Monarch Butterflies

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Many teachers from Vermont have written to ask what might have caused the monarch popuation to be so low across our state this year. Based on the low number of observations reported to our website from New England and southern Canada—last spring, this summer, and fall—the monarch population appears to be unusally meager in 2009. Why?

Dr. Taylor's Three Bears Hypothesis
When asked what causes the monarch population to fluctuate, Dr. Chip Taylor of Monarch Watch likes to refer to his "Three Bears Hypothesis" which states:

• Everything needs to be "just right" for the monarch population to do well.

Let's look at a "typical" year in Vermont, then consider some of the factors that might have made this year different. (These conditions apply beyond Vermont, to much of the northeastern extent of the monarch's breeding range.)

The "Typical" Year
In a typical year, the monarch population builds during the summer months as each new generation appears. For example, monarchs aren't usually very noticeable in Vermont when they first arrive in June. Monarchs begin to appear suddenly in July; that's when "Vermont's" first generation emerges (the offspring of the butterflies that reached Vermont in June). By August, a 2nd new generation appears; this causes monarch numbers to be at their highest in August and we see the greatest number of monarchs then.

The 2009 Breeding Season
This year's cool, rainy summer across the Northeast is probably a major factor behind the low numbers. How would it matter? First, the cool temperatures slowed the spring migration. The monarchs arrived late in Vermont and in small numbers. Second, the cool temperatures slowed the development of the first generation. The life cycle takes longer when temperatures are cool. For example, let's say it typically takes a month for the monarch to develop from egg to caterpillar to chrysalis to adult. The life cycle could take 6-7+ weeks when it's cool. Thus, this spring's late arrival and slow development could mean that the first generation of offspring didn't even emerge until late July or early August--perhaps a month late. Thus, the population may have missed a full generation, and the assocated building of numbers.

Other Factors?
Let's think about other factors: if it takes a long time for the young to develop, it means they are exposed to dangers (like mowing, disease and/or predation) over a longer period of time. There are countless variables and possibilities to consider--the condition of milkweed, the size of monarch predator populations, etc. (You might even wonder if monarchs had trouble finding mates when their numbers were so low!)

How Are Monarchs Doing Elsewhere?
Fortunately, different regions across North American tend to have different conditions each year, so a low population in one region does not necessarily mean the overall size of the population will be low.

At the end of every breeding season, when the monarchs have reached the overwintering sites in Mexico, scientists make an annual estimate of the overall size of the eastern population. This graph shows the results since winter 1994/1995.