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How Wet Should It Get?

Information about the moisture needs of nematodes will help in their effective use as biological control agents

Nematodes--microscopic roundworms that live in soil--have gained prominence as a biological control method in the past ten years. They infect many different insects and are potentially useful in any agricultural production system. But acceptance of this method by growers has been hindered by inconsistent results. Sometimes nematode sprays have resulted in astounding mortality rates of soil-dwelling pests; sometimes they have not.

Jennifer Grant and Michael Villani, of the Cornell entomology department in Geneva, undertook an IPM project in 1997 that has heightened our understanding of nematode ecology. "In order for nematodes to be effective pest hunters, they're going to have to have their needs met," explains Grant, a doctoral student. "We know that they need both high humidity and a layer of water in which to move through the soil, but just how much moisture is optimum? That's what we wanted to find out with this project."

Grant and Villani used both laboratory and field settings to test two nematode species whose Latin names will be abbreviated here as "HB" and "SG." The two species were stored at three different temperatures prior to testing them in soil because temperature is known to affect their activity quotient.

HB and SG were exposed to four levels of soil moisture ranging from very dry (6 percent) to very moist (15 percent). Waxmoth larvae were put in soil cups as food for the nematodes. Both species infected 80-100 percent of the waxmoths when moisture content was sufficient (in all but the driest soil). Their activity declined over time as the soils dried out but increased following re-wetting of the soils. HB that were in high moisture-content soils and that had been stored at the coolest temperature hunted and infected their waxmoth prey for the longest period. HB also seemed to tolerate both extremes of moisture (too little and too much) better than SG.

The field test showed similar results to the laboratory test, but the nematodes did not hunt and infect their prey for as long as was expected. While initial rates of insect mortality were between 80 and 90 percent in all but the driest soil, the rates dropped below 35 percent after eight days.

A beginning has been made, but more must be learned about the effects on nematodes of moisture and other soil characteristics. Mechanisms by which nematodes infect their prey must also be better understood. All of this will lead to much more certainty about their effectiveness at specific field sites.