Thorne's hairstreak butterfly (Callophrys gryneus thornei) occupies perhaps the smallest range of any butterfly within the continental United States. Thorne's hairstreak is a monophagous butterfly, apparently feeding exclusively on foliage of the endangered Tecate cypress (Callitropsis forbesii) as larvae. Tecate cypress is found in a very limited distribution in southern California, with valuable stands occurring within Otay Mountain Wilderness. Previous reports suggested that Thorne's hairstreak larvae require foliage from mature Tecate cypress on which to feed. However, recent large fire events on Otay Mountain have reduced the mature stands of Tecate cypress dramatically, and this study was conducted to determine whether Throne's hairstreak butterflies would oviposit on and whether larvae could feed successfully on foliage from Tecate cypress that were not reproductively mature, or that were growing in stands that were not comprised of fully mature trees.
Extensive field studies in spring 2008 documented the occurrence and distribution of Thorne's haristreak adults, eggs, and larvae on “young” cypress, and limited laboratory feeding studies were conducted to determine larval feeding behavior on foliage from “young” and “mature” cypress. Results supported the early findings by Fred Thorn (reported by Brown) indicating that Thorne's hairstreak may not be as obligately tied to fully mature cypress as was thought. Females laid eggs on young trees and larvae were found feeding on young trees. Positions of eggs on foliage of both young and mature cypress trees were very similar. In laboratory studies, larvae appeared to have no preference for foliage from mature trees and grew successfully on diets consisting of foliage from young trees. Conservation of the remaining few stands of Tecate cypress and the habitat remains critical, and these results suggest that stands of young cypress need careful management as well as mature trees and stands. In addition, these results may offer new opportunities for management and recovery planning for Thorne's hairstreak populations.