Reinvestigation of host location by western corn rootworm larvae (Coleoptera: Chrysomelidae): CO2 is the only volatile attractant

In strong contrast to earlier published results, we now conclude that the attraction of western corn rootworm, Diabrotica virgifera virgifera LeConte, larvae to corn roots is caused by CO2 alone, and that no other volatile chemical cues are involved in attracting the larvae. Choice test behavioral bioassays were conducted in the laboratory, with volatile corn compounds on one side of the bioassay apparatus and with different concentrations of CO2 on the other side (mass spectrometry was used to measure CO2 concentrations on both sides of the apparatus). Larvae were strongly attracted to volatile compounds from corn when ambient air was present on the other side of the bioassay. However, larvae chose equally between the 2 sides of the bioassay when volatile compounds from corn were present on one side and an equivalent concentration of CO2 was present on the other side. When given a choice between corn volatiles and a higher concentration of CO2 the larvae chose the CO2 side significantly more often. In an experiment conducted both with diapausing and nondiapausing strains, the headspace from germinating corn seeds was collected and continuously injected into one side of the bioassay apparatus, and a defined concentration of CO2 was continuously injected into the other side. We tested the possibility that compounds of limited volatility may be involved in larval attraction by preparing glass beads coated directly with volatiles produced by germinating corn seeds, and also by testing soil that was removed from corn roots. All these experiments indicated that compounds other than CO2 were not involved in larval attraction. In other experiments, the soil atmosphere surrounding the roots of growing corn plants was not as attractive as an equivalent concentration of CO2 alone, and the headspace from feeding damaged corn roots was not as attractive as an equivalent concentration of CO2 alone, indicating that weak repellents were present in these treatments together with the strong attractant CO2. Tests with solvent extracts and cryogenic extracts of germinating corn seeds in conjunction with CO2 also indicated the presence of weak repellents in corn for the larvae.