The Effect of Rain, Snow and Freezing Temperatures on Overwintering Monarch Butterflies in Mexico

Abstract

Approximately 2.5 million monarch butterflies in an overwintering colony in Mexico were killed by a severe winter storm in January 1981. Flight testing of butterflies sampled from four locations within the colony during and after the storm gave an overview of the general flux of butterflies from clusters to the ground and back due to the storm's effects. Normally adapative behavior to avoid freezing ground temperatures, including high roosting and crawling up onto low vegetation, were only partially effective against the severe cold. Selective wetting and subsequent freezing appears to account for the mortality patterns observed in the roosting clusters. EACH FALL TENS OF MILLIONS of monarch butterflies (Danaus plexippus L.) from eastern North America migrate to Mexico's transvolcanic belt (Urquhart and Urquhart 1976, Brower et al. 1977). Arriving in early November and leaving at the end of March, they pass the late fall and winter in this area. Within the protective canopy of high altitude forests, they aggregate in enormous numbers on trunks and branches principally of the oyamel fir, Abies religiosa H.B.K. One major cause of mortality of butterflies at the overwintering sites is bird predation (Calvert et al. 1979, Fink and Brower 1981), as evidenced by damaged body parts or missing wings. In two of the past five overwintering seasons we observed beneath trees with dense clusters accumulations from 2 to 15 cm deep of undamaged, dead butterflies which we assumed were killed by winter storms. In January 1981, a severe winter storm impacted a major overwintering colony in the area previously designated Site Alpha (Brower et al. 1977). We report here the effects of this storm.