“Rare” Butterfly Feeds on Oak Galls and Other Non-Nectar Sources
By Ed Ricciuti
In an era when reports proliferate about species careening towards extinction, field research by two University of Connecticut (UCONN) entomologists has turned up refreshing news: a subspecies of butterfly considered among the rarest in eastern North America may not be so scarce after all. The perceived rarity of the northern oak hairstreak (Satyrium favonius ontario) may be due to the fact that it lives covertly, hidden from the eyes of butterfly watchers, feeding and breeding overhead in the leaves and branches of the forest canopy.
“It is our guess that the adult’s behavior places it outside the typical zone for human detection and consequently that much of its rarity is one of perception,” wrote Benedict Gagliardi and David L. Wagner in a paper published in the Annals of the Entomological Society of America.
The two-year UCONN study of oak-feeding hairstreaks indicates that “adults spend much of their lives in the canopy and that ground-level sightings are not indicative of population numbers.” According to Gagliardi, scientists who have searched for the northern oak hairstreak have looked in the wrong places.
“Most butterfly watchers look around flowers near the ground,” he said. “They usually don’t look up.”
As a result, census takers overlook the butterfly when it plays hide-and-seek in the woodland’s roof.
Diet may reinforce the impression of the butterfly’s rarity. While most butterflies feed on nectar from flowers, near the ground and in plain sight, the researchers propose that the northern oak hairstreak feeds out of view, primarily amid the greenery on non-nectar sources such as oak galls and honeydew from aphids, mealy bugs, treehoppers, scale insects, and whiteflies.
The northern is one of four subspecies of the oak hairstreak that as a species ranges from southeastern Colorado to Michigan east to Massachusetts and south into Florida and Texas. The northern occurs from Georgia north through New England and west through the Great Plains. The subspecies are S. f. favonius, S. f. autolycus, and S. f. violae. The study by the UCONN scientists was undertaken because Massachusetts is at the northeastern margin of the oak hairstreak’s range, and it is listed as a species of special concern under the state’s Endangered Species Act. The researchers were curious whether increasing temperatures due to climate change might be expanding the butterfly’s territory but found no evidence one way or another.
The probability that northern oak hairstreaks prefer the canopy was inferred largely from observations of other species of hairstreaks that are close kin to the oak hairstreak. During their field research in the summers of 2013 and 2014, the researchers observed that four other Satyrium hairstreaks fed on the sugary goo that exudes from pea-size oak pip galls of the miniscule cynipid wasp Callirhytis balanacea. The observation is the first ever recorded of a New World hairstreak feeding at cynipid gall exudates. The hairstreaks may turn to flowers only when the normal sources are unavailable; when heavy rain washes away honeydew, for example.
If indeed the northern oak hairstreak is more common than thought, the authors suggest that conservation agencies seeking legal protection for the species should do so “guardedly.” In other words, they advise conservationists not to cry wolf if not warranted.
The study occurred in the pine-oak woodlands of Great Blue Hill, a 635-foot-high monadnock — an isolated, steep-sloped rocky hill or knob rising from flatland — only 10 miles from Boston, in Canton and Milton. The hill, a spot that draws butterfly watchers in droves, rises from a 7,000-acre state reservation and is the only location in New England where the northern oak hairstreak can be found in most years.
Soils on the flanks of Great Blue Hill, steep enough for ski runs on its northwestern slope, are so shallow, especially near the summit, that tree growth is limited. Near the top, bear or scrub oak (Quercus ilicifolia) usually stand less than about a dozen feet high. Some mature white oaks (Quercus alba) reach heights of only about 30 to 45 feet.
Each hairstreak species observed there favors a particular species of oak, vertically stratifying the levels of woodland at which they live, in the fashion of an apartment house. Edwards’ hairstreak, for example, feeds on the low-growing scrub oak, and is found on the first and middle floors of the forest, so to speak. The northern oak hairstreak resides in the penthouse, due to the fact that adults seem to prefer the crowns of post oaks and white oaks, largely hidden from the ground. When the scientists collected larvae on their research site, they discovered that the white oak, particularly, was the host tree for those of the northern subspecies.
The scientists did not spot any adult northerns during the 12 days afield in 2013. The next year they saw two adults, a female and a male. Both were first seen near the ground but then ascended into the canopy, as did adults that had been reared in the laboratory and then released. Edwards’ and banded hairstreaks, on the other hand, remained near the ground when disturbed.
Based on the location of the larvae and behavior of the adults, the scientists concluded that northern oak hairstreaks live mostly high up in white oaks. Given that these oaks and most other trees in Northeastern forests rely on the wind rather than nectar-feeding insects to pollinate their flowers, the researchers believe that hairstreaks rely on other sources of food. Most of the trees do not even bloom when hairstreaks are active.
The UCONN research has implications that go beyond the hairstreaks and the Northeastern forests. Honeydew from species such as mealy bugs, leafhoppers, treehoppers, scale insects, and whiteflies may be underappreciated in forests, barrens, deserts, and other communities in which nectar sources are few and far between.
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Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.