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Click on graph (at right) for larger view.Recently, we have looked at our volunteers’ monitoring data and have been doing some analyses of the Upper Midwestern monarch population. Above is a graph that shows a compilation of data - eggs per milkweed plant observed – throughout the upper Midwest. The data represented in the graph combine all sites in that region. The height of the bar is the proportion of plants occupied by a monarch egg on the peak July week for each year (eggs per milkweed) from 1996-2009. The data for 2009 are incomplete, since not all volunteers have reported in yet.
Overall, it's hard to see a clear trend - certainly the last few years have been below average (the average is 0.13 eggs per milkweed), and we haven't had a "big year" for a long time. Note that in both 2002 and 2004 there were freezes at the two largest monarch overwintering sites in Mexico, causing the deaths of tens of millions of monarchs. We can see the dips in the monarch population during those years on the graph.
Dina Kountoupes and Karen Oberhauser
MLMP
University of Minnesota
1 comments:
I was just checking out the chart on mlmp blog site, which raises some questions:
The chart shows just over .5 monarchs per plant in distribution mid-July in Minnesota. I understand that your milkweed plants there are much larger than those down here, but at the same time your populations are much higher.
So I'm wondering why we so frequently see many multiples of eggs per plant? Is it just because we don't have vast fields of milkweed? Is there some other factor at work? We regularly see (both spring and fall generations) 5-7 or even 15-17 eggs on relatively small plants (5-6 ramettes, 8-12" in height).
Is this a behaviour change? Environmental response? Are old generation monarchs dumping eggs? What would cause the same behaviour in the September breeding population down here? Is there a difference between egg laying here as compared to up 'there?'
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