Nematodes We Want in the Yard & Garden
Gardeners cringe at the mention of nematodes and plant diseases, but it turns out not all pathogens are bad. There are a number of insect-attacking pathogens (entomopathogens) that are natural enemies of yard and garden pests, and aid in pest suppression. Entomopathogenic nematodes are resident in soil and do not harm plants but are often found at low levels. Many beneficial nematode species, however, have been formulated so that they can be applied as a biopesticide.
From a couple of nematodes entering an insect, hundreds to a few thousand will eventually exit in search of a new insect host. Shown above is a waxworm infected with Heterorhabditis species nematodes.
Entomopathogenic nematodes are clear, nearly microscopic roundworms that enter their host insect through body openings, such as the mouth. These nematodes are associated with a bacterium they release that kills the insect within a couple of days and allows the nematodes to develop and reproduce. They are soil dwelling and perform best against insect pests, in particular larval stages, associated with a moist soil environment. Applications of entomopathogenic nematodes have also been shown to suppress plant-parasitic pest nematodes. Foliar applications of nematodes have been less successful and are not generally recommended, but may be used in greenhouses and against leafminers and thrips on ornamentals. Research on adjuvents, using surfactant-polymer formulations, and less disruptive spray nozzles is ongoing to improve foliar applications.
There are several beneficial nematode suppliers online (a list is provided in the cited references below), and nematodes can also be found at some garden centers. When purchasing nematodes, species will be associated with the genera Heterorhabditis or Steinernema and are labeled with a trade name like Nemasys, Millenium, and NemaShield and/or their species name. Species vary in their behavior, specificity to attack particular insects, and tolerance to various environmental conditions. These characteristics, often provided by the supplier, should be used to select the appropriate species for the insect being targeted. When nematodes are shipped from a supplier the container should have ice packs to keep them cool in transport. At a garden center, they should be kept refrigerated, otherwise the nematodes may not be alive. It is best to apply the nematodes immediately after they arrive, otherwise, refrigerate, and avoid freezing them. One way to determine whether your nematodes are alive is to place a small amount of the nematode product in a clear plastic baggie and add a few drops of water. Although nematodes are tiny, after a few minutes you should see nematodes squirming around either with a hand lens or the naked eye.
Beneficial nematodes are living organisms, so care is needed to ensure their effectiveness. Here are a few basic considerations.
- Irrigate before and immediately after a nematode application to help with their establishment in the soil.
- Apply nematodes in the evening or on a cloudy day to avoid UV exposure that can kill them.
- Remove fine screens and filters when using a sprayer and keep the spray pressure below 300 psi. Nematodes can also simply be applied with a watering can, just shake or stir the solution to keep the nematodes from settling at the bottom.
Other factors to consider are the compatibility of nematodes with other plant and pest management tactics. Nematodes are compatible with other pesticides but avoid applications with organophosphates, carbamates, nematicides and Do Not mix with fertilizer solutions. Some research has shown also that strategies used to reduce plant parasitic nematodes, such as mustard green manures, may decrease the effectiveness of entomopathogenic nematodes for insect management.
Table of nematode species suitable to certain pests (adapted from Cornell).
|Pest||Key Crop||Nematode species to consider|
|Armyworms||Vegetables||Steinernema carposapsae, S. feltiae, S. riobrave|
||Turf||Heterorhabditis bacteriophora, S. carpocapsae|
||Turf, vegetables||S. carpocapsae|
||Mushrooms, greenhouse||S. feltiae, H. bacteriophora|
||Turf, ornamentals||H. bacteriophora, S. carpocapsae|
|Strawberry root weevil
Once nematodes have been applied it can be difficult to determine if they are doing their job. One way to test that the nematodes are alive and capable of infecting is to collect soil in the area of the application, place a small amount of soil in a cup, and add a couple of Galleria waxworms (available where fish bait or live food for small animals is sold) on top of the soil. About 2-3 days later, nematode infected waxworms are dead and tan to brown in color for Steinernema species or reddish to orange for Heterorhabditis species.
There are several factors to consider from the supplier to the application and management compatibility when using entomopathogenic nematodes so that they successfully aid in pest suppression. Taking these precautions, however, can help add beneficial nematodes to a yard and garden pest management program.
-Ricardo Ramirez, Entomologist
For More Information:
University of Massachusetts fact sheet: Biological Control: Using Beneficial Nematodes.
Cornell University fact sheet: Biological Control - Nematodes.