  Although many people call all insects ‘bugs’ there is only one group of insects that are truly bugs. Most bugs can be recognized by the presence of a triangle on the back directly behind the head. Several different types of bugs damage vegetables, including the tarnished plant bug, some species of stink bugs, chinch bugs, and squash bugs. Bugs range in size from quite small, like the 1/6-inch-long chinch bug, to fairly large, like the 5/8-inch-long squash bug. Bugs tend to be fairly mobile, so scouting for them may require a sweep net, depending on the specific insect and the crop. Some bugs are predators of a number of pest insects.

• Leafhoppers.
  There are a number of sizes, shapes, and colors of leafhoppers. Adult leafhoppers are relatively small, rarely exceeding 1/4-inch in length. Some leafhoppers that are important pests of vegetables include beet leafhopper, potato leafhopper, and aster leafhopper. Adult leafhoppers are very mobile, and often must be sampled with a sweep net or some sort of sticky trap. The nymphs cannot fly, and therefore are much easier to find, usually on the underside of leaves.

• Thrips.
  Thrips are slender, very tiny insects that feed with rasping or scraping mouth-parts. The wings of adults are fringed with long hairs, but these may be difficult to see. Some species are capable of transmitting serious diseases of vegetables. Thrips also feed on small grains and weeds, and may move to vegetables in large numbers when these hosts dry down in summer. They tend to be more of a problem during hot, dry weather. Thrips tend to hide in secluded locations on the plant. On some crops, such as onions and cabbage, plants may have to be taken apart to assess populations. Some thrips that are important on vegetables are onion thrips, western flower thrips, and tobacco thrips.

• Beetles.
  Beetles can be readily distinguished by the hard covering that is formed by one of their pairs of wings. They vary greatly in size, shape, and color. The larvae may be grub-like or almost caterpillar-like. Both adults and larvae feed with chewing mouth-parts. The larvae of some species, such as bean leaf beetles, root-worms, cucumber beetles, and wire-worms, feed on underground portions of plants. Others, such as Colorado potato beetles and Mexican bean beetles, feed on plant foliage in much the same manner as the adults. Many species of beetles are pests of vegetables. Fortunately, most can be easily identified with the aid of the type of publications mentioned previously. Depending on the crop and species of beetle, sampling can be accomplished by direct observation, scouting for damage, and using a sweep net or beat cloth or sticky traps.

• Maggots.
  Maggots are the larval stage of flies. They are usually white, legless, soft-bodied insects that feed in moist locations. Most of the maggots of importance on vegetables feed on the roots or other underground plant parts. The most important maggot pests of vegetables are the cabbage maggot, onion maggot, and seed-corn maggot. The adult stages of these pests are flies, slightly larger than houseflies. Damage is usually more severe during cool, wet weather, and in fields with lots of decaying organic matter. These insects cannot be scouted for because once they can be found, it is too late to do anything. Preventive control must be practiced.

• Caterpillars.
  Probably the largest group of pests on vegetables are caterpillars, which are the larval stage of butterflies and moths. They may vary in size from the diamondback moth larvae (3/8-inch long) to the tomato and tobacco horn-worms (4 inches long). Correctly identifying which species of caterpillar is difficult. Again, make good use of the reference materials you have available. The first step in identifying a caterpillar is to note which crop it is feeding on. This will frequently reduce the number of possibilities to a half dozen or fewer. The size may not be very helpful, since all species start off as very small larvae. However, determining the maximum size may help. Look for distinguishing characteristics such as color, spots, stripes, and the number of prolegs. Prolegs are fleshy appendages on the abdomen that look like legs. Often the number of prolegs can help identify the insect. For example, most species of loopers have two pair of abdominal prolegs. The best ways to sample for most species of caterpillar is by direct observation on the plant, scouting for damage, or by using a sweep net or beat cloth. However, some insects, such as corn earworms on sweet corn, cannot be controlled after they are found. The moths must be trapped with pheromone traps to predict when the eggs will be laid so that protective control measures can be taken before the eggs hatch.

There are more than three-quarter of a million species of insects in the world. Correctly identifying which species are feeding on your crops can be a challenge. However, proper management usually depends on how well you identify the pests you are trying to control.