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

Abstract

The interaction between the moth, Cactoblastis cactorum, and the cactus, Opuntia stricta, is used as a model to examine the question of whether the CO sense of a herbivorous insect can detect the CO gradients associated with a plant's metabolic activity. Both the anatomical and the electrophysiological characteristics of CO-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 CO 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 CO 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 CO sensors. In a simulation of probing, pronounced responses of the sensors to the CO-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.

Carbon-dioxide sensing structures in terrestrial arthropods

“…In the pyralid moth Cactoblastis cactorum, the labial palps show a pronounced sexual dimorphism (Stange et al, 1995). In females (typical body length 21 mm), the palps point forwards and measure approximately 12% of body length.…”

A Plant Notices Insect Egg Deposition and Changes Its Rate of Photosynthesis

“…Female moths probe the plant surface with their CO 2 sensor, thus probably examining the suitability of the host plant. Most eggs are laid on the most vigorous plants (Stange et al, 1995). Furthermore, Thom et al (2004) showed that CO 2 plays an important role in the foraging behavior of the nectar-feeding moth Manduca sexta, which prefers surrogate flowers that emit CO 2 concentrations characteristic of a flowering plant.…”

Behavioral responses of Drosophila to biogenic levels of carbon dioxide depend on life-stage, sex and olfactory context

“…Lower doses are used by blood-feeding insects to locate their host (Gillies, 1980;Pinto et al, 2001;Barrozo and Lazzari, 2004;Dekker et al, 2005). Herbivorous insects use CO 2 to locate leaves, damaged fruits, flowers or roots (Hibbard and Bjostad, 1988;Stange et al, 1995;Stange, 1999;Thom et al, 2003).…”