Host-associated fitness variation in a seed beetle (Coleoptera: Bruchidae): evidence for local adaptation to a poor quality host

The geographic distributions of many generalist herbivores differ from those of their host plants, such that they experience coarse-grained spatial variation in natural selection on characters influencing adaptation to host plants. Thus, populations differing in host use are expected to differ in their ability to survive and grow on these host plants. We examine host-associated variation in larval performance (survivorship, development time, and adult body weight) and oviposition preference, within and between two populations of Stator limbatus (Coleoptera: Bruchidae) that differ in the hosts available to them in nature. In one population, Acacia greggii (Fabaceae: Mimosoideae) and Cercidium microphyllum (Fabaceae: Caesalpininoideae) are each abundant, while in the second population only C. floridum and C. microphyllum are present. In both populations, egg-to-adult survivorship was less than 50% on C. floridum, while survivorship was greater than 90% on A. greggii. Most of the mortality on C. floridum occurred as larvae were burrowing through the seed coat; very low mortality occurred during penetration of the seed coat of A. greggii. Significant variation was present between populations, and among families (within populations), in survivorship and egg-to-adult development time on C. floridum; beetles restricted to Cercidium in nature, without access to C. floridum, survived better and developed faster on C. floridum than beetles that had access to A. greggii. Large host effects on body size were detected for female offspring: females reared on A. greggii were larger than those reared on C. floridum, whereas male offspring wee approximately the same size regardless of rearing host. Trade-offs between performance on C. floridum and C. floridum were not detected in this experiment. Instead, our data indicate that development time and survivorship on C. floridum may be largely independent of development time and survivorship on A. greggii. Patterns of oviposition preference corresponded to the observed patterns of host suitability: in laboratory preference tests, beetles with access to A. greggii in nature tended to prefer this host more than beetles without access to this host in nature.