The CO2 sense of the moth Cactoblastis cactorum and its probable role in the biological control of the CAM plant Opuntia stricta.

The interaction between the moth, Cactoblastis cactorum, and the cactus, Opuntia stricta, is used as a model to examine the question of whether the CO2 sense of a herbivorous insect can detect the CO2 gradients associated with a plant's metabolic activity. Both the anatomical and the electrophysiological characteristics of CO2-sensitive receptor neurons in C. cactorum indicate an adaptation to the detection of small fluctuations around the atmospheric background. Evidence is provided that further rises in background will impair the function of the sensory organ. In the habitat of the plant, during the diurnal window of the moth's activity, two types of CO2 gradients occur that are detectable by the moth's sensors. The first gradient, associated with soil respiration, is vertical and extends from the soil surface to an altitude of approximately 1 m. Its magnitude is well above the detectability limit of the sensors. The notion that this gradient provides, to a flying insect, a cue for the maintenance of a flight altitude favourable for host detection is supported by field observations of behaviour. The second gradient, associated with CO2 fixation by the plant, extends from the surfaces of photosynthetic organs (cladodes) over a boundary layer distance of approximately 5 mm. Again, its magnitude is well above the detectability limit. The notion that this gradient provides, to a walking insect, a cue to the physiological condition of the plant is supported by the observation that females of C. cactorum, prior to oviposition, actively probe the plant surface with their CO2 sensors. In a simulation of probing, pronounced responses of the sensors to the CO2-fixing capacity of O. stricta are observed. We propose that by probing the boundary layer, females of C. cactorum can detect the healthiest, most active O. stricta cladodes, accounting for earlier observations that the most vigorous plants attract the greatest density of egg sticks.