Malcolm, SB; Cockrell, BJ; Brower, LP
ABSTRACT. Each March, monarch burrerflies, Danaus plexippus (L.), remigrate north from Mexican overwintering sites. By June they are distributed across most of eastern North America, north to southern Canada. Here we distinguish between two alternative spring recolonization strategies: (1) "single sweep recolonization," in which overwintered individual remigrants from Mexico were hypothesized to colonize the entire eastern North American breeding range, and (2) "successive brood recolonization" in which remigrants from Mexico were hypothesized to lay all of their eggs in the southern United States and then die, leaving their offspring to continue the migration northward and recolonize the northern breeding range.
Cardenolide concentrations, cardenolide "finger-prints," and wing wear of migrant butterflies captured along two latitudinal transects in the spring of 1985 provided three lines of independent support consistent with hypothesis 2. Monarchs captured in the southern United States in late March and early April, which lay eggs on the emergent milkweeds, had (1) low cardenolide concentrations, (2) cardenolide fingerprints identical to those of overwintering monarchs in Mexico, and (3) very worn wings. In contrast, monarchs from the northern United States in May and June had (1) high cardenolide concentrations, (2) fingerprints characteristic of the southern spring milkweed flora, and (3) wings in significantly better condition. Thus, monarchs recolonize their breeding range each spring by migration of successive broods and not by a single sweep of overwintered migrants.
The existence of concurrent migration and reproductive activity of the new spring brood is an exception to the "oogenesis-flight syndrome" of migratory insects, which holds that reproductive activity is suppressed during migration. The new evidence also indicates that chemical defense during the monarch's annual life cycle is dynamic and cycles from poorly protected autumn migrants, which overwinter and then recolonize the southern United States, to highly protected migrants of the first spring generation, which recolonize the northern United States.