Inverse size‐dependent phenology of female spruce budworms along reproductive and behavioral classes: an empirical study

Abstract

The incidence and causality of inverse size‐dependent phenology was investigated for spruce budworm, Choristoneura fumiferana (Clemens) (Lepidoptera: Tortricidae), in part due to large sample size [>22000 feral females with known body size (wing area) and timing of emergence/occurrence]. Data collected in three Canadian provinces (New Brunswick, Nova Scotia, Quebec) between 1976 and 2019 included a wide array of sampling procedures grouped along gradients of reproductive classes. (1) For resident females of local origin (collected as pupae and allowed to emerge in the laboratory), body size declined over time at seven of seven sites, indicative of inverse size‐dependent timing of emergence. (2) Females foraging on trees (collected by fogging host trees or with sweep nets) exhibited declines in body size over time at five of five sites. (3) For behaviorally active females collected at traps (malaise, light, and canopy traps), inverse size‐dependent phenology of flight was observed at 20 of 23 combination of trap types/sites; partition of variance indicated limited variation in body size as a function of trap type, thus all inflight females were pooled in subsequent analyses. (4) In contrast to other sampling procedures, dead females collected on drop trays exhibited size‐dependent timing of mortality at only one of five sites. The article includes first body size measurements of migrant females captured in the troposphere with a kite balloon; as with other sampling procedures, body size decreased during the interval of sampling. Ongoing studies combining measurements of body size and dry weight of females (wing load) aim at inferring consequences of inverse size‐dependent phenology on reproduction and migration of budworms, including linkages with population dynamics.