We tested the hypothesis that the conophagous Douglas-fir cone gall midge,
Contarinia oregonensis Foote (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 for C.
oregonensis.