Wing wear and body size measurements of adult spruce budworms captured at light traps: inference on seasonal patterns related to reproduction

Abstract

Adult spruce budworms, Choristoneura fumiferana Clemens (Lepidoptera: Tortricidae), were captured at light traps in New Brunswick (NB) in 2012, Quebec (QC) in 2013, and Newfoundland (NL) in 2014; population densities were low in NB–NL, and moderate in QC. Morphological parameters (wing score index, wing length, dry weight, residual fecundity) were estimated for thousands of adults. A very large number of budworms were captured during the night of 15–16 July 2013. The heavy weight and low proportion of eggs laid by these females suggest a synchronized, pulsed emergence of local adults rather than mass immigration. Variation in sex ratio and adult morphology suggest that populations were ‘closed’ at all locations (resident moths ⋙ immigrants), as indicated by an increasing proportion of females over time (protandrous emergence) and declining weight over time (reflective of the ageing of adults). The small size of adults in QC relative to NB–NL suggests that inverse-density-dependent fecundity affects population dynamics early in the transition between endemic and epidemic phases. Wing wear provides a useful proxy for the age of adults: individuals with fresh wings (limited wear) were heavier than those with damaged wings, which likely reflects the metabolic costs of survival and (for females) the accruing proportion of eggs laid over time.