Intraspecific competition between healthy and parasitised hosts in a host-parasitoid system: consequences for life-history traits

1. In two different treatments, groups of healthy hosts (Ephestia kuehniella) or hosts parasitised by Venturia canescens competed for a limited amount of food. The larva to adult survival in each group, as a function of the initial number of hosts and treatment, was fitted to the generalised Beverton and Holt and generalised Ricker survival functions, and a number of life-history traits of the parasitoids was measured. 2. Intraspecific competition was scramble-like, and the parasitised hosts were less susceptible to competition than were their healthy counterparts. 3. For both the healthy and the parasitised hosts, the number of larvae surviving to adulthood gave a good fit to both the generalised Beverton and Holt and generalised Ricker models, but the values of all the parameters differed between the two treatments. 4. Parasitoid size, egg load, and adult survival time decreased significantly with the initial host number. 5. Previous theoretical work suggests that both lower susceptibility to competition by parasitised hosts and scramble competition contribute to the dynamical instability of host-parasitoid systems. Changes registered in life-history traits may also affect host-parasitoid dynamics. These changes have not yet been incorporated into host-parasitoid models.