Weather again dampens this year’s butterfly count
After having some remarkable results for at least one species in the 2013 and 2014 Washington-Rappahannock annual butterfly counts, this year’s count was pretty much drama free, but the weather did seem to depress butterfly numbers.
Held on July 18, the 2015 count was managed by Old Rag Master Naturalists (ORMN) and held in conjunction with the North American Butterfly Association’s annual Fourth of July count, as in the previous four years.
Rain, depending on the location, was intermittent during the count. Despite the weather, a record 59 official adult counters and six children showed up to help, said ORMN member Caroline Watts, one of the count’s coordinators, in a recent email. Among the volunteers were 37 chapter members, most serving as team leaders, coordinators, scribes (who record the numbers) or photographers.
“We were really pleased with the turnout and enthusiasm,” Watts said.
The butterfly turnout, on the other hand, was “lower than we hoped, because rain was drizzling for much of the morning,” she added. “But our species count of 42 was in line with even our most abundant count year in 2013.”
While the total number of butterflies this year was almost double that of last year (2,123 versus 1,291) and some numbers went up or down significantly from last year, most are not too far out of line with the averages over the previous four years.
As I reported in my Aug. 14, 2014 column, one species stood out in the 2013 and 2014 counts, the eastern tiger swallowtail (ETS), which had a big impact on the totals. In 2013, an amazing 2,375 of the species were counted. Not only was that a huge increase from the two previous years (102 in 2011 and 615 in 2012), it was the highest recorded in any year, at any count location, since the start of the NABA international count began in 1975, originally under the auspices of the Xerces Society.
In fact, probably many more ETSs were out during the 2013 count than were actually recorded as such. Females of the species can have a dark form, which makes them hard to distinguish from the spicebush, black and pipevine swallowtails, especially from a distance. In 2013, 607 swallowtails were recorded as just “swallowtail spp.,” meaning an undetermined swallowtail species.
The huge 2013 boom of the ETS was remarkable enough, but last year brought an equally remarkable bust, with only 30 counted. Even with the possibility that some dark-morph females had been lumped into the swallowtail spp. category (down to 16 last year), the number was still incredibly low. While rainy weather did seem to dampen numbers pretty much across species last year, it is unlikely to account for such a low number.
The results for other nearby counts (Luray-Shenandoah and Fauquier) showed similar ETS booms and busts from 2013 to 2014, but their highs and lows did not come close to matching ours. The spicebush, which is also quite common, black and pipevine swallowtails also rose then fell a bit those years, but not to such a huge extent.
In doing research, including talking to a butterfly specialist at the Smithsonian, after last year’s count, I couldn’t find a good reason for such a huge boom and bust for the ETS. Even factoring in weather, possible changes in predator populations, and other variables, the exact cause (or causes) of the boom and bust remain a mystery. Some wasps are known to parasitize caterpillars, but they don’t seem to particularly target or avoid the ETS.
The good news is that ETS numbers rebounded considerably this year from last year’s, with 675 counted, putting the species first in the list of top 10 species in terms of numbers, as it has been for every year except last year. The other nine in the list are the silver-spotted skipper (263), eastern tailed-blue (175, bumping it down from last year’s first-place spot), pearl crescent (156), cabbage white (116), spicebush swal- lowtail (109), great spangled fritillary (91), clouded sulphur (51), least skipper (39) and orange sulphur (35). All but the least skipper were in the top 10 for at least three years of the count. Species that were in the top 10 other years but not this year are the sachem, silvery checkerspot, variegated fritillary, cloudless sulphur, black swallowtail and common buckeye.
The five species with the highest totals across all five years are the ETS (3,797), eastern tailed-blue (1,044), silver-spotted skipper (981), cabbage white (890) and silvery checkerspot (509), with the spicebush swallowtail not far behind.
Totals for all species combined for 2011-15 are, respectively, 1,258, 2,380, 4,798, 1,291 and 2,123. Taking out the ETS and the swallowtail spp. category numbers, the total for 2011-15 were 1,101, 1,635, 1,816, 1,245 and 1,356. These average out to 1,449 for 2011-14; adding in the 2015 total only drops the average down to 1,431, indicating the large role the swallowtails, particularly the ETS, play.
Monitoring butterflies and looking for trends are important in monitoring the health of the environment. Not only are butterflies important in the food web, serving as pollinators and as prey for other species, but the health of their populations can reflect the health of our ecosystems. However, any one-day count has enough variables that trying to discern any trends with just five years of data should really only be considered a heuristic exercise, to raise more questions for further study. The count is just a snapshot on one day in the summer. Butterfly experts hesitate to talk about trends without at least 10 years of data to analyze — and more data is always better.
Variables that can affect the numbers and species of butterflies that show up on the day of the count include weather, breeding and migration timing of specific species, number of generations typically produced by a particular species, and numbers and timing of other prey and predator species.
Keeping all that in mind in looking at the data so far, no particular trend seems to emerge beyond what might be considered the ups and downs common with butterfly populations. And who knows what’s going on with the ETS? With another five years of data, we might find that this species tends to have big booms and busts occasionally here, for whatever reason — or that the 2013 and 2014 numbers are truly outliers.