How They Spread
Whether they feed above- or belowground, most nematodes spend at least part of their life cycle in the soil. While they can’t move very far under their own power, they can swim freely in water, and they move more quickly in moist soil. They are also spread by anything that can carry particles of infested soil, including tools, boots, animals, and infected plants.
Nematodes as Pests
While most nematodes are beneficial (see Beneficial Nematodes below for more on their role in the garden), a few species of these translucent, unsegmented worms are plant parasites. Pest species are only about 1/50 inch long and cause root knots or galls, injured root tips, excessive root branching, leaf galls, lesions or dying tissue, and twisted, distorted leaves. Plants most commonly attacked at the roots include tomatoes, potatoes, peppers, lettuce, corn, carrots, and other vegetables. Plants that sustain leaf and stem injury include chrysanthemums, onions, rye, and alfalfa.
Slender, translucent, unsegmented worms, beneficial species of nematodes are 1/25 inch to several inches long. Their roles in the garden vary. Some are soil dwellers that break down organic matter and are common in compost piles. These decomposers are easily visible; they are about ¼ inch long.
Other nematodes (families Steinernematidae and Heterorhabditidae) attack and kill insects either by injecting bacteria (Xenorhabdus sp.) that kill the host within 24 to 48 hours or by entering the host, parasitizing, and feeding on it.
Beneficial nematodes are effective against a variety of pests, including weevils, clearwing borers, cutworms, sod webworms, chinch bugs, and white grubs. When purchasing and applying them, it is very important to select the right species of nematode, because different species are effective against different pests. In addition, nematodes require moist, humid conditions, and fairly warm soil to be most effective. Water application sites before and after spreading nematodes. When purchasing them, follow application directions carefully.