Exploitation of electromagnetic radiation as a foraging cue by conophagous insects

Many insects exploit sections of the electromagnetic spectrum as foraging or attraction cues, detecting wavelengths in the ultraviolet (UV; ~ 300–400 nm), human visible (400–750 nm) and infrared (> 750 nm) range. Two distinct types of receptors are involved. Compound eyes in the head detect UV and human visible light, and IR receptors on the thorax or abdomen detect radiant IR which contrasts against the background and is therefore dectable. I investigated the potential use of electromagnetic foraging cues in three members of the conophagous insect guild: the diurnal Western conifer seed bug, Leptoglossus occidentalis Heidermann (Hemiptera: Coreidae), the nocturnal fir coneworm moth, Dioryctria abietivorella Groté (Lepidoptera: Pyralidae), and the diurnal Douglas-fir cone gall midge, Contarinia oregonensis Foote (Diptera: Cecidomyiidae). In electrophysiological recordings, two-choice laboratory bioassays, and field trapping experiments, I tested the hypotheses that during (cone) foraging C. oregonensis, D. abietivorella and L. occidentalis (1) rely on IR receptors that receive and respond to cone-derived radiant IR, (2) receive and respond to cone or plant-derived colour cues, and (3) integrate radiant IR and insect visible light cues. My data support these hypotheses, at least in part. Male and female L. occidentalis were more attracted to radiant IR from heat sources within but not outside the natural cone temperature range. Male and female D. abietivorella have IR receptors on their ventral prothorax which help them detect and discriminate between IR stimuli. Both D. abietivorella and C. oregonensis are attracted to warm objects with IR signatures resembling or corresponding to those of tree branches but not those of cones. No insects oriented towards cone-reflected light as a singular foraging cue, but mated female L. occidentalis preferred the complete light spectrum of conifer needles (their oviposition site) to a narrow bandwidth of visible light. Visible (blue) light combined with radiant IR from a 40 ºC heat source were synergistic in attracting female L. occidentalis, indicating that the central nervous system of L. occidentalis is capable of processing and integrating information from compound eyes and IR receptors. Such integration of visible stimuli and IR was previously known only in viperid snakes.