Monarch Watch Blog

Monarch Population Status

Friday, August 3rd, 2018 at 3:36 am by Chip Taylor
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When it comes to estimating the size of the migration, each year is a series of experiments, with numerous hypotheses, during which I try to match what I know about monarchs with the progression of the seasonal conditions that influence both monarch behavior and plant growth. To make projections for each fall migration and overwintering population, I start with the numbers of monarchs measured at the overwintering sites in Mexico. Next, I focus on overwintering mortality, followed by the spring conditions as monarchs move northward from the overwintering sites to the milkweed areas in south and central Texas, and then the conditions in the South Region (TX, OK, LA, AR, KS) during the growth of the first generation in March and April. That is followed by attention to the conditions during the period from 1 May-9 June that allow, or don’t allow, first generation monarchs to reach the northern breeding grounds. Summer temperatures along with the seasonal distribution and amounts of rainfall are also in focus when estimating the fall and winter numbers. These stage and time specific assessments provide the context for a number of hypotheses or projections concerning the coming migration and the opportunities to tag monarchs each season. Sometimes I’m on the mark and sometimes I’m wrong. The point is to not only give those interested an idea of what to expect but to learn from my mistakes and few successes. Last year, I predicted a large population in the Northeast in general and for Cape May in particular. I was right on the money. However, I underestimated the impact of the drought that ranged from the eastern Dakotas through western Minnesota down through western Iowa. I also overestimated the production of monarchs in the rest of the Upper Midwest with the overall result that the overwintering population of 2.48 hectares was lower than the near 4 hectares I was expecting. These differences were reflected in the number of overnight roosts reported to Journey North through the migration and the relative success of taggers in the East and Midwest. Still, it was a great tagging season.

So, let’s see if I can do better this year. With respect to the Northeast, this should be another good season, although not as good as last year. In Canada, eastern Quebec will be down, but most of Ontario is on track to produce a substantial number of fall monarchs. The counts of monarchs per hour at Cape May will be lower this year, but will still be well above the long-term average. On the positive side, in the Upper Midwest, unless I’ve misjudged the situation once again, the migration should be the strongest since 2008 (a 5-hectare year) with the real possibility that the overwintering population could hit 5 hectares once again. Let’s see if I’m correct.

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