Sampling procedures and adult sex ratios in spruce budworm

Abstract

AbstractUnbiased estimates of sex ratios that reflect local abundance of adult insects are practically difficult to obtain because many gender‐specific behavioural adaptations differentially influence the catchability of males and females in commonly applied sampling procedures. Historic data on outbreak populations of spruce budworm, Choristoneura fumiferana Clemens (Lepidoptera: Tortricidae), the major pest of conifers in Nearctic boreal forests, include dozens of sex ratio observations for 10 different sampling procedures; these data illustrate the importance of understanding the reproductive ecology of adults to contextualize sex ratio assessments. Sex ratios of resident adults (assessed by rearing field‐collected pupae to adulthood or fogging host trees with insecticide) were not different from 1:1. Sex ratios of in‐flight adults collected using Malaise traps or light traps deployed in tree canopies were consistently male‐biased, which presumably reflects the higher level of flight activity for males relative to females. Sex ratios of moths captured outside the forest canopy (presumed migrants), in contrast, were consistently female‐biased, a trend which is expected because females seeking oviposition sites are more likely to undergo migration than are males. The sex ratio among adults that died from natural causes (collected on drop trays) was not distinguishable from 1:1. In pre‐outbreak (endemic) populations, sex ratios estimated by light trapping were much more strongly male‐biased than in outbreak populations. This surprising result should, however, be interpreted with caution because little is known of reproductive ecology in endemic budworm populations.