Date: 09/12/2009

Number: 1

Many people have written from Vermont to ask what might have caused the low number of monarch sightings across the state this year. When asked what causes the monarch population to fluctuate, Dr. Chip Taylor of Monarch Watch likes to refer to his "Three Bears Theory." Everything needs to be "just right" for the population to do well, he explains.

Let's look at a "typical" year in Vermont, then consider some of the factors that might have made this year different.

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.

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.

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 wonder if monarchs
had trouble finding mates when their numbers were so low.)

Luckily, 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 population. The graph below shows the results since winter 1994/1995.

Norwich, VT

Latitude: 43.8 Longitude: -72.3

Observed by:
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