Entomopathogenic Nematodes

Insect-pathogenic, or entomopathogenic nematodes, are a group of soil-dwelling roundworms which kill insects that live in, on, or near the soil surface, usually closely associated with plants. These nematodes can occur naturally in soil and are found in most places where plants grow. Research has demonstrated that entomopathogenic nematodes can be mass produced, have a narrow host specificity against pests, and are safe to plants and vertebrates; and, therefore, the U.S. Environmental Protection Agency has exempted them from all registration requirements and related regulation. Entomopathogenic nematodes have been available commercially to agriculturists and used in a variety of cropping systems.

There are two main groups of entomopathogenic nematodes: the steinernematids and the heterorhabditids. Both have similar life cycles, and only the free-living, infective juvenile stage is able to infect the target pest insect. It is the juvenile stage that is found in or on the soil, searching out a host to infect. In fact, the juvenile form is the only form found outside of the host.

Slug-pathogenic, or malacopathogenic, nematodes, are also a group of soil-dwelling roundworms, a novel method that has been used in Europe to kill slugs with some success. One of the most widely established, commercially-available slug biocontrol agents in Europe is the nematode Phasmarhabditis hermaphrodita (Schneider) sold as Nemaslug, and mixed with P. californica sold as Nemaslug 2.0. This nematode is associated symbiotically with a bacterium that uses an endotoxin that kills slugs. The nematode locates slugs in the soil and enters the slug’s mantle cavity. After the slug dies, the nematodes multiply over the decaying slug body and then migrate back into the soil where, if conditions are favorable, they infect more slugs. Phasmarhabditis hermaphrodita has also been found in CA and OR, as well as other Phasmarhabditis species which have shown promise for control of the grey garden slug. These nematodes must be demonstrated as not harmful to native species such as the banana slug before they can be commercialized. They are currently unavailable for purchase.

This document focuses on insect-pathogenic nematodes.

Nematode selection for Insects

The selected nematode (Steinernema spp. or Heterorhabditis spp.) depends on the target insect pest. In general, nematodes in the genus Steinernema are “sit-and-wait predators” or ambushers and are used against insects whose immature stages (larvae or pupae) spend most of their time at or near the soil surface. Other species are highly mobile and roam through the soil searching for hosts. The host-finding strategy of most Steinernema is to wait until the prey bumps into the nematode, and then infects it. In contrast, nematodes in the genus Heterorhabditis actively seek out or hunt for their prey, sometimes several inches below the soil surface, and stay in one spot for an extended period of time. Thus, nematodes in the genus Steinernema (S. feltiae) are the best choice against fungus gnat larvae, often found on the soil surface of potted plants, while the genus Heterorhabditis, (H. megidis, H. marelatus, or H. bacteriophora) are the best choices against the black vine weevil, deeper in the soil. Recently, H. bacteriophora, H. indica, Oscheius oniric, Steinernema carpocapsae, S. feltiae, and S. kraussei appeared promising for targeting spotted-wing drosophila. The nematodes can penetrate the larval and teneral adult stages of spotted-wing drosophila. Given the high dose needed and cost of nematode application, researchers are examining whether nematodes can be effectively delivered by drip irrigation to target flies that pupate in the soil. The fact that infected adult flies can disperse and spread the nematodes to other flies may make this a viable option.

There are over ten entomopathogenic nematodes commercially-produced as a biological insecticide for over 25 insect pests. There is some overlap between the various species with regards to host-finding ability. Consult a nematode manufacturer/supplier for selection of the proper entomopathogenic nematode product.

