More Insects Emerge

by Carl Strang

Phenology, the timing of natural history events, is an easy area of study that adults or schoolchildren can pursue. I have been sharing my phenology observations at Mayslake Forest Preserve for first flowers, first fruits, and spring arrivals of migrant birds. Today I would like to look at results comparing 2009 and 2010 spring first sighting dates of insects. Insects are small and easy to overlook. Their numbers can vary a lot between years. Consequently I am not going to place too much weight on small differences between years for individual species. A case in point is the hobomok skipper.

I have seen few of these at Mayslake. Nevertheless, last year’s first date of 5 June is very close to this year’s June 9. Similarly, the first prince baskettail dragonfly appeared on June 1 last year, June 10 this year.

They seldom land, so I’m resorting to a UFO photo here. The difference in dates for Peck’s skipper is large enough to suggest a brood difference. I saw this one on May 25 this year.

The first date last year was August 24. This represents last year’s second generation, and I did not see a first generation representative. I don’t see many of these butterflies, so this is not a surprising result. I also haven’t seen many lyre-tipped spreadwings

Thus I am not willing to put much emphasis on the between year difference in first sightings of 13 July 2009 vs. 10 June 2010 for this damselfly.

So much for individual species. I was interested in looking for patterns among seasonal groups of species. In March this year I had only 2 species to compare to 2009, with a median difference of 5 days earlier than last year. April numbers were 9 species, median 17 days earlier than 2009. The 12 May species had a similar 16.5 days earlier median. To date in June, 9 species are a median 4 days earlier. These results suggest that the warmer spring this year is producing emergence patterns in insects that are similar both in direction and amount to those of first flower dates in plants.

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