If typical host plants are absent, some herbivorous insects “dump” eggs on unsuitable substrates, even though this can cause complete larval mortality and reduced maternal life span. In the seed beetle Callosobruchus maculatus (F.) (Coleoptera: Chrysomelidae: Bruchinae), the tendency to dump eggs varies genetically both within and between populations. A previous study hypothesized that egg-dumping behavior facilitates host-range expansions, and suggested that such expansions have served to maintain “dumper” genotypes in beetle populations. We tested this hypothesis in two experiments. For both African and Asian beetle populations, full-sib females from >60 families were split among three treatments: no seeds, a less-preferred novel host, and a preferred host. Within each population, we found no correlation among families between the tendency to dump eggs and the tendency to accept the novel host. We also compared egg dumping between selection lines that had adapted to a novel host and a line that had remained on the ancestral host. Females from lines that had evolved greater acceptance of the novel host did not dump more eggs if hosts were absent. Thus, neither experiment supported the host-range expansion hypothesis. Egg distributions on the preferred host in the first experiment provided weak support for a more proximate explanation: family-level variation in the tendency to dump eggs is inversely related to the tendency to avoid superparasitism of seeds. Such a relationship is also evident in comparisons between populations. Given the considerable short-term costs of egg dumping, we suggest that the host-range expansion hypothesis requires unrealistically high frequencies of host deprivation and subsequent host shifts in C. maculatus.
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