The impact of temperature on the reproductive development, body condition and mortality of autumn migrating monarch butterflies in the laboratory

Overwintering populations of the monarch butterfly, <i>Danaus plexippus</i>, have been in decline for the past 30 years. Several hypotheses for the decline have been proposed, including summer and winter habitat loss and migration mortality due to non-senescing milkweeds and the parasite <i>Ophryocystis elektroscirrha</i> (OE). However, the impact of climate change on migrant physiology, has been understudied. This is surprising because warmer temperatures will probably destabilize reproductive diapause, a physiological strategy central to migration and overwintering success. Here, we exposed wild-caught migrants to different field-realistic migratory temperatures under laboratory conditions for 30 days, followed by different overwintering temperatures until death. During the migratory phase, warmer temperatures reduced male body condition, increased male mortality, increased mating frequency and caused females to prematurely abandon their reproductive diapause/dormancy and invest in oocyte production in the absence of milkweed. Monarchs that experienced warm migratory conditions prior to overwintering also exhibited greater overwintering reproductive development and mortality. Overall, reproductive development and OE burden were the best predictors of death. These data suggest warm migratory temperatures significantly alter monarch physiology and fitness and provide a mechanism by which climate change could facilitate migratory failure, winter-breeding and overwintering mortality, all of which can decrease overwintering population size.