Monarch Population Status

In early July I wrote the text for the pre-migration newsletter that goes out with the tagging kits in which I stated that there was no real basis for making a prediction about the size of the fall migratory population:

“If there is sufficient rainfall and normal to above normal temperatures in the northern breeding range through August, the migratory population may be the largest since 2003 – perhaps 6-7 hectares. On the other hand, a continuation of the extreme heat and drought conditions could result in another overwintering population in the 2-3 hectare range – well below the long-term average of 7 hectares.”

It is now clear that fall population will be on the low side with an overwintering population close to 3 hectares once again. I don’t need to tell you that the summer was hot and dry – too hot and dry in many areas for good monarch reproduction. We have received many comments on the poor quality of the milkweed available to monarchs for the last generation. In eastern Kansas, where the drought ranges from extreme to exceptional, milkweeds, even blue vine (Cynanchum laeve), suitable for egg laying, are scarce. The best areas for the production of fall migrants appear to be northwest and central Minnesota, northern Wisconsin and Michigan, and much of New England. Reports from New England suggest that the number of monarchs passing through Cape May will be higher than normal this fall. Elsewhere, we can expect the number of fall migrants to be similar to that seen over the last several years.

The low number of nectar sources that will be available to monarchs moving through the lower Midwest in September is a concern. Some fall flowers have already bloomed, some have died and many of the others are stunted and just barely alive. There will be nectar but it will be harder for the monarchs to find. Rain in the areas of Nebraska, Kansas, Oklahoma, Missouri, Iowa, Arkansas, and Illinois over the next three weeks could make a big difference in the proportion of the fall migrants that reach Mexico.