Behavioral responses of first-instar western corn rootworm (Coleoptera: Chrysomelidae) to carbon dioxide in a glass bead bioassay

A behavioral bioassay was developed to test responses of newly hatched (neonate) larvae of western corn rootworm Diabrotica virgifera virgifera LeConte to volatile compounds from corn plants, a major host for this insect. A glass Y-tube filled with glass bends was used to allow choice tests in a vertical direction and to reproduce the thigmotactic cues available to larvae in their natural soil environment A syringe pump was used to provide slow, consistent delivery of candidate compounds to the 2 sides of the apparatus, Significantly more larvae were attracted to the side containing a germinating corn seed than to the side containing ambient air. In addition, significantly more larvae were attracted to the side containing cut corn roots than to the side containing an ambient air control. Carbon dioxide (CO2) from corn roots previously has been implicated as an attractant for the larvae, and dose-response curves for larval attraction to CO2 were obtained using different sources (different dilutions of carbonated water? the headspace over a carbonated water dilution, and different concentrations of CO2 in air). The CO2 concentrations for all sources were measured by mass spectrometry with selected ion monitoring at m/e 44.Neonate larvae were significantly attracted to concentrations of CO2 as low as 1.125 +/- 0.04 mmol/mol (concentration of CO2 in ambient air on the control side was 0.99 +/- 0.02 mmol/mol). Larvae were optimally attracted to 2.51-4.20 mmol/mol CO2, but they were attracted to concentrations as high as 100 mmol/mol. Larvae were not attracted to 300 or 900 mmol/mol CO2, and they exhibited toxic symptoms at these high concentrations. The concentration of CO2 in soil near growing corn roots was 4.36 +/- 0.31 mmol/mol which was consistent with the behavioral optimum for the larvae. The concentration of CO2 in soil that contained no corn was 1.38 +/- 0.03 mmol/mol and the concentration in ambient air was 0.91 +/- 0.01 mmol/mol.