Nematodes Show Potential for Control of Soil Insect Pests

by Casey Hoy, Parwinder Grewal and Janet Lawrence
Ohio State University

Entomopathogenic nematodes (Steinernema and Heterorhabditis) 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 entomophthogenic 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 pests and can be easily mass-produced and applied using conventional equipment.

Under suitable environmental conditions, the infective 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 bacteriophora 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 osts 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 infective juvenile nematodes stand on their tails and wait for long periods for an insect to come close enough to latch on. Steinernema glaseri and 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 groubs 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, although they were isolated from 15-20% of the soil samples taken in the grassy border areas.

Three species of entomopathogenic nematodes were recovered: S. feltiae, H. bacteriophora, and H. megidis. These species all are 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 namatodes 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.