| Theory predicts that parasitoids with a greater fecundity will provide better control of their hosts, and will be better biological control agents. A simple host-parasitoid model, incorporating the effects of parasitoid fecundity-limitation and host density-dependence, supports this prediction. A taxonomically diverse data set obtained from the biological control record failed to support this theoretical prediction, while at the same time indicating a strong effect of host taxon on the outcome of biological control. The hypothesis that the fecundity of parasitoids is correlated positively with their ability to suppress host populations is supported by data exclusively from the host order Lepidoptera.; An Ephestia kuehniella-Venturia canescens host-parasitoid laboratory system was used to study the influence of host and parasitoid density variation on host and parasitoid life-history parameters. E. kuehniella intraspecific competition is of scramble type, and is driven by reduced growth leading to pupal mortality, rather than by juvenile mortality. This system exhibits a density threshold effect such that, at sufficiently high initial host densities, total (host plus parasitoid) survivorship to adulthood decreases due to the effects of juvenile host intraspecific competition, leading to weakened host larvae which are parasitized and killed, but which are subsequently unable to support parasitoid development to adulthood. Host self-limitation models with a density threshold provided the best fits to the data, with the generalized Beverton-Holt model providing the best fit. The Ricker model, with fewer free parameters, provided a poor fit to the data.; Theory predicts that competing species cannot coexist on a single, non-replaceable resource unless the resource is partitioned in some way. A simple one-host, two-parasitoid model indicates that stable three-species coexistence is possible under a wide range of conditions, and demonstrates that the R* rule, the generalization that the stronger competitor will draw down the resource to the point of excluding the weaker, does not apply under these circumstances. For biological control purposes, this analysis shows the potential conflict between the properties of a one-host, two-parasitoid system that provide the maximal absolute host suppression, and those properties that provide the maximal additional host suppression resulting from the addition of a second parasitoid. |
