Insect parasitic nematodes
Insect parasitic nematodes are small worms that are abundant in soil and require an insect host to complete their life cycle. These worms are colorless, measure 0.5-1.5 mm in length, and tend to be species-specific in what hosts they infect. Nematodes are commercially available and have been used to control soil-dwelling insects of turfgrass and gardens. It is important to note that they are most effective under specific temperature ranges, moisture content, sun exposure, and other abiotic conditions. Two genera of insect parasitic nematode, Steinerma and Heterorhabditis, are widely considered for managing insect pests and do not pose a threat to plants or vertebrate animals.
Species in genus Steinernema are the most widely researched for managing insect pests and are also more readily available for home use, owing to their relative ease of handling and rearing. Different species are most effective against certain insect groups. For example, Steinernema carpocapsae is most effective against caterpillars of sod webworm, cutworms, and certain borer species. Another species, Steinernema feltiae, can infect the larvae of certain flies such as fungus gnats. Steinernema spp. tend to be less effective against white grubs, root maggots, rootworms, and certain weevil species. Insects killed by Steinernema spp. turn brown or tan.
Heterorhabditis spp. are more susceptible to environmental extremes and are more difficult to rear, making them less commercially available than Steinernema spp. for insect management. Commercially available species include Heterorhabditis bacteriophora, H. megadis, and H. indica. Evidence suggests that Heterorhabditis spp. are more effective against white grubs and many root-feeding pests in plant nurseries. Insects killed by Heterorhabditis spp. turn red and often have a gummy consistency.
- Insect parasitic nematodes, also called entomopathogenic nematodes, are soil-dwelling organisms that can infect and kill a variety of insect hosts, depending on species. When an infected insect cadaver ruptures, hundreds of thousands of nematodes are released.
- Two genera of insect parasitic nematodes are of significance in pest management, Steinernema and Heterorhabditis. Collectively, they can infect white grubs, root maggots, rootworms, and the larvae of some weevils.
- Nematodes are highly sensitive to environmental conditions. Temperature, moisture, and sun exposure can influence their storage longevity and efficacy in the field. Certain pesticides will also limit their effectiveness.
The infective juvenile stage of an entomopathogenic nematode (Steinernema scapterisci). This species of nematode infects larvae of scarab beetles. Image credits: David Cappaert, Bugwood.org.
Two white grub larvae killed by the nematode Heterorhabditis bacteriophora (top). Two healthy larvae (bottom) are also present for comparison. Note the dark brown to red coloration of the cadavers, which is a distinguishing feature of insects killed by Heterorhabditis spp. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Larva of corn earworm killed by a species of insect parasitic nematode (Steinernema carpocapsae). Note the light brown or tan coloration of the cadaver, which is a distinguishing feature of insects killed by Steinernema spp. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Ruptured insect cadaver of greater wax moth (Galleria mellonella), from which thousands of infective nematodes (Heterorhabditis bacteriophora) have emerged. Image credit: Peggy Greb, USDA Agricultural Research Service, Bugwood.org
Life history and habits
Biocontrol products are formulated with juvenile nematodes. They move short distances when searching for a suitable host insect, and Heterorhabditis spp. are typically more mobile than Steinernema spp. The nematodes enter a host through natural openings such as the mouth, anus, or spiracles, and some can also penetrate body surfaces with a thin cuticle. After entering the host, the nematode releases a symbiotic bacterium that kills the insect in two to three days. The nematode then feeds on the liquefied host and bacteria before maturing into an adult.
Juveniles of Steinernematid spp. develop into males or females, while juveniles of Heterorhabditis spp. develop into hermaphrodites that self-fertilize and produce males and females in subsequent generations. One to two generations can occur in a single host. Hundreds of thousands of infective juveniles emerge from a ruptured cadaver and begin searching for new hosts.
Steinernema is used more frequently for yard and garden applications because it is easier to handle. In the field, Steinernema carpocapsae is very effective against caterpillar larvae, but can also be used to control webworms, cutworms, and some borers. Steinernema feltiae is effective against the larvae of some flies. Heterorhabditis is less commonly used because it is more susceptible to environmental stresses. However, this genus is very effective in controlling white grubs and other nursery pests such as black vine weevils, citrus-infesting weevils, and other insects that feed in the root zone.
Generally, biocontrol products with insect parasitic nematodes have a shelf life of several weeks or a few months and can be applied with a sprayer, injector, hose end sprayer, or watering can. Effective control with nematodes is highly dependent on environmental conditions during storage and field applications. Most notably, they are susceptible to desiccation and ultraviolet light and are therefore most active in moist, dark locations. Pretreatment irrigation is recommended as nematodes require free flowing water for mobility. Posttreatment irrigation can also be beneficial since it helps nematodes move through the soil. Soil composition is also important, as nematodes can move more freely through loose sandy soils in comparison with tightly compacted clay soils. Some pesticides are toxic toward nematodes, such as chlorpyrifos and mercurial fungicides. Carbaryl and bendiocarb tend to be moderately toxic, while diazinon displays low toxicity toward nematodes.
The expiration date of biocontrol products should be checked upon arrival, and storage instructions should be followed to maximize shelf life. To assess the viability of nematodes in a biocontrol product, a small amount of the product can be placed in a clear container or petri dish with one or two drops of room temperature water. After a few minutes have passed, actively moving nematodes will be visible. Placing the nematodes over a black background and using a hand lens will make them easier to spot with this method.
Pundt, L., and T. Smith. 2021. Biological Control: Using Beneficial Nematodes. University of Massachusetts Amherst. Available https://ag.umass.edu/greenhouse-floriculture/fact-sheets/biological-control-using-beneficial-nematodes
Shapiro-Ilan, D., and R. Gaugler. (n.d.). Insect-killing nematodes. Cornell – College of Agriculture and Life Sciences. Available https://cals.cornell.edu/new-york-state-integrated-pest-management/eco-resilience/biocontrol/biocontrol-biology/insect-killing-nematodes#pests