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

Sensory organs that detect CO₂ are common in herbivorous moths and butterflies, but their function has been unclear until now. As the CO₂ gradients in the vicinity of a host plant depend on its physiological condition, CO₂ could provide a sensory cue for the suitability of the plant as a larval food source. This study investigated whether changing the atmospheric CO₂ concentration affected oviposition by Cactoblastis cactorum on its host, the cactus Opuntia stricta. On host plants exposed to rapid fluctuations in CO₂ concentration, the frequency of oviposition was reduced by a factor of 3.2 compared to the control. As the fluctuations mask the much smaller CO₂ 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 CO₂, 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 CO₂-receptor neurons.