A novel method for estimating soybean herbivory in western corn rootworm (Diabrotica virgifera virgifera)

A variant strain of western corn rootworm, Diabrotica virgifera virgifera LeConte, poses a management problem to growers of rotated corn in parts of the Midwest by ovipositing in the alternate crop, which is normally soybean. Sampling methods are needed that accurately predict the potential for damage to first-year corn while being relatively easy and inexpensive to implement. Soybean herbivory has been shown to occur when adult rootworms migrate to soybean fields, and has been shown to be associated with oviposition therein. This behavior could be utilized as a marker in sampling programs designed to estimate the prevalence of variant activity in an area. A unique characteristic of adult rootworms that have fed on soybean foliage is the distinctive dark green color that appears in their ventral abdomen. A colorimetric assay is described that exploits this characteristic to estimate the rate of soybean herbivory within a sample of beetles collected from the field. The results of this assay when applied to beetles collected via a sampling program conducted throughout Indiana are discussed.