Some nematodes destroy plants. Others, known as beneficial nematodes, destroy the insects which destroy plants. How do you know which is which?
Nematodes are very tiny, microscopic “worms” that mostly live in soils, although “foliar” nematodes live in leaves. While some species harm plants by attacking roots, others are beneficial nematodes, attacking and killing pests such as grubs in our lawns and gardens and unwanted adult insects. Nematodes are easily applied, but the right timing and conditions are needed for them to be effective.
Beneficial nematodes are very effective for soil pest control. Feeding on over 200 insects from roughly 100 families, nematodes are safe to use around humans, children, and pets and won’t harm non-target organisms such as bees or other pollinators. They’re easy to apply (no protective gear is needed), quick to work (often within a couple days), and cost-effective – for under $20 you should get about 5 million—enough to treat 1,500 square feet.
How do beneficial nematodes kill pest insects?
The “mode of action” of beneficial nematodes is like something from a horror film, at least for the target pests. One species (Steinernema) uses an ambush strategy, waiting for the prey to come close before attacking. Another nematode (Heterorhabdtis) uses a “cruising” strategy, seeking out and destroying pests. The nematodes either follow the trail of excrement of the pest or seek it through changes in temperature and carbon dioxide levels.
Once found, the beneficial nematodes enter the grub or adult pest through body openings, such as the mouth or openings to the breathing tubes (“spiracles”). The nematodes carry a bacterium which is harmless to plants, humans, or the environment, and when this bacterium enters the pest’s body, the bacteria multiply, killing it in two days or less, and converting host tissue to food for the nematodes. Since these bacteria work to help their host– the nematodes– they are called “symbiotic”.
The infected grubs change from a beige-white to reddish-brown, die and turn slimy. The nematodes then feed on the dead pest from within (hence they are called “entomopathogenic”), and can go through several generations of adults in a couple of weeks. Once their food source is gone they exit and move on to new prey.
Which beneficial nematodes should be added to a garden?
Each species of nematode targets specific pests. For instance, one species (S. carpocapsae) is effective on larval stages of armyworm, weevils such as black vine, caterpillars, cutworm, and sod webworm. Another species (S. feltiae) is used against larvae of cabbage maggot, onion maggot, raspberry crown borer, and thrips. Both species are effective on larvae of codling moth, corn earworm, and cucumber beetle. You’ll want a species (H. bacteriophora) of the other nematode for larvae of tree and vine borers, European chafer, Colorado potato beetle, corn rootworm, flea beetles, and grubs including those of the Japanese beetle.
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Your local garden center, master gardener programs, online resources, books, and state extension diagnostic clinics (www.nepdn.org) are all good resources to identify target pests. If you’re not sure what pest you have, you may need to apply a couple of different nematode products.
Soil type can affect nematodes. Some work best in cultivated soil (Steinernema) while others work best in undisturbed soils (Heterorhabdtis). Other species work best in clay or silt (S. feltiae), another works best in sandy soils (S. carpocapsae). Soil type isn’t crucial for the other main beneficial nematode (Heterorhabdtis).
How to apply beneficial nematodes to your plants and soil.
Once you know which beneficial nematode species is appropriate, to be effective it must be applied properly. Timing is important, as the soil stages of your target pest need to be present for the nematodes to work on them. Late summer into early fall is a good time to target many grubs of plants, crops, and lawns, as they are usually small, feeding on grass roots near the soil surface, and have thin skins the nematodes can easily penetrate. This is also the time when armyworms fall to the ground to change stages (pupate).
Beneficial nematodes are living organisms, so should be applied very soon after you get them. They like moisture, so if it hasn’t rained, water thoroughly both before and after application, or apply during rain. If your nematodes arrive on a wet sponge, rinse them from the sponge into cool water, then spray on plants according to label directions. Soils must be kept moist for a week to 10 days after application.
Rainy, overcast weather is good for another reason—beneficial nematodes are quite sensitive to ultraviolet sunlight and can be killed within a minute or two if exposed to it. That’s why you’ll often see recommendations to apply in the early morning or late afternoon. They’re sensitive to temperature too, with ideal temperatures between 45 and 100 degrees (F).
For prevention, you can apply beneficial nematodes 2 to 3 times a year, such as in spring, summer, or fall. For control of existing pests, apply every 2 weeks until the infestation lessens or goes away.
A version of this article appears on the University of Vermont Extension website.