Consequences of larval tapeworm infection for the fitness of the intermediate hosts, flour beetles (Tribolium spp.)

Abstract

The problem of the effect of parasites on the fitness of their hosts is central to understanding the ecological and evolutionary consequences of parasitism. This study examined the consequences of infection with a tapeworm parasite, Hymenolepis diminuta, on the fitness of its intermediate hosts, Tribolium castaneum and T. confusum, and the correlation 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, this fitness measurement estimated the overall effects on various fitness components affected by parasites, except the beetles' susceptibility to predation by definitive hosts or to parasitism by other parasites. Three different methods of infection were used to produce various levels of parasite intensity. Overall, the tapeworm infection significantly 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 infected T. castaneum was probably associated with its greater susceptibility to the parasite. The negative correlation between beetle fitness and 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 influence competition between beetle species in ways that might benefit T. confusum. The evolution of parasite virulence in intermediate hosts is discussed.