Host‐mediated shift in the cold tolerance of an invasive insect

Abstract

While many insects cannot survive the formation of ice within their bodies, a few species can. On the evolutionary continuum from freeze‐intolerant (i.e., freeze‐avoidant) to freeze‐tolerant insects, intermediates likely exist that can withstand some ice formation, but not enough to be considered fully freeze tolerant. Theory suggests that freeze tolerance should be favored over freeze avoidance among individuals that have low relative fitness before exposure to cold. For phytophagous insects, numerous studies have shown that host (or nutrition) can affect fitness and cold‐tolerance strategy, respectively, but no research has investigated whether changes in fitness caused by different hosts of polyphagous species could lead to systematic changes in cold‐tolerance strategy. We tested this relationship with the invasive, polyphagous moth, Epiphyas postvittana (Walker). Host affected components of fitness, such as larval survivorship rates, pupal mass, and immature developmental times. Host species also caused a dramatic change in survival of late‐instar larvae after the onset of freezing—from less than 8% to nearly 80%. The degree of survival after the onset of freezing was inversely correlated with components of fitness in the absence of cold exposure. Our research is the first empirical evidence of an evolutionary mechanism that may drive changes in cold‐tolerance strategies. Additionally, characterizing the effects of host plants on insect cold tolerance will enhance forecasts of invasive species dynamics, especially under climate change.

Sublethal effects of subzero temperatures on the light brown apple moth, Epiphyas postvittana: fitness costs in response to partial freezing

“…), a response that could be considered “partial freeze tolerance” (e.g., Sinclair, ). This apparent mixed response to cold confirms previous observations for late instars (Morey et al ., , ), which we show here also occurs in pupae, albeit to a lesser extent than late instars. For both life‐stages, the individuals that survived partial freezing had relatively higher supercooling points than those that died (Fig.…”

“…If our findings from an artificial environment in the laboratory are also reflective of variation in, and effects of, extreme low temperature tolerance in a natural E. postvittana population, even low levels of putative partial freeze tolerance may have implications for expanding the distribution limits of invading populations over time (Morey et al ., ). This may be especially true for species that have a broad host range; diet can impact cold tolerance response, as we have previously seen in E. postvittana (Morey et al ., ). However, we show here that tolerance to initial ice formation is not without consequence for organismal fitness.…”

“…Larvae were transferred to individual gelatin capsules (size 4; 14.3 mm length, 5.1 mm diameter), as per Morey et al . (), and cooled at ∼1 °C/min from room temperature to the desired treatment temperature within individual polystyrene cubes inside a −80 °C freezer, as per Carrillo et al . ().…”

“…Bürgi and Mills () classify late instars as “chill susceptible” ( sensu Bale, ) because larvae experience substantial mortality at temperatures above the mean supercooling point. We have previously shown that a small proportion of late instars from a laboratory population survived partial freezing (i.e., acute exposure to the peak of their supercooling exotherm) and continued to develop into reproductively successful adults (Morey et al ., , ). In a laboratory setting, such tolerance to partial freezing affords an immediate advantage over the chill‐susceptible portion of the population when temperatures are cold enough to initiate freezing, but fitness costs to the survivors are currently unknown.…”

Modest Effects of Host on the Cold Hardiness of Emerald Ash Borer †

“…Because we did not measure the extent of ice formation, it would be presumptuous to conclude that these individuals were freeze tolerant. Such individuals might be classified as partially-freeze tolerant [46,56], but the ecological ramifications of such a cold tolerance strategy are debatable [56,57].…”