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Biological Control: Using Beneficial Nematodes

Applying Beneficial Nematodes

Growers that are interested in using biological control are encouraged to begin by using beneficial nematodes to manage fungus gnats. Beneficial nematodes are relatively easy to use and are applied similar to conventional pesticides with some special precautions listed in this article.

What are beneficial nematodes?

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 and most recently, thrips pupae in the soil. Steinernema feltiae does not control shore flies, however, the beneficial nematode Steinernema carpocapsae  is being sold under the trade name Millenium to control shore fly larvae.  Fungus gnat larvae may be parasitized in any larval stage.  Nematodes have traditionally been used against soil dwelling pests because they are sensitive to ultra violet light and desiccation.

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.

How to use beneficial nematodes

The nematode S. feltiae is sold under the trade names of NemaShield, Nemasys, Scanmask and Entonem. All of these products are labeled as a soil drench treatment against fungus gnat larvae. Preventative applications to moist soils work best. 

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 one oz. (or 1 tablet), blue dye in the mixed solution so they can see the mixed solution.

Unlike many traditional pesticides there is no REI (an added bonus in propagation houses), nor possibility that the target pest will develop resistance. No adverse effects have been shown against non-target organisms in many different field studies. But, beneficial nematodes are living organisms, so there are a number of precautions you need to follow for their successful use.

Check their viability before application
To do this, 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 or swimming nematodes. They have s slight J"curvature at the end of their bodies. Use a dark black background and a hand lens or field microscope to see the small (0.6 mm or 0.02 inches in length) nematodes. Dead nematodes will straight and still.

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

Pesticide Side Effects Database – Koppert Biological Systems
Pesticide Side Effects Database - Biobest Sustainable Crop Management

Keeping nematodes

Apply immediately after receiving them, if possible.  If you must store the nematodes, store them in a refrigerator (38-42°F). Avoid placing them in a small refrigerator where they may freeze and die!.

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

Specific Tips for Use Against Fungus Gnat Larvae

  • Treat as soon as possible (2 to 3 days) after sticking cuttings, planting plugs or starting seeds.  Some growers apply the nematodes to the media directly before sticking cuttings to insure that the nematodes reach the media. Injectors are placed directly on the planting line.
  • Apply as a media drench to target the fungus gnat larvae.
  • 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 temperatures).
  • Water the growing media before and after application. (Nematodes need moisture for movement). 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).
  • 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.

How to tell if they are working

The symbiotic bacteria breaks down the host insect's cuticle. The infected larvae rapidly disappears, so may be difficult to locate. Infected fungus gnat larvae are often opaque-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.

Use against  thrips

In addition, the product Nemasys is also labeled for use against western flower thrips. In the late 1990s in the U.K., it was reported that cut chrysanthemum growers who applied nematodes weekly as a foliar spray, noted a reduction in their thrips populations. More recent work in Canada, the U.K. and Germany showed that soil-dwelling stages of thrips (especially the pupal stages) were highly susceptible to several species of nematodes, and particularly to Steinernema feltiae. During the weekly sprays, a significant number of nematodes reached the growing media via runoff from the foliar sprays. Nematodes are very short lived on the foliage (significant reduction after one hour) but may persist for several weeks in the media. Mobile life stages on the plant (adults and larvae) appear to be less susceptible to attack. Thrips control noted in commercial crops probably occurred as a result of overspray and run-off into the soil after spraying. Special precautions are taken to help reduce potential desiccation: use of a non-ionic wetting agent, spraying in the late afternoon or evening, and the use of black cloth.  

Specific tips for use against western flower thrips (from the Nemasys label).

  • Nematodes require moist conditions to enhance effectiveness.
  • If plants are dry, provide light overhead irrigation prior to nematode application.
  • Ensure good foliar coverage of spray mix to enhance contact with the target pest.
  • Use of a wetting agent or surfactant will enhance wettability of the spray mix and encourage nematode movement. Some growers use 1 to 2 ounces of Capsil per five gallon bucket (stock solution).
  • Following application, ensure that the crop remains wet for at least two hours. Note:  
  • Do not apply in direct sunlight.

Note: The nematodes will dessicate after about one day, depending upon environmental conditions.  Grower feedback has been variable, with some observing excellent results and others less so. Efficacy will be variable depending upon the relative humidity, and temperature in your greenhouse, dose applied, frequency of application, and life stage of the thrips.  Some growers apply the nematodes with additional water in the summer months to ensure that the foliage stays wet to contact the thrips stages on the foliage. Depending upon the temperature, relative humidity levels and other environmental conditions, up to 2x the amount of water may be needed to keep the foliage wet for two hours. Regular monitoring, sanitation, proper spacing and judicious use of fungicides and biological fungicides may be needed to discourage foliar diseases.

Applying the nematodes as a heavy surface spray or "sprench" to young, incoming plant material will have an added benefit of targeting any incoming fungus gnats in the media as well as thrips pupae.  Growers who have had success with this application method, apply the nematodes on a weekly basis, and target the young growing point where thrips tend to hide. As with any biological control measure, they are most effectively used preventively in conjunction with good cultural practices for thrips control (sanitation, rigorous weed controls, etc).

For more information on Steinermena feltiae:

Rearing nematodes: Do-it-yourself guide, Michigan State University

New England Greenhouse Floriculture Guide Ordering information
Graesch, Julie. 2011. Shake It Up: American Nurseryman
NemaShield – BioWorks
Nemasys – BASF
McGaughey, Roger. 2010. Incorporating Nematodes into an Insect Control Program. Greenhouse Management
Murphy G. 2010. Nematodes in Pest Management. Greenhouse Grower.
Shapiro-Ilan, D. USDA-ARS, SEFTNRL, Byron, GA & Randy Gaugler, Department of Entomology, Rutgers University, New Brunswick New Jersey, NematodesCornell University.

2011 by Leanne Pundt, University of Connecticut Extension
Updated 2013, reviewed 2015

Tina Smith
Extension Greenhouse Crops and Floriculture Program
University of Massachusetts, Amherst