Distinguishing between Larvae of the Imperiled Callophrys irus (Godart, 1824) (Family: Lycaenidae) and Strymon melinus (Family: Lycaenidae)

Larval surveys can be an important tool in butterfly studies serving many purposes including determining species presence. Current survey recommendations for the imperiled frosted elfin Callophrys irus (Godart, 1824) (Family: Lycaenidae) include use of larval surveys for detecting species presence. But similar-appearing larvae of the gray hairstreak Strymon melinus (Hubner, 1818) (Family: Lycaenidae) also occur on C. irus host plants, which if misidentified as C. irus can result in false-positive detections. We reared C. irus and S. melinus larvae (n = 61 and n = 16 respectively) and assessed three characteristics of late instar larvae for distinguishing between them in the field. Larval body coloration overlapped for both species (pale green and pale pink) though C. irus was typically green while S. melinus was typically pale pink or red. Cranial pigmentation was also a helpful though imperfect distinguishing characteristic accurately identifying 65 (94.20%) of 69 larvae. The ratio of cranial width to distance between the sensory setae insertions on the prothoracic shield was a definitive distinguishing characteristic with C. irus having ratios <2.20 and S. melinus having ratios >2.20. We recommend observers determine the presence of C. irus at a given site based on multiple larvae (2-3 individuals minimum) if using a combination of body color and cranial pigmentation, or on one individual larva if using ratio measurements. Both larval body color and cranial pigmentation can be determined in the field; the former with the naked eye and the latter with a 60x hand lens. Ratio measurements require use of a dissecting microscope ex-situ.