Chrysalis Conundrum: Why One Butterfly Species’ Pupae Fare Better in Winter Than Summer
By Paige Embry
Pupae are the great enigma of the lepidopteran life cycle. Crawling caterpillars and winged adult moths and butterflies are easy to spy, and even the tiny eggs aren’t a huge cipher—look on the plants that the larvae eat—but then there are the pupae.
The pupal phase is that time when the insatiable larvae stop eating and find a place to hide out in a cocoon for the transition to adulthood, which may take a few weeks or many months. It is a time when the lepidopteran can’t move or defend itself, so finding a secret bunker is the primary job for the last instar larvae. They need a place that is likely to protect them from both biotic (predators, parasitoids, disease) and abiotic (flooding, cold) harm.
Moth and butterfly larvae tend to be so good at this job, says Martha Weiss, Ph.D., a biology professor at Georgetown University, that “for a lot of species we don’t even know where they go to pupate, much less how well they survive once they get there.” Weiss’s former doctoral student, Allison Brackley, Ph.D., set out to investigate the pupal survival of one butterfly species, the silverspotted skipper (Epargyreus clarus), as part of her dissertation. The results were published in July in Environmental Entomology.
The silverspotted skipper can be found throughout most of the continental U.S. as well as parts of Mexico and Canada. Weiss explains a little bit about how the skipper lives: As soon as the larvae hatch, they go off and look for a place to make their first house. “They’re very good architects,” she says. “They cut their leaves and lash them down with silk to make little tepees and bungalows and condominiums.” They are also, she says, “fantastic little poop shooters.” They stick their hind ends out of their houses and shoot frass, or insect excrement, out into the world—up to 40 times their body length away from their little abode. (Weiss looked into why they did this and found that the poop was a lure for predators.)
The silverspotted skipper is multivoltine (i.e., having multiple generations per year), with both nondiapausing generations in summer and diapausing generations in winter. The summertime pupal phase lasts only 10 to 14 days, whereas the winter one goes on for months. For the study, Brackley looked at the survival rates of both the summer and winter generations and whether their houses help them survive. Last, she offered up different leaf options for housing material to see if the skippers had a preference.
Brackley raised skippers in the lab. Once the larvae had made their final house and gone inside to pupate, she took them out and put them in leaf litter under host trees. She checked on the summertime cohort every 2 to 3 days and the longer winter cohort every few weeks. She removed some of the skippers from their houses to see how important housing was for pupal survival.
Brackley looked at the relative losses between summer and winter, and the disparity between the seasons was large. Weiss says, “The big surprise is it is much more dangerous in the short summer than in the long winter.” At least 75 percent of the summertime pupae were lost in merely 10 days. Losses of winter pupae in their first 10 days, however, was only 20 percent and didn’t exceed 65 percent despite their months-long exposure. “Even more striking is [that] the daily per capita mortality rate—how likely you are to die on any given day—was an order of magnitude higher in the summer than it was in the winter,” Weiss says.
The scientists thought that vertebrate predators (e.g., rodents and birds) were the primary cause of losses based on evidence left behind: half or all of a pupal case missing, tooth marks on the tag the pupae were glued to. The scientists speculate that the higher losses in the summer were due to seasonal differences in food availability or activity levels and nutritional needs of the predators.
Unhoused larvae were at higher risk than housed ones during the summer, but housing made little difference in the winter, which the scientists attribute to the protective value of fallen leaves. As for housing material, the pupae had a choice between kudzu, black locust (both host plants), and red oak. They showed no preference.
Weiss says few studies have been conducted “that look at the survival of pupae in the field.” But, for conservation purposes, knowing what the pupae need is just as important as providing nectar plants for adults and food plants for larvae. “So, how well the butterfly survives or how well the caterpillar survives—it’s relevant but immaterial if the pupae all die,” says Weiss. “Because its a cycle, and everybody has to get through.”
Paige Embry is a freelance science writer based in Seattle and author of Our Native Bees: North America’s Endangered Pollinators and the Fight to Save Them. Website: www.paigeembry.com.
Editor’s note: The headline of this post was updated September 20, replacing “cocoon” with “chrysalis” to more accurately reflect terminology for lepidopteran pupal-stage biology. As the Florida Museum’s FAQs on butterflies and moths explains, “While pupa can refer to this naked stage in either a butterfly or moth, chrysalis is strictly used for the butterfly pupa. A cocoon is the silk casing that a moth caterpillar spins around it before it turns into a pupa.” Entomology Today regrets the error.