Based on results of on-farm tests over several locations and years, nematode population thresholds have been updated for peanut, soybean and corn in Virginia. Treatment thresholds for cotton are being reported for the first time. Each table lists the crop to be planted and three levels of risk for yield loss according to population densities of nematodes in a 500 cc sample of soil. The risk thresholds are based on numbers of nematodes present during late summer or early fall (August 1 to November 20).
Risk levels can be used to assess the need to apply a nematicide or choose a crop that would carry little or no risk of yield loss to nematodes. When nematodes do not exceed levels within the A (little or no risk) category, the application of a nematicide is not likely to be profitable. The B (moderate risk) code, denotes a borderline situation in which the application of a nematicide may or may not be profitable depending on the growing season and the existence of other stress factors. However, where more than one nematode is present at the B level, the likelihood of a profitable response to nematicide increases. In cases where one or more nematode count falls into the C (high risk) category, the use of a recommended nematicide is almost always profitable.
Again the risk thresholds apply to soil samples collected in late summer or early fall. Soil samples collected during winter or spring always contain reduced levels of nematodes due to unfavorable temperatures and the absence of a host crop. During the winter and early spring, many nematode species are surviving in soil either as eggs or larvae within eggs and worm counts are not a reliable measure of risk for crop damage.