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 a ttraction of western corn rootworm, Diabrotica virgifera virgifera LeConte, larvae to corn roots is caused by CO2 alone, and that no other volatile ch emical cues are involved in attracting the larvae. Choice test behavioral b ioassays 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 concentration s on both sides of the apparatus). Larvae were strongly attracted to volati le compounds from corn when ambient air was present on the other side of th e bioassay. However, larvae chose equally between the 2 sides of the bioass ay when volatile compounds from corn were present on one side and an equiva lent concentration of CO2 was present on the other side. When given a choic e between corn volatiles and a higher concentration of CO2, the larvae chos e the CO2 side significantly more often. In an experiment conducted both wi th diapausing and nondiapausing strains, the headspace from germinating cor n seeds was collected and continuously injected into one side of the bioass ay apparatus, and a defined concentration of CO2 was continuously injected into the other side. We tested the possibility that compounds of limited vo latility may be involved in larval attraction by preparing glass beads coat ed directly with volatiles produced by germinating corn seeds, and also by testing soil that was removed from corn roots. All these experiments indica ted that compounds other than CO2 were not involved in larval attraction. I n other experiments, the soil atmosphere surrounding the roots of growing c orn plants was not as attractive as an equivalent concentration of CO2 alon e, and the headspace from feeding-damaged corn roots was not as attractive as an equivalent concentration of CO2 alone, indicating that weak repellent s were present in these treatments together with the strong attractant CO2. Tests with solvent extracts and cryogenic extracts of germinating corn see ds in conjunction with CO2 also indicated the presence of weak repellents i n corn for the larvae.