This year, the group spotted 3,475 butterflies, representing 49 species, she said.
“That number is pretty much on par with what we’ve seen in past years,” Hamilton said. “We’ve had some years where we’ve seen in excess of 5,000 [individual butterflies], but our maximum number of species has only ever been 55, so we’re pretty much on target with what we’d usually see in an average year.”
The conservancy was particularly interested in whether there might be any indication that the mild winter had an effect on the local butterfly populations, and some surprises were revealed in the count, Hamilton said.
Certain species, including the Cloudless Sulphur, the Eastern Tailed Blue butterfly and the Variegated Fritillary, had particularly high numbers.
“The Eastern Tailed Blue butterflies, we had the highest number we’ve ever seen,” Hamilton said. In past years, butterfly counters usually spot only about 200 of the species. This year, the number more than doubled, to more than 400.
The Variegated Fritillary had an even more dramatic jump, Hamilton said, from 63 last year to 216 this time.
Such dramatic shifts in a butterfly species population can be a powerful indicator of other changes in the environment, Hamilton said, and often reflect the health of the butterflies’ host plants and nectar sources, as well as changes in weather patterns.
Butterflies have different “overwintering” strategies, Hamilton said. Some live out the cold months in a chrysalis, while others are still in eggs or in caterpillar form. Other species, such as the famed monarch butterfly, will migrate to a warmer climate.
“They require their host plants and their nectar plants,” Hamilton said. “When you look at climate change, when you have something like this drought in the Midwest, there’s a huge impact on the plants. Then you look at the butterflies, and they’re an indicator of how much the drought is affecting the environment.”
The conservancy conducts the count within the same area every year, to minimize variables that could affect results. Other butterfly counts take place across the country at different times in the summer — many in early July — but Loudoun’s butterfly population appears to peak in early August, Hamilton said.
The butterfly counters were also focused on the number of monarch butterflies sighted. Last year, the monarch population was devastated by a drought in Texas during its southern migration, and the population that arrived in Mexico in November was the lowest number ever recorded, Hamilton said.
When the surviving monarchs started their journey north this spring, they benefited from unusually strong winds that helped propel them up into the eastern United States, Hamilton said.