Assessing Within- and Transgenerational Effects of Thermal Stress on the Baltimore Checkerspot (Euphydryas phaeton phaeton)

Extreme weather events such as heatwaves are increasing in frequency and intensity due to climate change, posing significant threats to ectotherms. The Baltimore checkerspot ( Euphydryas phaeton phaeton ), a univoltine butterfly native to eastern North America, is of particular conservation concern due to its rapid population decline in recent decades. Because insect responses to thermal stress are highly species-, stage-, and temperature-range-specific, the mechanisms by which heatwaves impact development and fitness in E. p. phaeton have not been fully explored. Transgenerational effects, where ancestral environments influence offspring performance, may provide a mechanism for adaptation to thermal stress, but their role under heatwave scenarios remains unclear. This study examined the within- and transgenerational effects of summer and winter heatwaves on the development, reproduction, and survival of E. p. phaeton . Using a split-brood design, we exposed pupae to one of four summer thermal treatments (10°C, 26°C, 28°C, 32°C), and their offspring to different frequencies of winter heatwaves. We found that higher pupal temperatures accelerated development, but extreme heat (32°C) significantly reduced adult longevity and reproductive output. Winter heatwaves led to reduced post-dormancy larval survival, regardless of frequency. While parental thermal treatment alone had limited effects on offspring performance, offspring of parents that also experienced heat stress (28°C and 32°C) exhibited higher survival under warm winters, suggesting a potential role for adaptive transgenerational effects when parental and offspring environmental conditions correlate.