CONSEQUENCES OF LARVAL TAPEWORM INFECTION FOR THE FITNESS OF THE INTERMEDIATE HOSTS, FLOUR BEETLES (TRIBOLIUM SPP)

The problem of the effect of parasites on the fitness of their hosts i s central to understanding the ecological and evolutionary consequence s of parasitism. This study examined the consequences of infection wit h a tapeworm parasite, Hymenolepis diminuta, on the fitness of its int ermediate hosts, Tribolium castaneum and T. confusum, and the correlat ion between host fitness and infection intensity. Fitness was measured as the proportion of offspring contributed by a focal individual in a test population, using beetle body colors as genetic markers. Thus, t his fitness measurement estimated the overall effects on various fitne ss components affected by parasites, except the beetles' susceptibilit y to predation by definitive hosts or to parasitism by other parasites . Three different methods of infection were used to produce various le vels of parasite intensity. Overall, the tapeworm infection significan tly reduced the beetles' fitness, by 9-16% for T. confusum and 29-32% for T. castaneum, and parasite-induced fitness changes varied between the sexes and infection methods. The greater fitness reduction in infe cted T. castaneum was probably associated with its greater susceptibil ity to the parasite. The negative correlation between beetle fitness a nd infection intensity was statistically significant for T. castaneum but not for T. confusum. The significant between-species variation in parasite pathogenicity suggests that the tapeworm parasite could influ ence competition between beetle species in ways that might benefit T. confusum. The evolution of parasite virulence in intermediate hosts is discussed.