Friday, June 20, 2014

Butterfly Aggregation

Most folks who have spent much time in forest and field have seen aggregations of butterflies. The aggregations are found in damp places, on decaying plant and animal material, on piles of droppings and occasionally in areas that don’t seem to offer anything special and, to human eyes, are no different than many nearby spots. Butterflies appear to aggregate to obtain moisture and/or nutrients from the spot where they’ve gathered. 
In this area the most commonly seen aggregations are of the butterfly with the common name of red-spotted purple (also known as the white admiral); but tiger swallowtails also frequently aggregate.
Riding along a gravel path we came upon an aggregation of what initially appeared to be the drabbest brown butterflies we’d ever seen. 

After looking at them more closely, that description didn’t seem too far off – although they weren’t quite as drab as they first appeared, having some small whitish spots on the wings and a tan margin on the wings’ trailing edges.

At the time we had no idea of the species and it took some searching to confirm that they were either wild indigo duskywings (Erynnis baptisiae) or columbine duskywings (E. lucilius). According to the literature, both of these duskywings have two broods a year and the mature larvae of the second brood overwinter to pupate in early spring. Indigo duskywings feed on wild indigo and crownvetch while the columbine duskywings feed on wild columbine. Neither wild indigo nor crownvetch are found near the spot we found the butterflies. However, wild columbine is common in the area, as is the deciduous forest habitat with rock outcrops that both the plant, and reportedly, the butterfly prefer. 
Columbine duskywing is considered critically imperiled in Pennsylvania, having been found in only two counties – these aggregating butterflies were not in either of those counties. Were these the rare species and did we find a new location for the species? We’ll never know because, not knowing what they might be, I didn’t collect a specimen to be identified by an expert.

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