Catastrophic Winter Storm Mortality of Monarch Butterflies in Mexico during January 2002

estimated that more than 80% of the monarchs had been killed. Heavy mortality had also occurred in the Sierra Chincua and Sierra Campanario colonies (C. Gottfried and Rosario guides, pers. comm.). We hypothesized that the butterflies had been killed by the combination of wetting and the subsequent clearing that results in extreme radiant heat loss to the cloudless sky. Anderson and Brower (1993, 1996) tested this hypothesis at a field laboratory in the Sierra Chincua. They measured the cumulative mortality that occurred as the body temperatures of individual monarchs were experimentally lowered to 15°C below freezing. The study determined that dry butterflies have a modicum of cryoprotection and can super-cool, but by -8.1°C, 50% had frozen to death and by -15°C, all were killed. When misted, they lost much of their natural cryoprotection: 50% died as their body temperatures dropped to only -4.4°C, and by -7.8°C, all were dead. Larsen and Lee (1994) showed that wetting also accelerates the freezing process: monarchs held at -4°C froze within 3 hours when wet, whereas dry individuals survived the same treatment for the 24-h duration of the experiment. These studies are definitive: wetting of butterflies prior to the short-term temperature plunges that follow winter storms in the overwintering region greatly increases the percentage of monarchs that will freeze to death. Continuing microclimatic research at the overwintering sites in Mexico demonstrated that forest thinning exacerbates the monarchs’ risk of freezing (Calvert and Brower 1981; Calvert and Cohen 1983; INTRODUCTION