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Biological Control with Nematodes: Increasing the Odds of Success

Soil types and watering regimes affect nematode survival in nursery and greenhouse settings

Nematodes are microscopic roundworms that live in the soil. Some are pests; some are beneficial because they prey upon other pests. Recent attempts to achieve biological control of insect pests using these predatory nematodes have often failed. Why? Explanations include poor strain selection and inattention to the nematodes’ habitat and moisture requirements. Elson Shields and Tony Testa, of Cornell’s entomology department, decided to make another attempt, this time with a focus on discovering just what soil and moisture conditions will optimize nematode survival.

The pest of interest: black vine weevil, an insect that can kill ornamental trees and shrubs by feeding on their roots. The biological control agent: a cold-tolerant nematode that was first found in the Oswego, New York, area in 1990 (scientific name Heterorhabditis bacteriophora ‘Oswego’). Shields chose the Oswego strain because it is known to be effective against a close relative of the black vine weevil. What he didn’t know was whether the nematodes can survive in the types of soils commonly used in woody ornamental production.

Shields and Testa put the nematodes in several types of greenhouse potting soils and in a sample of the sandy loam in which the nematodes were originally located, and kept track of the numbers that survived over a several-month period.

They soon learned that watering from the top down, a typical greenhouse and nursery production method, is likely to flush the nematodes out of the soil altogether. When the nematode populations disappeared from all soil types within 30 days of this kind of watering, nematode populations were replenished, and the watering regime was changed to sub-irrigation (pots are placed in water-holding trays). Nematodes persisted in all of the soil mixes for at least 120 days thereafter.

Population levels of nematodes varied among the soil types, but no conclusive results have yet been released. The soil samples will continue to be monitored for several months beyond this writing, but according to Shields, "It is clear even now that choosing the right soil mix–as well as the right watering regime–will have a major impact on the long-term presence of predatory nematodes in the nursery production system."