Steinernema feltiae

Beneficial nematodes are an effective tool against certain nursery and greenhouse pests, especially fungus gnats.

S. feltiae help control fungus gnats.
Photos: ABOVE: AdobeStock_391873132; BELOW: R. Buitenhuis (Vineland)

Nematodes are small, colorless, cylindrical round worms that occur naturally in soils throughout the world. Different species work best against different target pests. Steinernema feltiae is primarily used against fungus gnat larvae, but also used to combat thrips pupae in the soil.

The nematodes enter the insect host through body openings. They multiply within the host and release a symbiotic bacterium whose toxin kills the fungus gnats. The larvae are killed in one to two days by blood poisoning. More than one generation of nematodes may develop in dead host insect in the media. The infective juveniles then exit the dead body and search for new hosts to infect.

Nematode applications do not require masks or other safety equipment. And re-entry time, residues, groundwater contamination, chemical trespass and pollinators are not issues. Most biologicals require days or weeks to kill, yet nematodes, working with their symbiotic bacteria, can kill insects within 24-48 hours.

S. feltiae is sold under several trade names and are labeled as a soil drench treatment against fungus gnat larvae. Preventive applications to moist soils work best. Nematodes are compatible with several different pesticides. However, they are generally not compatible with organophosphates, carbamates, nematicides and hydrogen dioxide. Do not mix nematodes with your fertilizer solution.

Beneficial nematodes are an effective tool against certain greenhouse pests, especially fungus gnats.

Unlike many traditional pesticides, there is no restricted entry interval (REI), an added bonus in propagation houses. There is also no possibility that the target pest will develop resistance. No adverse effects on non-target organisms have been observed in field studies.

Nematodes (S. feltiae) preparing to enter an adult western flower thrips.

How to apply Steinernema feltiae

Check nematode viability before application. Place a small amount of the product in a small clear container or petri dish. Add one or two drops of room temperature water. Wait a few minutes and look for actively moving nematodes. They have a slight J-shaped curvature at the end of their bodies. Use a black background and a hand lens or field microscope to observe the nematodes. Dead nematodes will be straight and still.

For best results, apply nematodes immediately after receiving them. If you must store the nematodes, store them in a refrigerator at 38-42° F. Avoid placing them in a small refrigerator where they may freeze.

Check the expiration date on the package for the length of time they can be stored.

Treat as soon as possible (two or three days) after sticking cuttings, planting plugs or starting seeds. Injectors are placed directly on the planting line. Some growers apply the nematodes to the media directly before sticking cuttings or dip plugs in solution prior to transplanting.

Media temperatures should be above 50° F but avoid applying when soil temperatures are above 80° F. Optimum media temperatures are between 60-70° F. Use a soil thermometer to monitor temperature.

Water the growing media before and soon after application, but avoid over watering so they aren’t washed out of the container.

Apply in the evening or at dusk or on a cloudy, overcast day. (Nematodes are very sensitive to UV light and desiccation).

Apply nematodes with a sprayer (remove screens and filters), injector, hose end sprayer or even a watering can. If using an injector, set the dilution to 1:100. Remove all filters or screens (50 mesh or finer) in any spray lines so that the nematodes can pass through unimpeded and undamaged and spray pressure should be kept below 300 psi. Although nematodes are applied in water, they are not aquatic animals and therefore they need extra care while in stock and tank solutions, so adequate aeration of the nematode suspension during application is important. This can be done using a small battery powered submersible pump or even mechanically to keep the solution agitated. Some growers use a paint stirrer on the end of a regular cordless drill fastened to the side of the 5-gallon bucket (stock solution) to keep the nematodes in suspension. The small pump will also keep them from settling on the bottom, which they tend to do.

The suspension in the spray tank should be kept cool and applied as soon as possible after mixing. In warm weather, some growers may use an ice pack to keep the water cooler. This is especially important during the warmer months. The longer they are kept before spraying and the warmer the tank water, the more quickly their energy reserves are used up. Weaker nematodes are less robust during and after application, and less able to search for and infect a susceptible host.

Some growers include 1 ounce (or one tablet), blue dye in the mixed solution so they can see the mixed solution. For shore fly larvae control, look for S. carpocapsae.

How to tell if nematodes are working

The symbiotic bacteria carried by the nematodes break down the host insect’s cuticle. The infected larvae rapidly disappear, so they may be difficult to locate. Infected fungus gnat larvae are often opaque and white to light yellow in color.

Use slices of raw potato to monitor for fungus gnat larvae. Place slices on the surface of the growing medium two days before application in order to determine the population level prior to treatment, and again 3-5 days and 10-12 days after application. Leave the potato disks out for two days in each case before examining them for fungus gnat larval activity.

Efficacy of biological control with beneficial nematodes will be variable depending upon the relative humidity and temperature in your greenhouse, dose applied, frequency of application, and life stage of thrips. As with any biological control measure, they are most effective when used preventively in conjunction with good cultural practices such as greenhouse sanitation and regular monitoring.

Sources: University of Massachusetts, Amherst; Sarah Jandricic, Ontario Ministry of Agriculture, Food and Rural Affairs; Cornell University

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