I know it when I see it: Incidence, timing and intensity of immigration in spruce budworm

Abstract

Incidence of immigration in spruce budworm Choristoneura fumiferana (Lepidoptera: Tortricidae) was inferred with rules of thumb and numerical thresholds contrasting closed populations (residents >> migrants) versus populations subjected to major immigration event, based on attributes of daily time series of abundance at light traps (abundance, statistical mode, sex ratio and protandry level). The data set included time series with known migration event (Bas St‐Laurent, QC, 2013), which were used to calibrate numerical thresholds associated with immigration. The study site on west coast of Newfoundland provided a natural laboratory to infer long‐range immigration due to geographic isolation from the mainland, proximity to the coast and low population density at onset of the study. Time series between 2014 and 2016 exhibited trends consistent with scenario of closed population, including low numbers of budworms, limited variation in day‐to‐day abundance and strong level of protandry (males fly seasonally earlier than females). Time series with candidate major immigration event (Bas St‐Laurent 2013, Sally's Cove 2017, 2018) displayed specific combinations of parameters, including: (a) sharp increment (>one order of magnitude) in abundance of both males and females from one day to the next (statistical mode, approximately corresponding to timing of migration); (b) pure (100%) convergence of pulse detection interval (post‐immigration period when migrants with nonzero survival rate are collected at light traps) across traps for males and females and (c) overall absence of protandrous flight. Parameters related to immigration thresholds highlighted that females are more likely to migrate than males, as indicated by increased ratio of females per male during pulse detection interval. Aerial collection of adult spruce budworms with helikite in 2019 confirmed the preponderance of females among true migrants.

Characterizing Eastern spruce budworm’s large-scale dispersal events through flight behavior and stable isotope analyses

“…Furthermore, on average at a given location, SBW adult emergence occurs for a period of 2 weeks and local moth activity lasts 3 weeks, yet the influx of immigrants that remain active following dispersal can extend the duration of activity at any given site (Greenbank et al, 1980). Overlaps in immigration events and local emergence affect the reliability of captures around dusk, as surviving immigrants -which can live for about four days (Rhainds et al, 2022)- can be misidentified as locals without any additional tools for discrimination.…”

“…However, for SBW, adults don’t feed which means that there is no addition of dietary Sr during the adult stage. Additionally, SBW moths live for approximately 10 days following emergence (Greenbank et al, 1980) and up to 4 days after immigration (Rhainds et al, 2022) limiting the possibility of Sr intake from locations different from the larval stage site. Consequently, it is likely that the 87 Sr/ 86 Sr ratios in short-lived SBW reflect the larval stage and can be applied for geolocation.…”

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

“…Collection of migrants in the troposphere was conducted with a Skyhook helikite as described in Rhainds et al (2022). For ascension, the helikite was hauled on the ground using a 1‐km‐long 280‐kg tension‐resisting Dyneema flying line attached to a 4000 kg 12V electric winch fixed to the back of a pickup truck.…”

Mass Trapping Lepidopteran Pests with Light Traps, with Focus on Tortricid Forest Pests: What If?

“…(Lepidoptera: Tortricidae), is a severe epidemic defoliator of conifers [balsam fir, Abies balsamea, and spruce trees, Picea sp.] in eastern Canada and the northeast states in the U.S.A. [1][2][3]. Concomitant with the large-scale outbreak of budworms in the province of Québec, two massive immigrations of females were detected in light traps (LTs) deployed in Sally's Cove in 2017 and 2018 (red dot in Figure 1), which led to a fortyfold two-year increase in the abundance of overwintering larvae from 0.2 to 8.6 individuals per branch [2] and nearly one million adults in 16 LTs in 2019 (Section 3).…”

“…in eastern Canada and the northeast states in the U.S.A. [1][2][3]. Concomitant with the large-scale outbreak of budworms in the province of Québec, two massive immigrations of females were detected in light traps (LTs) deployed in Sally's Cove in 2017 and 2018 (red dot in Figure 1), which led to a fortyfold two-year increase in the abundance of overwintering larvae from 0.2 to 8.6 individuals per branch [2] and nearly one million adults in 16 LTs in 2019 (Section 3). The initial research question at hand (the recognition of immigration patterns in LTs) was defeated by the a posteriori obvious reality that immigration is hard to discern in a high local density of adults.…”