Many different species of plant-parasitic nematodes occur in cornfields throughout the Midwest. Most of these microscopic, parasitic worms must reach a damage threshold number before they cause yield loss in corn. The damage threshold numbers for most nematodes that feed on corn in Iowa (see Common Corn Nematode Characteristics, ICM News, Aug. 3, 2009) are 100 or more worms per 100 cm3 , which is a little less than a half-cup of soil. The exceptions are the needle and sting nematodes, which damage corn even if only one or two worms per 100 cm3 soil are present. Fortunately, needle and sting nematodes only occur in soil with at least 70 percent sand and, therefore, do not occur in most Iowa fields.
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