Monarch Population Status

If you have been following the news about monarch populations over the last 8 months, you know that the 2009 overwintering monarch population in Mexico covered a forest area of only 1.92 hectares. This figure represents an all time low for overwintering monarchs and is well below the long-term average of 7.44 hectares (1994-2010). We worried about these low numbers because of the possibility that a devastating storm could drive the population even lower. And then it happened…a storm of the worst possible dimensions hit the overwintering area starting on 2 February. Fifteen inches of rain fell over a four day period. Large hail hit some areas and snow and freezing temperatures were reported subsequently during another period of precipitation. The consequences were devastating for the residents of the monarch overwintering area. Accounts of the flooding and landslides can be found in the Mexico category of the Monarch Watch Blog.

Attempts to find out how the monarchs fared following these winter storms were unsatisfactory. No measurements were made of the numbers of monarchs killed by the storms. Accounts by visitors to the colonies after the storms suggested that substantial numbers of monarchs died during these events. Reports from reliable observers suggested that as much as 80% of the butterflies died at some colonies while others suggested moderate mortality at one large colony of perhaps 20%. Overall we estimated that at least 50% of the monarchs died during the winter months, recognizing that this value could have been low.

Our next concern was the number of butterflies coming north in late February and March. Would the number of surviving butterflies be sufficient to establish a first generation in Texas and the South large enough to lead to a recolonization of the northern breeding areas and a recovery of the population?

Fortunately, the conditions encountered by the monarchs that reached Texas were favorable. Rainfall over the previous 6 months signaled an end to the long drought and produced an abundance of milkweed. The fire ants were down due to the drought and temperatures were lower than normal which seems to favor production of first generation monarchs. The result, in spite of the low number of returning monarchs, was a substantial first generation. These butterflies colonized much of the northern breeding area from late April to mid-June; however, not all areas were well colonized.

The numbers of monarchs reaching the northeastern area (New England and New York) seemed to be well below average, leading to the expectation that the fall migration in this region will be low. Rainfall and temperatures in the central area (Ohio, Indiana, Tennessee, etc.) were not ideal as monarchs arrived from the south and the fall population in this region is likely to be below average except for those areas receiving butterflies from eastern Ontario, which reported many arriving monarchs in May and early June. Based on the numbers of arriving monarchs and the abundance of eggs and larvae found by observers, the fall migration in the east north central region (eastern Dakotas to Michigan) should be above average – assuming that temperatures do not reach the 100s before the end of August.

In summary, it appears that the monarchs are making a modest recovery and I expect the overwintering population will measure close to 3 hectares.