Entomopathogenic nematodes
(steinernema and heterohabditis) are microscopic roundworms that parasitize
and kill insects in the soil. Unlike their plant eating distant relatives,
entomopathogenic nematodes do not harm plants, they only attack insects.
These nematodes have demonstrated potential for biological control
of insect pests, particularly those that live in the soil. The entomopthogenic
nematodes occur naturally in almost all soils and reproduce in dead
insect hosts. The nematodes cause widespread mortality of insects
in the soil and cause insect populations to crash conspicuously.
More than 30 of these nematode species have been discovered worldwide.
Due to the ease in nematode mass-production, several nematode-based
products have been developed for use as biological insecticides. Entomopathogenic
nematodes are well suited for pest control because they attack a broad
range of pest and can be easily mass-produced and appled using conventional
equipment.
Under suitable environmental conditions, the inefective juvenile nematodes
seek insect larvae and pupae in soil. They penetrate host insects
through natural body openings (mouth, anus, and spiracles or breathing
holes) and release a bacterium that kills the insects within a day
or two. Insects killed by nematodes are flaccid, do not give off foul
smell, and have conspicuous colors. For example, insects killed by
steinernema carpocapsae are yellow and those killed by heterorhabditis
baceriophora are reddish brown. After the death of the host, the nematodes
feed on the bacteria and insect body contents, and they reproduce.
Within two to three weeks hundreds to thousands of infective juveniles
are released into surrounding soil to seek out new insect hosts and
continue their life cycle. For commercial purposes, they are mass-produced
either in live insects or in fermenters.
Nematode species and strains differ in their activity against different
insect pests. These differences are due to the different search behaviors
of nematode species, and also the type and number of bacteria carried
by the infective juveniles. Steinernema carpocapsae will kill more
mobile insects that live in the upper soil or thatch layer, including
cutworms and armyworms. This nematode species uses an ambush approach
to find insect hosts. The inefective juvenile nematodes stand on their
tails and wait for long periods for an insect to come close enough
to latch on. Steinernema glaseri and H. bacteriaphora on the other
hand use a more active search strategy, called cruising, to find insect
hosts. These nematode species are, therefore, more effective against
less mobile hosts found deeper in the soil, such as maggots, white
grubs and weevil larvae. This distinction should be used as an overall
guide for matching the right nematode species with the target pest.
Dozens of insect pests are susceptible yet many non-target beneficial
insects are unaffected. Studies show that entomopathogenic nematodes
have good potential for pest control in vegetables, including root
maggots, carrot weevil and flea beetles. These studies also indicate
that the nematodes are less effective than conventional pesticides
if they are applied for immediate control of damaging pests. Given
their life cycle, however, they appear to have some potential for
population buildup in soils, where they can suppress a wide range
of pests over time.
In preliminary studies, we examined entomopathogenic nematode population
survival in a high-organic matter muck soil with vegetable pests as
hosts. H. bacteriophora, S. carpocapsae “ALL” strain,
and S. feltiae could be recovered for many weeks after application.
In one preliminary experiment, we released H. bacteriophora and S
carpocapsae in small field plots containing radishes and green onions,
normally attacked by cabbage and onion maggots, respectively. Although
subsequent samples determined that very few maggots were present in
the plots both nematode species survived in the soil and H. bacteriophora
in particular was recovered 52 days after the release. Because most
vegetable crops grown in this area mature in less than 60 days, and
each crop tends to bring with it a new set of insect pests, nematode
populations appear to have the potential to persist through multiple
crop cycles, surviving on various insect species.
We have completed a preliminary survey for entomopathogenic nematodes
in the Celeryville, Ohio vegetable production area, which has very
high organic matter muck soil. Growers in this area have never applied
entomopathogenic nematodes so any found would be naturally occuring.
No entomopathogenic nematodes were isolated from samples taken within
vegetable fields, altough they were isolated from 15 - 20% of the
soil samples taken in the grassy border areas.
Three species of entomopathogenic nematodes were recoved: S. feltiae,
H. bacteriphora, and H. megidis. These species are all raised commercially
for pest control and they are known to infect vegetable pests. We’ve
tested them in the laboratory and found that they are as effective
as commercially sold nematodes in controlling onion maggots and cutworms.
This survey demonstrated that multiple species of entomopathogenic
nematodes can survive and persist in this production area if conditions
are suitable, but the conditions in the vegetable crop area itself
apparently are unsuitable. We want to explain the differences between
fields and the grassy borders, where the entomopathogenic nematodes
seem to persist naturally. The ultimate goal is finding ways to build
this population of beneficial nematodes in the soils within the vegetable
fields to help control a wide range of soil-dwelling insect pests.
