Effects of changes in atmospheric carbon dioxide on the location of hosts by the moth, Cactoblastis cactorum.

Sensory organs that detect CO are common in herbivorous moths and butterflies, but their function has been unclear until now. As the CO gradients in the vicinity of a host plant depend on its physiological condition, CO could provide a sensory cue for the suitability of the plant as a larval food source. This study investigated whether changing the atmospheric CO concentration affected oviposition by Cactoblastis cactorum on its host, the cactus Opuntia stricta. On host plants exposed to rapid fluctuations in CO concentration, the frequency of oviposition was reduced by a factor of 3.2 compared to the control. As the fluctuations mask the much smaller CO signals generated by the plants, this suggests that those signals constitute an important component of the host identification process. On host plants exposed to a constant background of doubled CO, oviposition was also reduced, by a factor of 1.8. An increased background reduces host signal detectability, partially as a consequence of a general principle of sensory physiology (Weber-Fechner's law), and partially due to other factors specific to CO-receptor neurons.