D3221: Facultative ant mutualism in the rare hops azure butterfly, Celastrina humulus (Lycaenidae)

Conservation of rare insects relies on understanding how abundance is determined by biotic and abiotic factors. The hops azure butterfly, Celastrina humulus (Lycaenidae) is ranked by NatureServe as a G2 Imperiled species. Its range is restricted to areas containing native hops, Humulus lupulus var. neomexicanus. Hops azure larvae feed on the male inflorescences of hops. Like other lycaenids, hops azure larvae can form a mutualism with several species of ants. Ants protect larvae from natural enemies in exchange for larval secretions. However, past field surveys revealed that roughly one-third of hops azure larvae are not ant tended. Our objective was to assess the factors associated with ant tending. To this end, we monitored larvae on 134 unique bines in 143 patches of the host plant in 2020 and 2021. We then measured a variety of biotic and abiotic factors including light, density of larvae, patch area, and floral phenology. Using a model selection approach, we found that the likelihood of ant tending increased with (1) the density of larvae on the bine, (2) the area of the hops patch, and (3) the phenological stage of the flowers. These results suggest that large patches of the host plant are important both as larval food resources and to support the ant mutualism. Follow up experiments showed phenological matching with host plant flowers was important for larval growth.