Determinants of clutch size and seed preference in a seed beetle, Stator beali (Coleoptera: Bruchidae)

Many insects lay their eggs in clutches on discrete patches of resource that represent the only food available to progeny. The ability of females to distinguish resource patches of variable size and adjust their clutch size accordingly may enable these insects to reduce sibling conflict and exploit resources optimally. Stator beali Johnson is an unusual seed beetle because it lays eggs in clutches rather than singly. Within a natural population, clutch size varied from 1 egg to >19 eggs on seeds of Texas ebony, Chloroleucon ebano (Berlandier). We examined the heritability of clutch size in S. beali, and then determined the extent to which females can adjust clutch size in response to seed size. We obtained conflicting evidence as to whether clutch size variation is heritable; a standard parent-offspring regression analysis suggested that the heritability of clutch size was not different from 0 (h(2) = 0.10 +/- SEM 0.18), whereas an analysis of full sibs suggested that it was substantially greater than 0 (h(2) = 0.33 +/- 0.12). This discrepancy is likely the result of large nongenetic effects on clutch size. When presented with both a large and a small C. ebano seed, females preferentially laid eggs on the large seed. When forced to lay eggs on seeds of a specific size, females presented with 5 large seeds laid larger clutches (mean +/- SEM, 6.7 +/- 0.4 eggs) than those presented with 5 small seeds (5.2 +/- 0.5 eggs). Females also actively avoided superparasitism in the laboratory when alternative, nonparasitized seeds were available, However, most seeds collected in the field (56%) were superparasitized, and many (15%) had >2 clutches on them, suggesting that seeds are a limited resource for ovipositing females.