Nematodes are microscopic soil-dwelling worms, many less than 1/16-inch long.
There are beneficial nematodes and pest nematodes.
Beneficial nematodes help turn organic matter into plant nutrients. They also prey on soil-dwelling plant pests such as white grubs and root maggots.
Pest nematodes feed on plant roots, stunting and sometimes killing plants including many vegetables.
Nematodes are slender, translucent, unsegmented worms. Pest nematodes can be as small as 1/50-inch long. Beneficial nematodes that parasitize pest insects are larger, 1/25-inch long to several inches long.
Nematodes live in the film of water that coats soil particles; they thrive where the soil is rich, moist and warm. Nematodes can’t move more than about 3 feet on their own in the course of their lives, but they often travel around the garden in water, in shifted soil, in soil surrounding transplants, and on garden tools and even ants.
Predatory nematodes either have teeth or long spear-like structures which they use to stab and suck the juices out of plants or their insect prey.
Nematode reproduction is usually sexual, though some individuals are capable of self-fertilization. Eggs are laid in a gelatinous mass. From hatching to full-formed adulthood, nematode development usually consists of four molts that take about a month; the life cycle of some pest species is only 3 or 4 weeks. There are many generations over the course of a year. When the soil grows cold, nematodes overwinter as eggs or adults. Where winters are not cold, nematodes are active year round.
Nematodes are found throughout North America.
Some pest nematodes attack the roots of tomatoes, potatoes, peppers, lettuce, corn, and other vegetables. Other pest nematodes attack the stems and leaves of onions, rye, and alfalfa.
Pest nematodes include root-knot nematodes, lesion nematodes, ring nematodes, and sting nematodes.
Damage inflicted by these pests includes root knots or galls, injured root tips, excessive root branching, leaf galls, lesions on drying tissue, and twisted, distorted leaves. This damage can result in stunted growth and sometimes plant death.
Feeding Habits and Damage: Nematode damage often looks like a plant disease: leaves may turn yellow and become wilted and stunted. Roots of plants pulled from the ground often look lumpy and stunted.
The root knot nematode is perhaps the most destructive of the soil-dwelling pest nematodes in vegetable gardens. It moves in soil water to find the roots of susceptible crops. When it enters a root, it begins to feed by siphoning water and nutrients away from the plant. It secretes a substance that causes root cells to weaken and enlarge into firmly attached, knotty galls. After feeding the nematode lays a mass of eggs, new larvae hatch. The infested root grows large until it splits open and releases a new population of nematodes into the soil. The split root often becomes infected with fungi and bacteria and rots. The plant is left weakened, often near death.
Pest Nematode Controls: There are no organic methods to permanently eliminate pest nematodes but the population can be reduced. Here are some control strategies:
- Remove and destroy infected plants along with their roots from the garden. Place plants and roots in a clear plastic bag and place it in the sun for a week to kill the pests. Then dispose of the bag in the trash, not in the compost pile.
- Plant nematode resistant varieties: There are tomato and other crop varieties that have been developed to minimize nematode damage; damage will still occur but be less severe. Planting resistant varieties will indirectly reduce the pest nematode population over the course of a few years as there is less for the pests to feed on.
- Plant crops that are not injured by pest nematodes. Where pest nematodes are present, plant crops that are not harmed by nematodes; this also will gradually reduce the number of pests in the soil. Crops that can survive in nematode infested soil include broccoli, Brussels sprouts, mustard, onions, leeks, garlic, rutabagas, and glob artichokes.
- Confine pest nematodes to specific problem spots. Don’t allow nematodes to move around the garden. Nematodes cannot move more than about 3 feet on their own. Clean tools used in infested areas; wash the tools over the problem spot. Don’t carry pest nematodes around the garden.
- Add organic compost to garden beds. Beneficial fungi and bacteria that attack pest nematodes thrive in organically rich soil.
- Plant French marigolds (Tagetes patula) or African marigolds (Tagetes erecta) in the garden as a cover crop; grow only these plants for two months across vegetable planting beds. After two months, cut them down, let them dry in place, then turn them into the soil. Chemicals in these plants repels nematodes. You will likely have to repeat this treatment in two years.
- Solarize the soil. Cover the soil with clear plastic during the hottest part of the summer to kill both pest nematodes; this will rid the soil of pest nematodes for a year or two. However, beneficial organisms are also harmed or destroyed by solarization of soil.
- Add chitin to the soil. Chitin is a natural component of nematodes bodies. Fungi attack nematodes by breaking down the chitin in the body. Adding chitin to the soil will stimulate fungi to attack nematodes.
- Add ground sesame stalks to planting beds. Sesame will suppress nematodes.
- Leave the garden fallow for a year. The nematode population will decline if denied food for a year.
- Till the soil in winter to expose nematodes to killing sunlight and dryness.
Not all nematodes are pests; some are beneficial to soil and plants. These nematodes eat organic matter in the soil helping to decompose it and turn it into nutrients for plants. They also attack and kill harmful insect pest, ingest the remains, and turn it into nutrients—especially nitroten–plants can take up.
Beneficial nematodes that are insect parasites are harmless to humans, wildlife, bees, and earthworms. They attack cutworms, root weevils, corn and stem borers, and squash vine borers and some pest root nematodes.
The beneficial nematodes attack soil-dwelling insects through their natural body openings. Once inside, these beneficial nematodes release a bacterium that paralyzes and kills the insect. The nematodes then feed on the tissue of the insect carcass and also eat the bacteria. They reproduce inside the carcass and then move on to a new host.
Three beneficial nematodes are: Steinernema carpocapsae attacks cutworms, armyworms, corn rootworms, and fire ants; Steinernema fetiae attacks root knot nematodes, ring nematodes, and string nematodes; Heterorhabditis bacteriophora attacks cabbage root maggots, Colorado potato beetle larvae, white grubs, and root weevils.
Beneficial nematodes can be purchased for use in the garden. Beneficial nematodes come packaged in a gel, in a power, or mixed with peat and vermiculite. They are commonly mixed with water before being applied with a watering can or sprayer to the soil or plants, or injected into plant stems by syringe. It is important to follow label instructions when applying nematodes to the soil or plants. Commonly applications are repeated every two to three weeks if needed.
The efficacy of nematode application can be affected by hot weather, cold soil, and heavy rain. Nematodes are most active in temperatures between 72°F and 82°F.