The outcome of in-vitro contests between larvae of the endoparasitoid Venturia canescens depends on both their relative and absolute ages

An empirical model of the sympatric coexistence of two strains of the endoparasitoid wasp Venturia canescens

Recent research has demonstrated that a laboratory culture of the asexual solitary endoparasitoid wasp Venturia canescens Grav. (Hymenoptera: Ichneumonidae) contains two genetically and phenotypically distinct lines, coexisting on their host the flour moth Ephestia kuehniella Zeller (Lepidoptera: Pyralidae). The basis to the coexistence of the two lines appears to be differences in their reproductive success under single parasitism and superparasitism. Furthermore, examination of field-derived wasps from several locations has shown that the phenotypes displayed by the laboratory colonies also co-occur in field populations. Historically, the impossibility of showing that two species do not occupy separate niches has precluded any demonstration of sympatric coexistence in the field. Here we present the results of an iterative model that uses a range of experimental life history data to predict the stable composition of a mixed population of two lines displaying the laboratory phenotypes under different rates of superparasitism. The model predicts that sympatric coexistence of the two lines is possible when the overall rate of superparasitism is between 4 and 12% or greater. These values are within the rates reported for other solitary endoparasitoid wasp species in the field, and so demonstrate that the sympatric coexistence under natural conditions of two species that display the phenotypes observed in the laboratory lines is, in principle, possible.

The development of the endoparasitoid wasp Venturia canescens in superparasitised Ephestia kuehniella

Using a molecular marker that allows the differentiation of two strains of the solitary endoparasitoid wasp Venturia canescens, the study investigated the influence of host mass and the time interval between ovipositions on the survival and development of larvae from both the first and second laid eggs in superparasitised Ephestia kuehniella. As the time interval between ovipositions increased both overall and superparasitism success decreased, however, time between, and order of, ovipositions had little effect on other developmental parameters. Adult size increased with host mass under both parasitism and superparasitism, while host mortality decreased with host mass under superparasitism. In addition, wasps emerging from superparasitised hosts were larger than wasps from parasitised hosts. The results confirm that for V. canescens on the host E. kuehniella both self- and conspecific-superparasitism will be an adaptive strategy when hosts are the limiting factor.