Monarch Population Status

Monarch Watch turns 20! We started our monarch tagging program in September of 1992 – it doesn’t seem that long ago but I guess that we have been so busy that we lost track of the passing years. We began by recruiting our first taggers through notices in newspapers that called for volunteers and sending out tagging kits as fast as we could put them together. The response by the public was overwhelming and the positive feedback from the participants led to the creation of Monarch Watch, an organization we didn’t envision at the outset.

We started tagging in a down year for monarchs but we didn’t realize it then. Only later did we recognize that the dust veil created by the eruption of Mount Pinatubo had led to a significant reduction in summer temperatures and monarch numbers as well. A lot has happened since 1992. We’ve seen the overwintering population in Mexico increase each year from 1994 to 1996, only to crash inexplicably in 1997. We have seen ups and downs in overwintering numbers – but mostly downs since 2003. In fact, the population has been below the long-term average over the last seven years. Ominously, the four lowest populations recorded to date occurred in the last 11 years. The downward trend is now statistically significant (Brower, et al. 2011) and it is clear that we have entered a new era of monarch numbers.

The great migrations of the 90s are a thing of the past. In the future, we can expect overwintering populations in Mexico of 2-6 hectares. The main reason for the decline is loss of habitat. Monarch habitat has been reduced by at least 140 million acres in the last 10 years – about a fifth of the total breeding area available to monarchs has been lost. At least 100 million acres of habitat has been lost due to the adoption of herbicide resistant corn and soybeans. The herbicide tolerant (HT) crops allow growers to spray their crops with herbicides without affecting the crops. The result has been the near elimination of milkweeds in these row crops and a reduction in monarch numbers – monarch production in these fields was higher when measured in 2000 than in roadsides, old fields, conservation reserve lands and other habitats (Oberhauser, et al PNAS 2001). The adoption of HT crops began slowly in 1996 but has been increasing rapidly since 2003. By 2010 80.7% of the corn and soybeans planted in the United States (161 million acres) were herbicide tolerant. Since these crops are used in rotation, it is likely that milkweeds have been eliminated in more than 81% of the total acreage.

So, where does this leave us and what does this mean for tagging? It means that we will have another year like the last seven and a year not unlike 1992 when we started the program. Specifically, we can expect a low year, perhaps not as low as 2009 (1.92 hectares) or 2004 (2.19 hectares) but close to these numbers. The migration should be particularly low in the New England area and the numbers at Cape May will be low as well. The central region (Ontario, MI, OH, IN, IL) will see a modest migration and could produce more monarchs than the area defined by the eastern Dakotas, MN, WI, and IA. Even though the population will be down from historical highs, there will still be plenty of monarchs to tag. And, as always, you and your fellow taggers will not only have fun but you will also contribute to our knowledge of the monarch migration.