Background
The Hessel’s hairstreak is a butterfly found in spotty distributions on the Atlantic Coastal Plain from southern Maine to Florida. They are a species of special concern in Massachusetts, with the greatest densities occurring in the southeastern part of the state, including Worcester, Norfolk, and Bristol Counties (Natural heritage). The Hessel’s hairstreak is typically in flight in Massachusetts between May and June ⁴. This species is highly reliant on the Atlantic white cedar (Chamaecyparis thyoides) ³: it lays its eggs on the branch tips, the larvae feed on the new growth, and the pupae overwinter in the white cedar swamps ⁴.

Hessel’s hairstreak is threatened by exotic invasive plants and parasitoids, insecticides, and loss of its limited habitat through harvest, excessive feeding by deer ⁴. This species is also not able to disperse through upland habitat to colonize different Atlantic cedar swamps ⁶.

Climate Impacts
Hessel’s hairstreak is highly reliant on Atlantic cedar swamps that have already seen sharp declines because of human impacts. These forests are considered less vulnerable to climate change firstly because they extend along a broad latitudinal range into regions that already reflect climate conditions that Massachusetts is likely to experience in the future, and secondly because they benefit from wildfire, which is expected to increase under climate change ². However, Atlantic white cedar seed growth and establishment is impacted by surface water, which could be altered under climate change ².

Seasonal changes in timing (phenology) could adversely impact the Hessel’s hairstreak. Butterflies might have particular issues with phenological mismatches in the coming decades. Caterpillars will have to sync their timing with changing food availability. Host plants may be shifting north in response to changing temperatures, with caterpillars potentially responding to different cues. For instance, there is evidence that between the years 1895 and 2009, Lycaenid butterflies in general have been emerging significantly earlier in the spring. Since temperature was the most important factor determining the date of first sighting (index of emergence) for hairstreaks in Massachusetts, this trend is believed to be related to temperature changes ⁵. Moreover, leaf quality may be decreasing, requiring longer feeding times. Larvae could also be affected directly through increasing temperatures and changing moisture availability. Habitat specialists, like this species, are expected to be most vulnerable ¹.