Douglas-fir cone gall midges respond to shape and infrared wavelength attributes of host tree branches

Abstract

AbstractWe tested the hypothesis that the conophagous Douglas-fir cone gall midge,Contarinia oregonensisFoote (Diptera: Cecidomyiidae), responds to infrared (IR) radiation and other electromagnetic wavelengths associated with cones of Douglas-fir,Pseudotsuga menziesii(Mirbel) Franco (Pinaceae). Early-season (March–April) thermographic images showed that cone orientation (upright, horizontal, pendant) and cone colour (green, purple, green/purple) did not affect apparent cone temperature (inferred from thermographic images). Tree components significantly differed in apparent temperature with foliage being coolest and branches warmest. There was no significant difference in the number of larvae in cones of different colours, and adult midges were equally attracted to traps painted green or purple, suggesting that cone colour does not affect oviposition decisions by gravid females. Adult midges were more strongly attracted to warm traps with IR frequency emissions higher than the background than to cold traps with IR frequency emissions lower than the background. They were also more strongly attracted to warm branch-shaped traps than to warm can-shaped traps. Collectively, these data indicate that the shape and IR attributes of Douglas-fir branches may serve as foraging cues forC. oregonensis.