Biological Control: Using Beneficial Nematodes
Growers interested in using biological control are encouraged to begin by using beneficial nematodes to manage fungus gnats. Beneficial nematodes are relatively easy to use but must be applied properly in order to be effective.
What are beneficial nematodes?
Nematodes are tiny (about 0.5 mm), 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 and western flower thrips pupae in the soil. Fungus gnat larvae may be parasitized in any larval stage. Note that Steinernema feltiae does not control shore flies; however, the beneficial nematode Steinernema carpocapsae is available under the trade name Millenium to control shore fly larvae.
The nematodes enter the insect host through body openings. They multiply within the host and release a symbiotic bacterium which produces a toxin that kills the fungus gnat larvae. The larvae are killed within one to two days. More than one generation of nematodes may develop in dead host insects in the media. The infective juveniles then exit the dead body and search for new hosts to infect.
How to use beneficial nematodes
Steinernema feltiae is sold under the trade names of NemaShield, NemAttack, Nemasys, Scanmask, and Entonem. All of these products are labeled as a soil drench treatment against fungus gnat larvae and all but Scanmask are labeled for thrips pupae. Preventative applications to moist soils work best. Like most biocontrols, nematodes are best used to keep pest populations low and will not remedy a serious outbreak already in progress. Foliar applications are generally not as effective because 1.) fungus gnat larvae and thrips pupae reside in the soil, and 2.) nematodes are highly sensitive to UV light and desiccation.
Always follow label directions carefully.
Apply nematodes with a sprayer, injector, hose end sprayer, or even a watering can. If using an injector, set the dilution to 1:100. It is important to remove all filters or screens (50 mesh or finer) in any spray lines so that the nematodes can pass through unimpeded and undamaged. Keep spray pressure below 300 psi. Although nematodes are applied in water they are not aquatic animals, 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 cool. This is especially important during the warmer months. The longer the nematodes are kept before spraying and the warmer the tank water, the more quickly their energy reserves are used up. Weakened nematodes are less robust during and after application, and less able to search for and infect a susceptible host.
Unlike many traditional pesticides there is no 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.
Beneficial nematodes are living organisms, so there are a number of precautions that should be followed for successful use.
- Check nematode viability before application. Place a small amount of the product in a small clear container or petri dish. Add 1 or 2 drops of room temperature water. Wait a few minutes and look for actively moving nematodes. They have a slight J 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.
- Apply in the evening or at dusk or on a cloudy, overcast day.
- 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.
- 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.
- Repeated applications are often needed. Reapply in 2 to 4 weeks under moderate to heavy infestations. For longer term crops, apply at the beginning and at mid-crop.
- Nematodes are compatible with a number of different pesticides; however, they are generally not compatible with organophosphates, carbamates, nematicides, and hydrogen dioxide. Do not mix nematodes with your fertilizer solution.
- For more detailed information on pesticide compatibility, consult with your supplier or with the following resources on the Internet:
How to tell if they 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 potato disks to monitor for fungus gnat larvae. Place disks 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 for two day 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.
For more information on Steinermena feltiae:
John Sanderson, Suzanne Wainwright-Evans & Ronald Valentin. Grower Talks, 2021. Best Practices for Biocontrols Part 4.
Insect Pathogenic Nematodes. greenhouseipm.org
David I. Shapiro-Ilan, GA Byron, and Randy Gaugler. Nematodes (Rhabditida: Steinernematidae & Heterorhabditidae). https://biocontrol.entomology.cornell.edu/pathogens/nematodes.php
Association of Natural Biocontrol Producers. http://anbp.org/index.php
Rearing nematodes: Do-it-yourself guide, Michigan State University