Monitoring Results

Monarchs on the Move

By comparing data from several locations in a single year, we can track monarchs as they move and breed throughout their summer range. The three graphs below show 2002 MLMP data from three states.

Migrating monarchs arrived in Texas about the third week in March, and volunteers begin seeing eggs on their milkweed plants immediately.

Per plant densities are typically very high early in the season, perhaps because there are few plants available for egg-laying.

Adults from these eggs repopulated the northern states, arriving in Minnesota in late May.

As in Texas, early per plant densities were high. The peak in egg abundance dropped off as the migrant adults died and before the first northern generation matured.

The Northeast had lower monarch densities (as it has in all years of the project) and was colonized later than the Midwest.

There are several possible explanations for the timing and low abundance of monarchs in the northeastern US. Malcolm et al. (1993) suggested that fewer monarchs migrate to this region because of the barrier of the Appalachian Mountains. They found that the majority of adult monarchs captured in Pennsylvania and Massachusetts had fed on A. humistrata as larvae, a milkweed species restricted to the southeastern US. This pattern suggests that the monarchs whose offspring colonize the northeastern US are a relatively small subset of the entire population that fly east from Texas and then lay eggs in the southeast.

Although monarchs appear to vacate Texas and other southern states in the hot summer months, MLMP volunteers have recorded egg laying on native milkweeds and the non-native tropical milkweed again in the fall (top). Where the monarchs that lay these eggs originate, and to what extent the adults from this last generation migrate are questions that still need to be investigated.

References

Malcolm, S. B., B. J. Cockrell, and L. P. Brower. 1993. Spring recolonization of eastern North America by the monarch butterfly: successive brood or single sweep migration? In: Malcolm, S. B. and M. P. Zalucki, eds., Biology and conservation of the monarch butterfly. pp. 253-67. Los Angeles: Natural History Museum of Los Angeles County.