Monarch Population Status

Those of you who follow monarchs closely are aware that the monarch population has been declining for the last 10 years with significant drops in the population each of the last three years. The number of trees and total area occupied by monarchs in the oyamel fir forests in Mexico was at an all time low last winter – a mere 0.67 hectares. This decline has given rise to a great deal of concern about the future of the monarch migration. These concerns have resulted in many meetings and plans, and even a Presidential Memorandum directing federal agencies to devote resources to offset the decline in monarchs and pollinators. We have addressed this issue through our Monarch Waystation, Bring Back the Monarchs and Milkweed Market programs. These programs are growing but they need to become much, much larger to sustain the monarch migration. Large-scale habitat restoration, particularly in the upper Midwest, also needs to become a priority. In the meantime, we need to keep tagging monarchs as a way of monitoring their numbers and tracking any shifts in the origins of monarchs that reach Mexico. If you are a long-term tagger, you know it has been increasingly difficult to find enough monarchs to tag, especially during the last two years. The totals tagged each year roughly parallel the numbers recorded in Mexico each winter, giving us an independent assessment of the numbers in the migration. Regional tagging success also helps in that it demonstrates how monarchs respond to the physical conditions and quality of the habitats in these areas. Thus, tagging is an important tool to help us understand the overall dynamics of the monarch population.

So what should we expect this year? I usually like to wait until August to make predictions about the numbers in the migration and avoid bold assertions about the size of the overwintering population until well into the migration. However, this year I’m on record as early as the 3rd of May on our discussion list Dplex-L as predicting that there will be a modest increase in the number of monarchs in the migration and at the overwintering sites this winter. “Modest increase” is a vague term and I can’t put a number to it; however, all of the factors I’ve researched indicate that there will be more monarchs migrating this fall and at the overwintering locations by mid December when the colonies are measured. All in all things are looking up for monarchs this year.

Here is a brief overview of factors supporting my prediction of a modest increase in the number of monarchs this fall:

1. I looked at reports from those visiting the overwintering sites about the mortality seen at the colony locations. I also watched the weather in Mexico to be sure that there were no unusual weather events at the overwintering sites that would contribute to mortality. Fortunately, though the population was low, it seemed to winter well.

2. I followed the first sightings reported to Journey North and monitored, as closely as I could, the conditions of the milkweed and flowering of nectar sources in Texas. Since lower than normal temperatures in Texas in March and early April are also associated with an increase in the population, I monitored the temperatures in the region as well. Again, all the signs pointed to a slight growth in the population.

3. The temperatures during May and early June are another key to population growth. These temperatures largely determine when the first generation monarchs coming out of Texas and Oklahoma will reach the northern breeding area. Past records have shown that the timing of arrival of these monarchs in the north (i.e., >40N) is also critical. First sightings suggested fair numbers arrived during the critical period in the upper Midwest, smaller numbers arrived later in MI, OH and ONT and, still later, even fewer reaching the New England area. All of which suggested that most of the increase would come from the upper Midwest assuming that summer temperatures were normal or above normal.

4. A look at the longer-term temperatures suggested that normal temperatures could be expected over most of the northern breeding area, further supporting my optimistic outlook through May and early June.

5. The reports so far this July have not been disappointing. The 4th of July butterfly counts supported by the North American Butterfly Association, and other counts, have generally reported better numbers of monarchs in their surveys than last year.

Good luck with your tagging and thanks to everyone that participates in our programs!

ADDENDUM: 2 September 2014

Reports from throughout the breeding range indicate an increase in monarch numbers roughly along the lines projected in May. The migration is already underway having started at 50 N around the 12th of August. The leading edge should be in southern MN at this time and in Ames, IA around the 6th of Sept. Fall roosts have been reported to Journey North in the Dakotas, MN, WI, MI and NY as of 28 August. No roosts had been recorded by the 29th of August last year (see Monarch Roosts Fall 2013 and Monarch Roosts Fall 2014). There will surely be more monarchs to tag over the next two months and the overwintering population in Mexico is certain to be larger. At a minimum, I expect the population to be twice as large as last year or roughly 1.4 hectares but it could be twice that size. We still have to hear about monarchs from many areas and the conditions during the migration will likely determine how many of the migrants reach the overwintering sites. It will help to watch the reports of overnight clusters recorded by Journey North and to watch the weather conditions and note the availability of nectar sources as monarchs migrate through the United States and northern Mexico.