Combined totals for all four years: Eastern tiger swallowtail (3,122), eastern tailed blue (869), swallowtail family (species undetermined, 804), cabbage white (774), silver-spotted skipper (718), silvery checkerspot (496), spicebush swallowtail (346), great spangled fritillary (324), clouded sulphur (278) and orange sulphur (226).
In the Top 10 for all four years: Eastern tiger swallowtail, cabbage white, great spangled fritillary, silver-spotted skipper and swallowtail family.
New to the list this year: The least skipper (increased tenfold over last year, to 55)
After a steady rise in the total number of butterflies recorded during the Rappahannock butterfly counts, and record numbers recorded for two species, numbers fell dramatically for some species this year.
Old Rag Master Naturalists started the local count — officially listed as the “Washington, Virginia” count — in 2011 as part of the annual North American Butterfly Association (NABA) count held across the country each year in the summer. Last week, ORMN finished tallying the results for the local count, held on July 19, and count coordinator Robin Williams shared them with me.
There are several things to keep in mind when looking at the statistics: Although every year tells us more about the health of butterfly populations, four years is not enough to clearly delineate trends. Furthermore, factors in a particular year can cause temporary fluctuations that don’t portend a trend.
With these caveats in mind, we can still see some interesting patterns that could spell trends or just short-term booms or busts, which are typical in populations of such short-lived animals. The total number of butterflies had basically been doubling each year of the count, from 1,254 in 2011, to 2,380 in 2012, to 4,798 last year.
Last year’s numbers likely would have been even bigger if it hadn’t rained in the afternoon, Robin says, since butterflies tend to hole up in such weather. (See my column last Aug. 22 for more on the 2013 count.) This year, the total number fell dramatically — to 1,291, or about 25 percent of 2013.
However, on closer inspection, one butterfly, the eastern tiger swallowtail (ETS), is primarily responsible for the dramatic fluctuation in the numbers here. Its numbers had shot up in the last couple of years, from 102 in 2011 to 615 in 2012 and a remarkable 2,375 in 2013 — half of the total number of butterflies counted here and the highest number of ETS ever recorded since the NABA count was initiated in 1975 (then under the auspices of the Xerces Society). The number of ETS recorded here in 2012 was also higher than for any of the other 450 NABA counts that year.
The actual number of ETS spotted last year was probably even higher than recorded because of an interesting phenomenon exhibited by this species. Some butterflies, especially when they’re on the wing, are hard to tell apart, particularly within a family. Swallowtails, with their distinctive shape and large size, are generally among the easiest to identify here.
And within the family, the bright yellow-and-black coloring of males and some females make the ETS easy to distinguish from spicebush, pipevine and black swallowtails, which are black with mostly blue markings. However, the female ETS can also take on a dark form with similar markings to these other three species. Such a dramatic difference in coloration between genders is unusual with butterflies.
If the exact species cannot be determined, but its family can, a butterfly is recorded under its family name. Last year, 607 butterflies were determined to be swallowtails, but the species was unconfirmed, and Robin said these were the dark swallowtails. With the high number of butterflies that were identified as ETS, many among the 607 are likely the dark-form ETS female.
According to Robin, other nearby counts, including those in Madison, Fauquier and Shenandoah National Park (SNP), which includes part of Luray, showed similar crashes in some butterfly species, particularly ETS. The ETS was deposed here this year by the tiny eastern tailed blue, with 243 recorded. However, when totals are combined for all four years of the Rappahannock count, the ETS still reigns supreme, at 3,122, while the eastern tailed blue comes in second, at 869.
The little silvery checkerspot’s numbers fluctuated in line with the ETS over the four years, falling to 13 this year, from a peak of 366 in 2013, the highest number reported for that species in any NABA count that year. Some other species rose and fell in a pattern similar to, but less dynamic than, the ETS and silvery checkerspot, including the cabbage white, silver-spotted skipper, spicebush swallowtail, black swallowtail and red-spotted purple. However, most butterfly populations showed only minor fluctuations over the four years. A few species, such as the cloudless sulphur, have shown a small but steady decline.
The number of monarch butterflies, which are of concern internationally for their precipitous decline (see my July 3 column), has fluctuated slightly in the Rappahannock counts, but overall remained low. Four were counted this year, the average for the four years. Robin says the SNP count’s monarch numbers are generally higher (34 this year) and with more fluctuation than in Rappahannock.
So what accounts for the crash in numbers for some species here this year? Factors affecting butterfly numbers are complex and interrelated. They include weather, timing of breeding, availability of host plants, mortality from external factors (such as pesticides and herbicides) and fluctuations in parasite or predator populations.
By all accounts, the cool, late spring likely had a big effect this year. Numbers for a particular count could be down because of the day’s weather; the various counts in our area are held on different days, which gives more weight to trends that occur across them.
On the optimistic side, Robin points out that the low numbers during the count could just mean a delay in breeding. The ETS and spicebush swallowtail numbers seem to be picking up now, and the least skipper showed up in great numbers this year, making the top 10 list for the Rappahannock count for the first time.
There are also “healthy populations” of cabbage whites, clouded and orange sulphurs and silver-spotted skippers, Robin says, and some other skipper species, as well as the tiny sachems, are now “coming out in force.”
Some species, such as the great spangled fritillary and the clouded sulphur, have shown steady, if small, increases over the last four years. And while species may come and go here, the overall number recorded has remained about the same (around 40, with a spike to 50 in 2012). The SNP count has had similar numbers, Robin says, but the spike there was in 2011.
With no baseline before 2011 to give us a clearer picture of changes in butterfly populations within Rappahannock, we can only compare the data we have now to nearby counts and continue to monitor this important pollinator going forward.
Look for more on how butterfly populations are doing and what affects their rise and fall in an upcoming column. To see more photos and other butterflies that made the top 10 list in the Rappahannock count, go to the “Wild Ideas” page on rappnews.com.
© 2014 Pam Owen