The evolutionary ecology of generalization: among-year variation in host plant use and offspring survival in a butterfly

The majority of phytophagous insects are relatively specialized in their food habits, and specialization in resource use is expected to be favored by selection in most scenarios. Ecological generalization is less common and less well understood, but it should be selected for by (1) rarity of resources, (2) resource inconstancy, or (3) unreliability of resource quality. Here, we test these predictions by studying egg distribution and offspring survival in the orange tip butterfly, Anthocharis cardamines, on different host plants in Sweden over a five-year period. A total of 3800 eggs were laid on 16 of the 18 crucifers available at the field site during the five years. Three main factors explained host plant generalization: (1) a rarity of food resources in which the female encounter rate of individual crucifer plants was low and within-year phenological succession of flowering periods of the different crucifers meant that individual species were suitable for oviposition only within a short time window, which translates to a low effective abundance of individual crucifer species as experienced by females searching for host plants, making specialization on a single crucifer species unprofitable; (2) variation in food resources in which among-year variation in availability of any one host plant species was high; and (3) larval survivorship varied unpredictably among years on all host plants, thereby necessitating a bet-hedging strategy and use of several different host plants. Unpredictable larval survival was caused by variation in plant stand habitat characteristics, which meant that drowning and death from starvation affected different crucifers differently, and by parasitism, which varied by host plant and year. Hence, our findings are in agreement with the theoretical explanation of ecological generalization above, helping to explain why A. cardamines is a generalist throughout its range with respect to genera within the Cruciferae.