Agricultural and Forest Entomology (2010), 12, 353–362 DOI: 10.1111/j.1461-9563.2010.00484.x Biotypes of the seed beetle Callosobruchus maculatus have differing effects on the germination and growth of their legume hosts Charles W. Fox, Marsha L. Bush and Frank J. Messina* Department of Entomology, S-225 Agricultural Science Center North, University of Kentucky, Lexington, KY 40546-0091, U.S.A. and * Department of Biology, Utah State University, 5305 Old Main Hill, Logan, UT 84322-5305, U.S.A. Abstract 1 Populations of the seed beetle Callosobruchus maculatus (F.) exhibit considerable differences in body size and larval behaviour. We examined whether such variation modifies the relationship between beetle infestation and host plant performance. 2 Larvae from African and Asian biotypes were reared in seeds of four hosts that represented an almost four-fold variation in seed mass. We estimated mass lost to larval consumption, and compared germination rates and seedling growth between infested and control seeds. 3 In seeds bearing a single larva, the larger-bodied, contest-competing larvae of the Asian biotype caused a 38–47% greater reduction in seed mass compared with the smaller-bodied, scramble-competing larvae of the African biotype. The amount of seed mass lost per larva remained similar in seeds with one or two scramble- competing larvae but decreased significantly in seeds bearing two contest-competing Asian larvae. 4 Differences in larval consumption and behaviour produced striking differences in the frequency of germination. Germination of singly-infested mung bean (i.e. the smallest host) was 71% for African-infested seeds versus 11% for Asian-infested seeds. In cowpea (i.e. the largest host), 76% of Asian-infested seeds germinated, whereas the germination rate of African-infested cowpeas (92%) was similar to that of uninfested seeds. 5 Effects of beetle origin persisted after germination. Seedlings derived from Asian- infested seeds had greater cotyledon damage 7 days after germination, and displayed lower height and less biomass 15 days after germination. Cotyledon damage was a good predictor of seedling performance (i.e. better than seed mass consumed) 15 days after germination. 6 Previous studies have suggested that population differences in larval size and burrowing behaviour (‘centripetal tendency’) reflect adaptation to different-sized seeds. The present study demonstrates that these differences in turn influence the impact of larval feeding on host viability. Strong biotypic variation makes it difficult to generalize about pest impacts at the level of pest species. Keywords Callosobruchus, germination, grain legume, larval competition, seedling growth, Vigna spp. Introduction Many insects have greatly expanded their geographic ranges as a result of human activity (Malacrida et al., 2007; Smith et al., Correspondence: Charles Fox. Tel: +1 859 257 7474; fax: +1 859 323 1120; e-mail: fox@uky.edu 2007; Kenis et al., 2009). If the distribution of such species is discontinuous and gene flow is sufficiently reduced (Garant et al., 2007), introduced populations may diverge genetically from their ancestral population, and from each other, as a result of genetic drift or local selection (Phillips et al., 2008; Ahern et al., 2009; Diamantidis et al., 2009). In pest species, genetic differentiation is sometimes recognized by the formation of  2010 The Authors Agricultural and Forest Entomology  2010 The Royal Entomological Society