Nematodes are microscopic, un-segmented, threadlike, wormlike creatures that swim in the moisture surrounding soil particles and plant roots. Many nematodes are beneficial, preying on other nematodes that actually attack plants and other pests. They live in moist soil, decaying organic matter, water and even inside plants and animals.

There are thousands of species of nematodes. Although “wormlike,” they are not related to actual worms.

Parasitic nematodes lay eggs on plant roots that hatch into tiny larvae. The larvae molt a few times and then mature into adults. They puncture plant cell walls, inject a body fluid and suck out the plant cell’s contents. Some species feed on the outside of plant roots, while others enter the roots and live inside the plant, slowly destroying it. Symptoms include reduced growth, wilting and just an overall unhealthy look. They can also attack leaves.

Root-knot nematodes are very common, especially in lettuce, beans, tomatoes, potatoes, peppers and corn.

Good organic matter in the soil helps fight nematodes by promoting populations of beneficial fungi that feed on them. Companion plants such as marigolds repel nematodes. Susceptible crops should always be rotated.

Beneficial nematodes, available in most garden centers, kill soil-dwelling and boring insects such as weevils, grubs, beetles, maggots and cutworms. More than 230 different species of insects can be destroyed by nematodes. They also hasten the decomposition of organic matter in compost. Once they attack an insect, nematodes release bacteria that paralyze and kill the insect within a couple of days and then feed on it.

Since nematodes travel in a moist environment, the area to be treated should be well watered before application and for a few days after application. Direct sunlight will kill them, so the best time to apply is in the evening or in cloudy conditions.

As temperatures drop in the winter, nematodes travel deeper into the soil to hibernate. In spring, warmer temperatures bring closer to the soil surface. Since their return lags behind the earlier arrival of spring pests, beneficial nematodes should be reintroduced every year.

In areas where nematodes are common in the soil, plant susceptible crops as early as possible or very late to take advantage of lower soil temperatures. Most nematodes are unable to penetrate plant roots when the soil temperature is below 64 degrees.