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Variety of factors ‘compacts’ butterfly season, Keiper says
Above, an eastern tiger swallowtail butterfly drinks nectar from a flower. A combination of environmental factors has increased both the number and variety of butterflies in the area this summer, according to Virginia Museum of Natural History Executive Director Dr. Joe Keiper. (Bulletin photo by Mickey Powell)
A combination of environmental factors has increased both the number and variety of butterflies in the area this summer, according to Virginia Museum of Natural History Executive Director Dr. Joe Keiper.
There are two main factors at play in this summer’s explosion of butterflies, Keiper said: the number of butterflies and the variety of butterfly species.
He believes that the cool, wet spring may have played a role in the variety of species seen this summer.
“Bugs are dependent on temperature,” Keiper said, “so cooler temperatures mean slower development.”
For example, he said, zebra swallowtail butterflies normally are seen in the spring, but this spring, he didn’t see any. Around June or July, however, he began seeing zebra swallowtails frequently.
“It suggests to me that something’s happening that changed the timing of the butterflies this year,” he said.
The cool, wet spring also delayed the blooming period for the host plants that the butterflies lay their eggs on and their caterpillars feed on, Keiper said. Butterflies prefer specific plants to lay their eggs on; for example, tiger swallowtails, the Virginia state insect, lay their eggs on poplar trees, and zebra swallowtails lay their eggs on pawpaw trees.
“I think all that led up to a compaction, so to speak,” Keiper said. “You’ve got your summertime or late summer species overlapping with those that normally you only see in the beginning of the year, so you’re almost getting a compressed butterfly season, where everyone’s active during a relatively short period of time.”
However, Keiper said, this explanation only addresses the variety of species, not the number of individual butterflies. While it’s more difficult to pin down exactly why butterflies have been so abundant this year, Keiper has a theory.
Because there has been so much rain, he said, “plants are not stressed, so they’re growing really well. When they’re growing well and they’re focusing all of their effort on growing and producing seeds and therefore reproducing, they might be spending less time producing protective toxins.”
Just about every plant produces some sort of toxin to make it unpalatable to creatures that might want to eat it, Keiper said. As an example, if someone broke open a leaf from a black cherry tree and tasted it, it would taste bitter. That bitterness comes from a trace amount of cyanide that the cherry tree produces. As a result, Keiper said, dairy farmers don’t like to keep cherry trees near their cows because the cows can eat the leaves and become sick.
Similarly, Keiper said, the caterpillars of the familiar orange-and-black monarch butterflies feed only on milkweed, which produces several toxins.
“Very few insects — I only know of three or four — can really overcome that and digest the plant matter and not get sick or die from eating the leaves,” he said. “It might have just been a healthier summer for the (butterflies) because of the quality of food.”
Unfortunately, Keiper said, while most butterfly species are abundant this year, the numbers for one species are down, according to the Butterfly Society of Virginia: monarch butterflies are at a historic low.
“I know that’s something that people are really concerned about,” Keiper said. “For whatever reason, the monarch’s numbers go in this cyclical pattern.” However, he said, the current numbers are abnormally low.
Monarch butterflies migrate to central Mexico to survive the winter, Keiper said. There are certain evergreen forests that they prefer, and they hang in the limbs of the pine trees by the millions. After the winter is over, they migrate back to the United States.
Unfortunately, Keiper said, there has been a substantial amount of illegal logging in the forests where the monarchs winter, which may be one of the causes of their decline, although conservation efforts are being made to protect and re-plant the forests.
“The second thing is,” Keiper said, “their host plant is milkweed. They don’t lay their eggs on anything else. Nothing else in the world will attract the female and stimulate her to drop her eggs, so it’s got to be milkweed. A lot of people are saying right now, and it’s true, that there’s a lot of habitat loss, so a lot of grasslands that might support milkweed have been developed or otherwise utilized.”
Keiper, however, isn’t so sure that this is contributing to the monarch’s decline.
“This summer,” he said, “I’ve traveled around Ohio, Pennsylvania, Virginia and West Virginia, and I’ve looked at patches of milkweed all around. In fact, this past weekend in southwest Pennsylvania, I was looking at a big patch of milkweed. Not a single monarch. Not a single caterpillar. Very little feeding damage. There was just nothing there, and this was a repeating pattern. So my suggestion is that if the lack of milkweed is a problem, then what milkweed is available would be heavily utilized.”
Ultimately, Keiper said, it’s difficult to pinpoint a single cause — or even several causes — for the decline or proliferation of any species.
“There’s still so much about the world we don’t know,” he said. “You can’t just pick one or two explanations and say, ‘Now I understand why the species numbers are declining or increasing or staying the same.’ There are so many factors that can influence their numbers. Mother Nature will take care of things. The numbers will sometimes decline; the numbers will sometimes climb. But generally speaking, as long as you’ve got some healthy ecosystems, they’ll take care of themselves. They don’t need us.”