Entomopathogenic Nematode News

How Much Moisture Do Nematodes Need to Be Effective?

Entomogenous nematodes are potentially useful in many agricultural production systems, but their acceptance by growers has been hindered by inconsitent results. Sometimes nematode sprays have resulted in astounding mortality of soil-dwelling pests, and in other cases have had almost no impact on the pest population. Nematodes need both high humidity and a layer of water to move through the soil, but just how much moisture is necessary?

Two species of nematodes (Heterorhabditis bacteriophora and Steinernema glaseri) were exposed to four levels of soil moisture ranging from very dry (6%) to very moist (15%) in lab trials. The two species were stored at three different temperatures prior to testing them in soil because temperature is known to affect their activity. Both species infected 80-100% of the test insects when moisture was adequate. Their activity declined over time as the soils dried out, but increased when the soils were re-wetted. H. bacteriophora seemed to tolerate both extremes of moisture better than S. glaseri. The nematode that was effective for the longest period was H. bacteriophora that had been stored at the coolest temperature and was tested in high moisture-content soils.

Field tests showed similar results, but the nematodes were not active for as long. While initial rates of insect mortality were between 80 and 90% in all but the driest soil, the rates dropped below 35% after 8 days.

These preliminary results suggest that nematodes are most effective when soil moisture is high, but more must be learned about the effects of moisture on nematodes to better understand and predict their efficacy.


New York State Integrated Pest Management Program Annual Report, 1998

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