October 26, 1999
A total of 34 species of butterflies were observed on the Fermilab site during the summer of 1999. Four of these species (Black Dash Skipper, Eyed Brown, Bronze Copper, and Purplish Copper) are considered to be remnant-dependent and another seven species are remnant-associated. All of the remnant-dependent and most of the remnant-associated butterflies seen this year at Fermilab were wetland species. Perhaps there were enough wetland remnants at the time of the founding of Fermilab to "re-seed" with butterflies the new wetlands that have formed on site. This appears not to have happened for prairie species. Although the prairie restorations now total many hundreds of acres, no really prairie-dependent butterfly species were seen.
Banded Hairstreaks are found at the edge of an old, mature oak woods (ELM-24) in a route accessible to the public that includes the "Interpretive Trail" through a newer prairie restoration (in ELM-25). The woods edge and savanna also included numerous Great Spangled Fritillaries. It is a good place for spotting Giant Swallowtails; I have also seen them there in previous years. The Pipevine Swallowtails in ELM-25 last June were probably some kind of anomaly. The nearest population is probably in Waterfall Glen Forest Preserve, near Argonne Lab. (Strangely, that same day in June, I saw a Pipevine Swallowtail in my back yard in St. Charles. Perhaps someone released a bunch of them.)
A couple of small, wandering Pieridae, Little Yellow (Eurema lisa) and Dwarf Yellow (Nathalis iole) are known to gradually disperse to the north through the summer, but then die back during winter since they cannot survive the winter this far north. Although they are not permanent residents here and tell us little about the site, they are rather unusual and were nice to find. Some other butterfly migrants, Buckeye and Fiery Skipper (Hylephila phyleus), were unusually common this year and may have benefited from a relatively disturbed areas. The Purplish Coppers were usually flying in patches of Polygonum coccineum. It is likely that Polygonum coccineum is serving as a larval host plant for the Purplish Coppers.
A sedge meadow inside the Main Ring circle (in ELM-1 near ELM-3, not accessible to the public) is home to Black Dash Skippers and Eyed Browns. Larvae of both species feed exclusively on sedges.
Perhaps the most exciting finds for me were the Coppers, both Bronze and Purplish. I had seen Bronze Coppers at the intersection of Feldott Rd. and Main Ring Rd. in June, and also observed them near A2 on the Main Ring last year (1998). I had not seen Purplish Coppers here before. They seemed to be a bit more restricted to certain pond edges, and they are not quite as conspicuous in flight. It was only after chancing upon one Purplish Copper near some sandbar willows where I was looking for Cecropia and Polyphemus moth caterpillars in late August that I started searching similar pond-edge areas for Purplish Coppers.
In the butterfly list, the last column shows the number of northern Illinois butterfly monitoring network sites reporting each species. The fact that the Purplish Copper is only found at two out of the 29 sites monitored combined with its being very local (as opposed to wandering like the Little Yellow) make it an exciting find. The Purplish Copper is more common west of Illinois; we are on the eastern end of its range. The Bronze Copper and Purplish Copper are two of the 27 North American butterfly species "that have declined over at least part of their range in the past 25 years,"1 due to habitat destruction. References.
1. "Butterflies through Binoculars, the East," by Jeffrey Glassberg, Oxford University Press, New York, 1999.
Butterfly Data - Peak Count Data for 1999 - Map of Fermilab