Know Your Friends

The Nematode Heterorhabditis bacteriophora

The entomopathogenic nematode Heterorhabditis bacteriophora was first described from an infected caterpillar, Heliothis punctiger, collected in Brecon, Australia. This nematode is widely distributed in North and South America, Australia, and Europe, but has many strains that differ in behavior and physiology. There are six other described species of Heterorhabditis.

The life cycle of H. bacteriophora consists of an egg, four juvenile stages and the adult. Only third-stage juveniles attack and infect host insects. This stage is the only free-living stage in the life cycle of the nematode, and is adapted to remain in the environment without feeding for a prolonged time. All other stages exist only inside the host.

The infective juveniles move through soil in search of hosts. Once a host is encountered, the nematodes enter though natural openings or use their dorsal tooth or hook to break the outer cuticle of small, fragile insects to allow the juvenile to enter.

Once the infective juveniles are in the host insect, they begin development. Their alimentary canal becomes functional and they release symbiotic bacteria to multiply in the insect. These bacteria are consumed and digested by the developing nematodes.

The symbiotic bacterium Photorhabdus luminescens is only pathogenic to insects when introduced into the insect body, not if it is ingested. The bacterium is unable to survive in soil or water, so the nematode provides protection for the bacterium outside the insect host and a means of transmission to new hosts. The nematode is unable to reproduce without the nutrients the bacterium provides.

The bacteria kill the host, usually within 24-48 hours. Nematodes feed on the bacteria and host remains, and each infective juvenile develops into a hermaphroditic female. These females then produce eggs which will develop into both males and females. Only a portion of the eggs are laid outside the female; the remainder hatch inside the female and the juveniles destroy their mother as they develop. These nematodes mature, mate and produce infective juveniles that emerge from the cadaver 12-14 days after infection.

When infective juveniles leave the host, they move around in the soil, searching for a new host to infect. Soil texture, moisture, and temperature all affect dispersal and infection. Heavy soils impair movement, with less dispersal occurring as the percentage of silt and clay increases in the soil. These same factors all affect survival as well. Survival (at least in lab studies) is best under moderately low soil moisture and cool temperatures (60F).

These nematodes occur naturally in soil, but not at high enough levels to provide effective pest control. Inundative releases are necessary to reduce insect populations below economic thresholds. In some cases recycling through host insects will occur to minimize the need for subsequent applications. Some target pests that have been controlled (to varying extents) by H. bacteriophora in field tests are white grubs (Japanese beetle, northern masked chafer), black vine weevil, strawberry root weevil, citrus root weevil, Colorado potato beetle, cucumber beetles, red imported fire ant, several other soil-inhabiting insects, and filth fly maggots in manure. Some attempts have been made to utilized this and other species of nematodes against foliar pests, but problems of desiccation, exposure to sunlight and high temperatures that are fatal to exposed nematodes have limited such applications.

- Susan Mahr, University of Wisconsin, Madison

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