Life cycle

An infective juvenile may at first move randomly, and then find their insect host via carbon dioxide, host odor or damaged plant odors. Once a juvenile locates an insect, the juvenile enters via a natural opening; or it may penetrate a weak spot in the insect’s cuticle. Insect larvae and pupae are more susceptible to nematodes since adult insects are often more mobile. Once inside the host’s blood system, the juvenile releases a symbiotic bacterium that it carries. The bacteria are released into the blood of the host, rapidly multiply, and produce compounds that kill the host insect generally within 48 hours. The bacteria protect and preserve (via antibiotics) the dead insect from invasion by unwanted, contaminating soil microbes and the nematodes provide shelter for the bacteria. The infective nematodes complete one to several generations inside the host, feeding on the bacteria and nutrients within the dying host. Only when all the host tissues have been consumed does a new generation of juveniles emerge, all carrying the symbiotic bacteria with them in search of new hosts (see Figure 1).

One generation from egg to egg typically takes from 4 to 7 days. In most instances, there are at least two generations inside a host before the new juveniles emerge seeking a new host, so from the time of first infection by juveniles to the time “new” juveniles emerge may be from 8 to 14 days. The length of time is determined by the temperature of the soil, the size of the host, and which nematode is involved. A large host such as a cutworm will support several generations before conditions become too “crowded” and juveniles emerge, compared to a strawberry root weevil larva, where there may be only one or two generations before juvenile emergence. Similarly, a large nematode such as S. carpocapsae has fewer generations than S. feltiae when infecting similar-size hosts.

Application methods

Though the adult stage of some insect pests also is susceptible, entomopathogenic nematodes generally are used for controlling the soil-borne larval or pupal stages of a pest. Therefore, entomopathogenic nematodes most often are applied by drench or band application. While broadcast application has been used at times, the immature pest insect usually is not located between the crop rows as there is usually no food source there. If, however, the crop has a closed canopy like cranberries or mint, a broadcast application may be warranted. An adjuvant may help. Select your application method wisely, as it may impact greatly the success of host location, infection, and control by the entomopathogenic nematodes.

Entomopathogenic nematodes come in a variety of formulations: water-dispersible granules, nematodes on gel, micronized vermiculite, nematode wool, and an aqueous suspension of nematodes. These formulations are intended to be mixed with water to release the nematodes through common application equipment such as small pressurized sprayers, mist blowers, electrostatic sprayers, or even helicopters (aerial application). Some more promising methods for applying entomopathogenic nematodes are emerging. One uses irrigation systems in a manner similar to chemigation. Another uses nematode-filled capsules which include attractants or feeding stimulants for the pest; this draws in the pest for infection rather than relying on the nematode to find the pest.

Regardless of the method, nematodes can withstand application pressures of approximately 300 psi and can pass through most spray nozzles without difficulty, though operating pressures between 20 to 60 psi generally are sufficient. Keep in mind that nozzle orifices should not be smaller than 50 microns (0.00019685 inch), and that any screens in the system should have an opening of at least 50 mesh (0.0117 inch) or larger to allow the free passage of nematodes through the system. In any case, follow the manufacturer’s directions.

Nematodes require a film of water around soil particles to move through the soil profile in search of a host. Therefore, pre-irrigate the soil in the treatment area with about 0.25 to 0.5 inch of water no later than a few hours before application of the nematodes. Following the application, “water in” the nematodes with an additional 0.5 inch of water to wash them off of foliage and protect them from damaging UV radiation. Further irrigation to maintain adequate soil moisture for at least 7 days following nematode application also is recommended. Be careful not to over-irrigate, because excess water inhibits the movement of oxygen in the soil, and the nematodes will drown. A good rule of thumb is to avoid standing water in your fields.

Key points for success in using entomophathogenic nematodes

1. When applying agrichemicals in the area where entomopathogenic nematodes are to used, be sure that there is enough separation time between applications of toxic compounds and entomopathogenic nematodes (Table 1). Some chemicals have been found to affect nematode efficacy when nematodes are exposed to them. These should be applied with care when used in conjunction with nematodes.
2. Entomopathogenic nematodes require a moist, not saturated, soil environment so they can move around and locate their host.
3. Soil temperature where nematodes are to be applied should be above 55°F and less than 90°F. Nematodes are also affected by suboptimal soil type, thatch depth, and irrigation frequency.
4. Protect nematodes from excessive exposure to ultra violet (UV) rays which can inactivate and kill them.
5. Time application of entomopathogenic nematodes to target the susceptible stage of the pest.
6. Select the proper nematode species to match the most susceptible pest stage.
7. Storage of formulated nematode species varies: Steinernematids at 39 to 46°F; Heterohabditids at 50 to 60°F. Do not leave in a hot vehicle.
8. Select the application rate and method to maximize contact between entomopathogenic nematodes and the target pest.
9. In all cases, refer to the manufacturer’s label for recommendations.

