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

Abstract

Eastern spruce budworm moth (Choristoneura fumiferana(Clem.)) mass outbreaks have widespread economic and ecological consequences. A key explanation for the large-scale spread and synchronization of these outbreaks is the long-distance dispersal (up to 450km) of moths from hotspots (high-density populations) to lower-density areas. These events have proved difficult to monitor because dispersal flights occur only a few times a year, have no consistent routes, and commonly used tracking methods (e.g., population genetics, mark-recapture, radio telemetry) are inadequate for this system. Distinguishing between local and immigrant individuals is a crucial step in identifying the physical and ecological drivers of moth dispersal. Here we test whether isotopes of hydrogen (i.e., delta notation:δ2H) and strontium (i.e., strontium isotope ratios:87Sr/86Sr), known to independently vary in space in a predictable manner, can be used to distinguish between local and immigrant adult spruce budworm moths. We used an automated pheromone trap system to collect individuals at six different sites in eastern Canada within and outside the current outbreak area of budworm moths. We first use moth flight behaviour and time of capture, currently the best available tool, to determine putative local vs. immigrant status, and then evaluate whether individual87Sr/86Sr andδ2H differ between putative classes. At all sites, we detect immigrant individuals that differ significantly from putative locals. Saliently, sites where putative locals were sampled before the occurrence of potential immigration events (~10 days) showed the strongest differences between immigrant individuals and the locals87Sr/86Sr andδ2H values. Sites where the collection of putative locals was close in time (hours) or following an immigration event had a less-clear distinction between putative immigrants and locals, and showed signs of mixing between these two groups. We speculate that recent immigration could have led to the misclassification of immigrants as putative locals.87Sr/86Sr andδ2H data generally support the adequacy of current approaches using capture-time to detect immigration events, and provide enhanced resolution to distinguish between local and immigrant individuals. We discuss the broader implication of adding isotopes to the toolkit to monitor spruce budworm dispersal and suggest next steps in implementing these tools.

Metals and metal isotopes in insect wings: Implications for diet, geolocation and pollution exposure

“…Strontium isotope ratios have already been applied to the geographic assignment of monarch butterflies and other lepidopteran species and showed great promise for increasing the precision of isotope-based geographic assignment (Reich et al 2021, Dargent et al 2022. Here, we confirmed the assumption that the ⁸⁷Sr/⁸⁶Sr of monarch wings remain the same over time and continue to represent the larval signature despite adult feeding, at least in males (Figure 3B).…”

“…Strontium isotope ratios have already been applied to the geographic assignment of monarch butterflies and other lepidopteran species and showed great promise for increasing the precision of isotope-based geographic assignment (Reich et al . 2021, Dargent et al . 2022).…”

A hydrogen isoscape for tracing the migration of herbivorous lepidopterans across the Afro‐Palearctic range

“…The isoscape would also be applicable to trace the one-way dispersal and population connectivity of invasive insect pests. 9 The average uncertainty (17.3‰) obtained from the calibrated δ 2 H wing isoscape is higher than previous δ 2). The Brattström et al 29 Considering the scarcity of Mediterranean samples in Brattström et al, 29 the different transfer functions are not totally surprising as simulation exercises performed by Contina et al 40 showed that lack of sampling of isotope extremes of the range could lead to large difference in calibration lines.…”

“…Hydrogen isotope composition (referred to subsequently as δ 2 H 8 ) is a promising tool to trace insect mobility and has been applied to a few insect species to study specific dispersal events, [9][10][11][12][13] single generations within the migratory cycle [14][15][16][17][18] and more rarely multigenerational migration. [19][20][21] The basis behind this approach is that the δ 2 H in water varies spatiotemporally and predictably on the landscape with temperature, elevation and continentality.…”

Metals and metal isotopes incorporation in insect wings: Implications for geolocation and pollution exposure

“…Ultimately, many sources of variability in metal composition need to be further explored, including the effects of host plant (Bowden et al, 1984), microbial communities (Consuegra et al, 2020), parasitism (Sures, 2004), type of metamorphosis and life-stage, reproductive status, homeostatic mechanisms, and their interactions (Tibbett et al, 2021). Strontium isotope ratios have already been applied to the geographic assignment of monarch butterflies and other lepidopteran species and showed great promise for increasing the precision of isotope-based geographic assignment (Reich et al, 2021;Dargent et al, 2022). Here, we confirmed the assumption that the 87 Sr/ 86 Sr of monarch wings remain the same over time and continue to represent the larval signature despite adult feeding, at least in males (Figure 4B).…”