Spring remigration of the monarch butterfly, Danaus plexippus (Lepidoptera: Nymphalidae) in north-central Florida: estimating population parameters using mark-recapture

Monarch butterflies (Danaus plexippus) of the eastern North American popula tion migrate each fall from the northern U.S.A. and southern Canada to over wintering sites in Mexico and return the following spring to the southeaste rn U.S.A. where they lay eggs and then die. The spring remigration is the l east studied phase in the annual migration cycle. We therefore conducted a mark-recapture study and examined population recolonization dynamics and re sidence time in a north-central Florida pasture where the monarch's milkwee d host plant (Asclepias humistrata) was abundant. Beginning in late March 1 995 two waves of monarchs arrived, their numbers peaked at 71 individuals b y mid-April, and the butterflies disappeared in early May. After arriving, the adults remained for 3-5 days, laid eggs and then continued to migrate. We also compared population sizes and arrival times in 1994 and 1996. We fo und no evidence of a second spring generation, which was also consistent wi th the deteriorating quality of the A. humistrata plants. Individuals of th e new spring generation disappear shortly after eclosion. The arriving popu lation was approximately nine times greater in 1995 than in 1996. Our findi ngs support two recent hypotheses: (1) the bird-like migration of the monar ch butterfly in North America evolved with the northward expansion and phen ology of milkweeds; and (2) monarchs appear to be migratory throughout thei r annual cycle of several generations. By lingering for only a short time a t each milkweed patch they encounter, the old monarchs returning from Mexic o locate the resurgent milkweed flora over an extensive area in the souther n states. Then, within less than a month, their fresh offspring continue th e migration and exploit the unfolding cornucopia of milkweeds as the spring advances northward. The more we discover about the biology of this insect, the more remarkable is its annual migratory, breeding and overwintering cy cle. (C) 1999 The Linnean Society of London.