The ninth annual Rappahannock County butterfly count, held last Saturday (July 20) during the recent brutal heat wave, may have broken a national record for one ubiquitous species — the eastern tiger swallowtail. The ETS was designated the Virginia state insect in 1991 for what is increasingly becoming obvious reasons.
The Rappahannock count was established in 2011 by Old Rag Master Naturalists, which continues to manage the count. It’s one of many held across North America as part of the annual North American Butterfly Association (NABA) July 4 count. The local count is coordinated by a committee now headed by ORMN member and Rappahannock resident Jane Smith.
While butterfly numbers overall look good so far, huge numbers for the ETS were apparently reported, according to Jane. “Despite the sweltering heat, this year's butterfly count could possibly result in our largest numbers ever,” she noted in an email to the chapter the day after the count. “Our teams had a combined total number of over 3,000 eastern tiger swallowtails! That number beats the record we've held since 2013 for that species of swallowtail!” The total ETS reported for Rappahannock that year was 2,375, the highest number of this species recorded since the NABA count was initiated in 1975 (then under the auspices of the Xerces Society).
The preliminary numbers for the ETS jibe with anecdotal reports I’ve been getting and with what I’ve been seeing where I live, which is mostly forest. A generalist that uses a variety of trees and shrubs as host plants, the ETS far outnumbers other butterfly species here, although spicebush swallowtails, silver-spotted skippers and great-spangled fritillaries are also numerous.
Getting an exact number for the ETS is always difficult because, while the males have the species’ distinctive yellow coloring with black stripes, many females take a dark form, making them hard to distinguish from other black swallowtails, including spicebush and black swallowtails. The coloration of all these is thought to be a form of mimicry: the other local dark swallowtail, the pipevine, is poisonous, so mimicking it can provide protection from predators.
The easiest way to distinguish the dark-morph ETS female from other dark swallowtails is by the lack of spots on its body, which the other species have. But in the field, butterflies are often on the move, making such details harder to discern. During the count, when species of dark swallowtails cannot be confirmed, they are just recorded as a swallowtail species.
After the committee compiles the count data, which I’ve been told will probably be this week, they will report the official numbers to NABA. And, as in previous years, they will also share the data with me. While some numbers may change slightly, those for the ETS would have to change dramatically to not set a new record for this local count, possibly also beating NABA counts in other areas, too.
The downside to having a high ETS count is that, as with the 2013 boom, its numbers are likely to crash next year, as they did in 2014, when only 30 were counted. In writing about the count, I consulted butterfly experts about the phenomenon and the possible reasons for such booms and busts. With only nine years of data from the count, it’s hard to nail down trends, but I, for one, will be looking forward to seeing the official results for the ETS this year, and next year.
The ETS was not the only butterfly recorded in big numbers on Saturday, according to Jane. Silvery checkerspots, common buckeyes and “a large variety of others were out in droves,” too, she wrote. Robin Williams, one of the count’s founders and leaders, shared some of her thoughts on this year’s preliminary data, noting that silvery checkerspot populations seem to also boom and bust here in the years ETS populations do.
The ETS has two to three broods a year, and “there was no real break” between them this year, Robin writes, with numbers seeming to hold steady since April. She added that she is just now seeing the silvery checkerspots, which sync their breeding to the abundance of the many different composite wildflowers that serve as the host for this species’ larvae, including black-eyed susan (Rudbeckia), sunflowers (Helianthus) and wingstem (Actinomeris alternifolia).
Kudos to all the volunteers who ventured out into the hellish weather on Saturday to help monitor these important pollinators!
© 2019 Pam Owen