Seed treatment with rubidium for monitoring wireworm (Coleoptera: Elateridae) feeding on corn

A method of labeling corn plants with the trace element rubidium (Rb) was developed for monitoring wireworm feeding on germinating corn seed and seedlings. The method involved treatment of corn seed with an adhesive solution containing Rb. In a greenhouse experiment, only small amounts of Rb were extracted from soil adjacent to treated plants. In field evaluations, corn was labeled at time of planting and remained labeled for less than or equal to 32 d after planting. Rb was transferred to larvae of four species, and concentrations in all wireworms found while feeding on labeled plants exceeded 101 ppm Rb. One-sided 99% tolerance limits calculated from Rb concentrations in wireworms from untreated plots were used as cutoff levels for separating labeled and unlabeled larvae. Percentages of three wireworm populations that became labeled were similar in plots that received seed treatments containing 15, 25, and 35% Rb and in samples obtained between 2 and 32 d after planting. Portions of two populations were obtained from soil adjacent to labeled plants and contained Rb levels similar to naturally occurring concentrations. Seed treatment with Rb provides a sensitive tool for estimating percentages of wireworm populations that feed on corn during its early stages, and also may permit measurements of differences in damage potentials between species and dispersal of larvae after feeding.