In the soil or in encrypted habitats such as the pockets behind the bark of trees, the juvenile nematode waits for (or sometimes actively seeks) an unsuspecting host - a grub or a larva - to jump on it and penetrate it through the insect's natural openings - mouth, anus, spiracles. Or the nematode may enter the host directly by using a dorsal tooth.

Once inside the insect, the nematode vomits the pathogen, which kills the host within 24 to 48 hours and even digests its tissues, creating a perfect environment for the EPN to grow and multiply. One or more adult generations live entirely inside the decaying insect. The third stage infective juvenile is the only one that can live outside the insect host. Numbering about 150,000 strong or more, these juveniles exit the dead larva, carrying the bacteria, and look for other hosts to begin the cycle again. These juveniles also can survive dry conditions in soils for long periods of time before they infect more insects.

This naturally-occurring relationship between the nematodes and their mutualistic bacteria has existed for millennia. EPNs are found in terrestrial environments, including deserts, rainforests, grasslands and other ecological systems, offering a tremendous array of possibilities for study. Stock, who has been researching and interpreting the evolutionary relationships of nematodes for the past 15 years, has collected them in Arizona and from other locations worldwide.

In Costa Rica, she is working with a collaborative team from four universities: the University of Vermont, University of Florida, University of Nebraska and University of Costa Rica, to learn where EPN communities are concentrated. The work is funded by the National Science Foundation.

"We're looking at all groups of nematodes in tropical rainforests," Stock says. "We sweep from the tops of the trees all the way to the ground, searching for nematodes that are potential insect pathogens. The misconception is that they are concentrated more in temperate zones, but this is not true. We're trying to unveil the mystery of nematode diversity in the tropic regions."

Once the right nematodes are identified, they can be suspended in a gelatinous matrix, or dried in powder, then mixed in water and sprayed, broadcast or irrigated onto crops. Large numbers of infectious juveniles are released to inundate and kill the pest insects quickly. Depending on climate conditions, this method works best on greenhouse ornamentals and vegetables, citrus, cranberry, turfgrass and other crops, rather than on high-acreage crops like cotton and soybeans.

"The beauty of this is that in the last 20 years nematodes have been formulated and commercialized," Stock says. "They are more expensive than a chemical product, but so far they have been demonstrated not to harm humans, livestock, beneficial insects or the environment. Nematodes usually have to be underground; their targets are soil insects."

The formulations keep improving as newer isolates of nematodes are found, and there is a lot of commercial interest in matching nematodes to pests, Stock says.

"Yet these nematodes are so powerful and pathogenic not in and of themselves, but because they live in symbiosis with bacteria," Stock concludes. "Both the bacteria and the nematode need each other to survive, making them not only good as biological agents, but also as model systems for understanding basic questions in biology." Given the number of nematodes that exist in the world, the possibilities for discovery are immense.

"The whole nematode phylum is estimated to have 500,000 to a million species. About 25,000 species have been identified so far," Stock says.

 

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