Determinants of Host Range In Bruchid Beetles

Abstract

We asked what factors keep the seed beetles Stator limbatus and S. pruininus (Bruchidae) restricted to different species of Acacia (Leguminosae) in sympatry. Factors assessed were plant microenvironment, interspecific competition, suitably of seeds for food, and enemy impact. Females selectively oviposited on seeds of their normal host when given a choice of acacia seeds in their normal host microenvironment. This result occurred whether seeds of the other acacia were offered with or without potential competitors in them. Host suitability was determined by comparing full—siblings on both acacias. Three performance variables were measured: (1) emergence per egg laid, (2) time to emergence, and (3) size of females. Stator limbatus performed best on its normal host in all three variables measured, while S. pruininus performed best on its normal host in emergence time but showed no difference in percent emergence or female size. We also assessed the impact of the egg parasite Uscana semifumipennis (Trichogrammatidae) on the two species of bruchids when the bruchids occurred in their normal hosts. Uscana attacked both bruchids at the same level (20% of eggs parasitized). Thus, the proximate determinant of locality restricted use of hosts is oviposition behavior, which we suggest relates best to differences in the suitability of hosts for larvae.