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.
“Thank goodness that happened — those winds actually pushed the monarchs past Texas and past a lot of the drought areas faster than they would have otherwise traveled,” she said. “The numbers were actually kind of decent. Last year, we had 52; this year, we had 57.”
The results of Loudoun’s butterfly count will be sent to the North American Butterfly Association. The organization compiles similar data from across the entire country.
“Using their protocol and the same type of count circles across the nation, they’re able to aggregate the findings at that macro level,” Hamilton said. “This year will be particularly interesting to look at, given the massive drought that’s happening in the Midwest.”
Hamilton said she was relieved to see the resilience of the local butterfly populations, despite changing environmental variables and the threat posed by pesticides. Many Loudoun farms spray pesticides to protect their crops, and the chemicals can have the unwanted side effect of killing butterflies and their host plants. Also, the county this year instituted a controversial approach to spraying a pesticide designed to kill ticks, with the aim to prevent the spread of Lyme disease.
“With the farming aspect, that’s such a part of Loudoun’s culture and history, that when we do our count, I think that’s just one of the stable, known quantities,” Hamilton said. “But something that’s new, like spraying for Lyme and ticks, that’s above and beyond.”
Hamilton said that the conservancy is opposed to broad-based spraying but that the localized nature of the county spraying program appeared to minimize its effects on the environmental. A bigger concern, she said, was what might happen if the county actively promotes pesticide spraying to homeowners to help combat Lyme-carrying ticks.
“If homeowners start spraying all of their areas, that would be really bad,” she said. “Especially if it’s encouraging [homeowners associations] to start spraying across communities — that would be really horrible. The insecticide kills all insects, not just ticks.”