Local distribution factors and sampling effort guidelines for the rare frosted elfin butterfly

Callophrys irus (Frosted Elfin) is threatened under New York State conservation law and has fewer than 5 secure populations within the state. Published research on these populations is needed to support the development of a state recovery plan and monitoring program for the species. We assessed the relationship between adult occupancy (patch use) in the Albany Pine Bush Preserve and a suite of potential controlling factors. We then used the results in a simulation framework to quantitatively inform how many sites and surveys are needed for Frosted Elfin occupancy monitoring. Patch use was best explained by a model that assumed the same occupancy probability for each patch. The species was more likely to use patches with limited shrub cover and greater host plant density, yet showed a good chance (≥76%) of using even the smaller patches (<1 ha) with relatively sparse density (<1000 ramets ha ^-1). Detection probability depended primarily on observer and survey date, ranging from 0.34 to 0.94 among observers and from 0.35 to 0.96 across surveys. In the worst-case scenario (i.e., low detectability and low intrinsic occupancy rate), minimum effort for adult Frosted Elfin occupancy monitoring in habitat similar to the Albany Pine Bush may require at least 20 habitat patches surveyed 6 times each or at least 10 habitat patches surveyed 8 times each. Less effort (e.g., 10 sites × 4 surveys) will likely suffice if surveys are restricted to the period of peak abundance. Adult occupancy (or patch use) is probably the most efficient state variable for monitoring Frosted Elfin populations, and changes in detection-corrected occupancy rate or proportion of area occupied could be useful for conservation planning.