This large family of small roundworms contains both beneficials and pests. Beneficial nematodes aggressively attack the young of various pest insects, and can persist in the soil for long periods of time. They are sometimes referred to as entomopathogenic nematodes (insect-disease-causing nematodes), or EPNs. Nematodes used for insect pest control are safe for humans and other vertebrates.
Nematodes occur naturally in soil; much greater-than-normal concentrations are required to control serious pest problems. Drying out will kill most nematodes, and as such, they are most effective in moist and shaded locations. Juvenile nematodes actively seek out prey, which they kill within 48 hours; they multiply rapidly in hospitable environments.
Nematodes are small, but large enough to be visible with a magnifying glass, and some adult forms even to the naked eye. They quickly disappear into the soil, not to be seen again. That said, squeamish homeowners may opt for other pest control solutions: let the buyer beware.
Most useful for biological pest control are:
For lawn-damaging grubs, combinations of Steinernema carpocapsae and Heterorhabditis bacteriophora are frequently used, and more effective than either species alone.
Nematodes work only on larval forms, and as such, timing the application depends on the life cycle of target pest. Timing is important. For some, this means an application in May or June for many insects, one in August or September for others, and sometimes both. Read the directions.
Nematodes should be used as soon as possible, as time will affect viability. If stored, make sure they are stored per the manufacturer's recommendations; avoid baking in a car, or exposing to freezing temperatures. Post-application, UV light, fertilizers, insecticides, and extreme temperatures can affect viability and effectiveness.
Nematodes are usually sprayed or delivered by irrigation systems, though other formulations and methods may be used. (A watering can, for example, will often work fine.) Follow the directions.
Apply solutions in late evening, as nematodes require moist, cool and dark conditions to survive. Temperatures should be 13C/55F or higher. Consider aerating densely-packed soil prior to treatment. Water well before the application: this creates a hospitable environment for nematodes, and helps drive pest larvae closer to the soil surface. Water again after application, more lightly, to carry nematodes into the soil, but not hard enough to flush them through it.
Note that nematode species can compete with, and can displace, other beneficial nematodes, though they can also complement native soil nematodes. There is some impact on non-pest beetles and flies, though this appears to be minimal. As with any pest control method, consider pros and cons carefully, use complementary strategies, and use them discriminately.
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