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Gardening Articles: Edibles :: Vegetables

Beneficial Nematodes (page 2 of 4)

by Dan Hickey

How Nematodes Work

The life cycle of beneficial nematodes consists of six distinct stages: an egg stage, four juvenile stages, and the adult stage. The adult spends its life inside the host insect. The third juvenile stage, called a dauer, enters the bodies of insects (usually the soil-dwelling larval form). Some nematodes seek out their hosts, while others wait for the insect to come to them. Host-seeking dauers travel through the soil on the thin film of water that coats soil particles. They search for insect larvae using built-in homing mechanisms that respond to changes in carbon dioxide levels and temperature. They can also follow trails of insect excrement. Other species have a "sit-and-wait" strategy, like a praying mantis. When the mobile insect tunnels by them, they attack.

After a single dauer nematode finds and enters an insect (its skin or natural openings), the nematode releases a toxic bacteria that kills its host, usually within a day or two. In less than two weeks, the nematodes pass through several generations of adults, which literally fill the insect cadaver. (Steinernema reproduction requires at least two dauer nematodes to enter an insect, but a single Heterorhabditis can generate offspring on its own.) Nematode adults feed until they exhaust their food supply (the insect carcass), and as that time nears, the life cycle is halted.

As if they know time is running out, the entire host population of nematodes--as many as 200,000--become dauer nematodes, each with the ability to kill another insect. Eventually, they leave the insect carcass and begin to search for another host. Since it is nonfeeding, the dauer nematode can last for weeks in the soil.

How to Use Nematodes

For the home gardener, localized spraying is probably the quickest and easiest way to get beneficial nematodes into the soil. Although there's no need to worry about wearing rubber gloves or protecting your clothes, take reasonable precaution against splashing them on you. Beneficial nematodes, unlike many of their cousins, are harmless to mammals.

Producers ship beneficial nematodes (dauers) in the form of gels, dry granules, clay, and water-filled sponges. All of these dissolve in water and release the millions of nematodes. A typical spraying will introduce hundreds of millions of nematodes--each ready to start seaching for an insect--into your garden.

Nematodes should be sprayed on infested areas at a time when the targeted pest is in the soil. Timing is important, or else you'll have to repeat the application.

Northern gardeners should apply nematodes in the spring and fall, when the soil contains insect larvae. "Most of the beneficial nematodes are adaptive to cold weather," says Cate, at Integrated Biocontrol Systems. "In fact, the very best time to control white grubs is in the fall."

If you're in a warmer climate, beneficial nematodes are most effective in the summer. In any case, if you're unsure of when to apply beneficial nematodes, call your Extension service. Find out when and for how long the soil-dwelling stage of the target insect will be present and plan your nematode application for that time.

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