Host Density Responses of Mastrus ridibundus, a Parasitoid of the Codling Moth, Cydia pomonella

The response of Mastrus ridibundus (Gravenhorst) (Hymenoptera: Ichneumonidae), an introduced parasitoid of the codling moth, to host density was investigated by three different approaches. In a field-release experiment in six walnut orchards, the probability of a host patch (tree) being attacked increased with host density in four of the orchards, although there was no consistent pattern of percentage parasitism. When data from all orchards were combined, the probability of attack increased asymptotically with host density, and percentage parasitism was inversely density dependent. In a release–recapture experiment in a walnut orchard, female parasitoids were found searching and ovipositing more frequently on trees with a high host density (20 per tree) than on trees with a low host density (5 per tree). This was consistent for all distances at which parasitoids were recaptured and for all time periods during the course of the 8-h experiment. In a laboratory functional-response experiment, the number of hosts parasitized over a 24-h period by a single parasitoid female increased from 0.4 (SE 0.09) hosts at a density of 1 host to 1.8 (SE 0.17) hosts at a density of 8 hosts. At the same time, after 1 day of experience, clutch size declined from 5.8 (SE 0.7) individuals at 1 host per female to 3.4 (SE 0.3) individuals at 8 hosts per female. The implications of the aggregative response of M. ridibundus to host density, its limited functional response, and its ability to adjust clutch size are discussed in relation to its potential for the biological control of the codling moth.