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

Abstract

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.

Evaluation of thermoregulation of different pine organs in early spring and estimation of heat reward for the western conifer seed bug (Leptoglossus occidentalis) on male cones

“…anemophilous, the color of cones would not need to attract pollinators. In female cones, this has been discussed in the context of three hypotheses: thermoregulation, photoprotection, and anti-herbivory [21][22][23]34]. However, there are no studies that focused on male cone color and its adaptive implications.…”

Insects of Reproductive Structures

“…However, pheromone-baited traps are indirect estimators of female abundance. Zahradnik et al (2012) found that adult midges were attracted to specific infrared radiation and traps constructed to emit this radiation attract both males and females, providing a better estimate of midge numbers.…”

“…Visual cues: These are used when there is a difference between cone and foliage colour. Colour cues are important in some species (e.foliage is used by L. occidentalis in finding its host(Takacs et al 2008), though this was later challenged by Schneider (2014).• Shape and size of reproductive structures: Though this has been implicated in host defense and utilization, little has been found regarding host orientation Zahradnik et al (2012). found that branch-shaped (long and thin) but not barrel-shaped (short and wide) silhouettes are attractive to C. oregonensis, possibly indicating that branches are used in orientation towards the cone-bearing portions of Douglas-fir.…”