Insect First Appearances

by Carl Strang

Yesterday I detailed the phenology of first flowering dates at Mayslake Forest Preserve. Today I want to share some equally impressive first appearances of insect species there. Most of these observations were made in the warm period in mid-March.

Before this year I never had seen a common green darner earlier than April 2, anywhere in DuPage County. This year the first migrants showed up on March 19, and tandem pairs already were laying eggs in the stream corridor marsh.

One question in the back of my mind relates to the relatively mild but snow-free winter we experienced. Some species may have benefited from the warmer winter, but others may have been set back by the lack of insulating snow. One species that may have benefited is the spring azure. I have never seen so many of those little blue butterflies as I have counted already this year.

This year’s first sighting of a spring azure came 54 days earlier than last year, 23 days earlier than in 2010, and 21 days earlier than in 2009.

So, here are the statistics. Species counts are smaller than for the plants that bloomed in March, at 5-6 species per year. First appearances ranged 16 to 80 days earlier in 2012 than in 2011, with a median of 30.5 days earlier. The range for 2012 vs. 2010 was 17 to 36 days earlier, median 23 days earlier. The range for 2012 vs. 2009 was 21 to 40 days earlier, median 28 days earlier. These medians were similar to those for first flower dates, despite the smaller numbers of species.

Tomorrow I’ll conclude March’s remarkable phenology with migrant bird arrival dates.


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