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Ever wonder about beneficial nematodes in your garden?

One morning while walking my dog, I overhead some men discuss adding nematodes to their gardens. I wondered why anyone would do that.

"Nematodes are so abundant," my invertebrate zoology professor once said, "that if you got rid of everything except for nematodes, you would still see a shadowy outline of the living world." Nematodes tend to be tiny (50 micrometers to 1 millimeter long) and thus easily overlooked. I knew a Thai scientist who needed an extra fancy microscope just to identify them. A hundred cubic millimeters of soil could contain several thousand nematodes, so we're talking billions in an average backyard. 

Nematodes come in many feeding styles, but the best-known nematodes seem to be the plant parasites that upset gardeners. More recently, an anthropocentric gaze has turned to the "beneficial" nematodes that control insects that are considered pests.

Hundreds of nematodes parasitize insects, but so far only a few entomopathogenic species are produced commercially. Others are under study and in development. The most common are Steinernema carpocapsae and Heterorhabditis bacteriophora, which the City of Vancouver recommends for dealing with Chafer Beetles. 

Nematodes can infect all kinds of insects because they don't require specific host nutrients. Fortunately, they are still particular enough that people, pets and even earthworms are safe. Beneficial insects are unaffected because they tend to occur in small numbers and have different lifestyles.

Steinernema and Heterorhabditis share similar life histories that include an egg stage, four larval stages and adulthood. Steinernema requires that at least two different sexes are present to reproduce, but Heterorhabditis has both sex bits together and can reproduce individually. In less than two weeks, several generations pass through the juicy insides of an insect larva. When food becomes depleted, the third juvenile stage, called a dauer, heads out in search of a new host.

Companies distribute live dauers, so you need to use them soon after receiving them. They have a double thick skin for protection but they are still sensitive to high temperatures and ultraviolet light, so you should add them in the evening. They travel in the thin film of water surrounding soil particles, so you should apply them with water in moist soil. They respond to telltale changes in carbon dioxide and temperature and excrement trails, so you should only add them if you actually have pest insects in the soil, like in the spring.

Dauers enter through an available opening of a suitable host, or in the case of the Heterorhabditids, make their own with a built-in chisel. Once inside, they deploy the secret weapon carried in their guts—symbiotic bacteria that produce anti-immune proteins (to protect against host defenses) and enzymes that break down the host. The nematodes feed off this soupy goodness and kill off their host in two to three days. Steinernemae carry species of Xenorhabdus bacteria, while Heterorhabditids carry a closely related Photorhabdus luminescens that bioluminesce. The glowing insect carcasses may attract fresh meat!

If you have experience with beneficial nematodes and especially if you have seen glow-in-the-dark insect carcasses, please do tell.

Still curious? Ever wonder about how dogs smell so well? Or why humans get old?