Lipid accumulation by migrating monarch butterflies (Danaus plexippus L.)

The migration of Danaus plexippus during the late summer in southern Ontario in 1986 lasted for about 8 weeks and consisted of three phases, an early phase characterized by increasing abundance, a middle phase of peak abundance, and a late phase characterized by declining abundance. As the season progressed, systematic changes were observed in wet mass, dry mass, lean dry mass, lipid mass, and forewing length. Wet mass, lean dry mass, and forewing length were similar for early- and middle-phase individuals, but declined in late-phase migrants. Lipid mass peaked in the middle phase of the migration and then declined abruptly in the late phase. Dry mass also peaked in the middle phase, reflecting changes in lipid mass and lean dry mass. We hypothesize that the observed changes in lipid mass and lean dry mass over the 8 weeks resulted from changes in population structure as well as seasonal changes in the weather, and in availability of nectar. Opposing conclusions reached in previous studies of lipid accumulation in D. plexippus are probably the result of failure to control for phase of migration.