Geographic variation in larval cold tolerance and exposure across the invasion front of a widely established forest insect

Abstract

As the global climate changes, high and low temperature extremes can drive changes in species distributions. Across the range of a species, thermal tolerance can experience plasticity and may undergo selection, shaping resilience to temperature stress. In this study, we measured variation in the lower thermal tolerance of early instar larvae of an invasive forest insect, Lymantria dispar dispar L. (Lepidoptera: Erebidae), using populations sourced from the climatically diverse invasion of the Eastern United States. In two chill coma recovery experiments, we recorded recovery time following a period of exposure to a non-lethal cold temperature. A third experiment quantified growth responses after chill coma recovery to evaluate sublethal effects. Our results indicate that cold tolerance is linked to regional climate, with individuals from cold climate populations recovering faster from chill coma. While this geographic gradient is seen in many species, detecting this pattern is notable for an introduced species founded from a single point-source introduction. We demonstrate that the cold temperatures used in our experiments occur in nature from cold snaps after spring hatching, but negative impacts to growth and survival appear low. We expect that population differences in cold temperature performance manifest more from differences in temperature-dependent growth than acute exposure. Evaluating intraspecific variation in cold tolerance increases our understanding of the role of climatic gradients on the physiology of an invasive species, and contributes to tools for predicting further expansion.

Forecasting the spread of an invasive forest‐defoliating insect

“…In models that predict potential distributions of invasive species, there is increasing recognition that increasing complexity does not always translate into an increase in model performance (Gallien et al, 2010). Although we chose variables to represent long-distance transport and developmental performance that were grounded in prior research (Gray, 2009;Hafker et al, 2021;Thompson, Powers, et al, 2021;Walter, Johnson, et al, 2015), it is possible that other representations of these mechanisms, or of other mechanisms We also found that forecast accuracy tended to be highest for lower occupancy thresholds and shorter Omniscape radii. One possible explanation for the stronger performance of models using shorter Omniscape radii is the short spatial scale of natural dispersal by L. dispar.…”

“…We represented the temperature‐dependent developmental performance of L. dispar in each grid cell for each year by combining gridded climate data (Thornton et al., 2020) with three measures of physiological performance: (1) a numerical model of temperature‐dependent growth and development (Gray, 2004, 2009); (2) cold exposure risk based on experimental data on cold tolerance (Hafker et al., 2021); and (3) the pupal mass of L. dispar reared in different thermal environments (Thompson, Powers, et al., 2021). Because previous studies provide evidence of local adaptation in L. dispar responses to temperature (Faske et al., 2019; Hafker et al., 2021; Thompson et al., 2017; Thompson, Powers, et al., 2021), we incorporated this variation into the second and third components of developmental performance. In the first component, the model of Gray (2009) was used to evaluate whether a typical individual would complete its life stage in a given year (See details in Data S1).…”

“…This model also provided estimates of onset and completion of larval development in a given year and these dates were used in the estimation of the two other components of local developmental performance based on climate. The second component of developmental performance was cold exposure risk, which accounted for the number of cold days during larval development and local adaptation in cold tolerance using data on chill coma recovery, an index of cold tolerance (MacMillan & Sinclair, 2011), from 15 populations collected across the invaded range (Hafker et al., 2021). The third main component of developmental performance was projected pupal mass, taken as an index of fitness (Faske et al., 2019), which was estimated taking into account temperatures during larval development and local adaptation in thermal performance using data from 14 populations collected across the invaded range (Thompson, Powers, et al., 2021).…”

“…Because previous studies provide evidence of local adaptation in L. dispar responses to temperature (Faske et al, 2019;Hafker et al, 2021;Thompson et al, 2017;Thompson, Powers, et al, 2021), we incorporated this variation into the second and third components of developmental performance. In the first component, the model of Gray (2009) was used to evaluate whether a typical individual would complete its life stage in a given year (See details in Data S1).…”

“…We represented the temperature-dependent developmental performance of L. dispar in each grid cell for each year by combining gridded climate data (Thornton et al, 2020) with three measures of physiological performance: (1) a numerical model of temperature-dependent growth and development (Gray, 2004(Gray, , 2009; (2) cold exposure risk based on experimental data on cold tolerance (Hafker et al, 2021); and (3) the pupal mass of L. dispar reared in different thermal environments (Thompson, Powers, et al, 2021).…”