Note: We appreciate the contributions of past employees of Oregon State University, Peter Guthro and Ralph Berry, to this document.

References for Table 1

Alumai, A. and P. Grewal 2004. Tank-mix compatibility of the entomopathogenic nematodes, Heterorhabditis bacteriophora and Steinernema carpocapsae, with selected chemical pesticides used in turfgrass. 2004. Biocontrol Science and Technology, 14(7). DOI: 10.1080/09583150410001724334.

Shetlar, D.J. 1999. “Application Methods in Different Cropping Systems,” in Proceedings of Workshop—Optimal Use of Insecticidal Nematodes in Pest Management, Aug. 28–30, 1999. S. Polavarapu, ed.

Smith, K. 1999. “Factors Affecting Efficacy,” in Proceedings of Workshop—Optimal Use of Insecticidal Nematodes in Pest Management, Aug. 28–30, 1999. S. Polavarapu, ed.

Additional information on entomopathogenic nematodes and their application can be found in:

Labaude, S., and C.T. Griffin. 2018. Transmission success of entomopathogenic nematodes used in pest control. Insects 9, 72, https://doi.org/10.3390/insects9020072

Mc Donnell, R.J., A.J. Colton, D.K. Howe, and D.R. Denver. 2020. Lethality of four species of Phasmarhabditis (Nematoda: Rhabditidae) to the invasive slug, Deroceras reticulatum (Gastropoda: Agriolimacidae) in laboratory infectivity trials. Biological Control 150: 104349. https://doi.org/10.1016/j.biocontrol.2020.104349

Miles, C., C. Blethen, R. Gaugler, D. Shapiro-Ilan and T. Murray. 2012. Using entomopathogenic nematodes for crop insect pest control. PNW Extension Publication 544. https://pubs.extension.wsu.edu/using-entomopathogenic-nematodes-for-crop...

Table 1. Chemical-use patterns with nematodes

May be tank-mixed with nematodes
Compound	Trade name
acephate	Orthene
azadirachtin2	Azatin, Neem2
Bacillus thuringiensis (Bt)	M-One, Dipel
bifenthrin	Talstar
sodium bromide	Unibrom
carbaryl2	Sevin2
bifenthrin	Talstar
chlorothalonil	Daconil
dimethyl tetrachloroterephthala	Dacthal
copper hydroxide	Kocide
cyfluthrin	Tempo
malathion	Malathion
diazinon	Diazinon
diflubenzurion	Dimilin
endosulfan	Thiodan
esfenvalerate	Asana
etridiazole	Terrazole
fertilizers	various
fipronil2	Chipco Choice2
glyphosate	Roundup
insecticidal soap2	various
iprodione	Chipco 26019
isofenphos	Oftanol
kinoprene	Enstar
metalaxyl	Subdue
methidathion	Supracide
methomyl2	Lannate2
methoprene	Apex
oryzalin	Surflan
oxamyl2	Vydate2
oxazoidinedione	Ornalin
pentachloronitrobenzene (PCNB)	Terrachlor
thiophanate-methyl	Zyban
triademefon	Bayleton
1-wk separation
Compound	Trade name
anilazine	Dyrene
chlorpyrifos	Dursban
dimethyl benzyl ammonium chloride	Physan 20
fenarimol	Rubigan
2,4-D	Various
triclopyr	Turflon, Confront
2-wk separation
Compound	Trade name
ethoprop	Mocap
isazophos	Triumph