Nutritional ecology of predaceous Tetanocera elata larvae and the physiological effects of alternative prey utilisation

Tetanocera elata (Fabricius) (Diptera: Sciomyzidae) is an obligate mesoparasitoid of pestiferous Deroceras spp. slugs in the first and second larval instars and then emerges to become a free-living predator of terrestrial slugs in the third instar. To determine the biological control potential of T. elata, naïve third-instar larvae were exposed to a range of prey slug species (Deroceras reticulatum, Arion hortensis, and Tandonia budapestensis) in no-choice, pairwise two-choice, and three-choice feeding assays. While larvae showed little prey preference, typically attacking the first individual with which they came into contact, A. hortensis was significantly preferred over T. budapestensis in two-choice trials (P = 0.048). Larvae were also more efficacious at predating D. reticulatum, in that significantly fewer larval attacks preceding feeding were required for D. reticulatum than for A. hortensis or T. budapestensis (P = 0.0008 and P = 0.0059, respectively). Larvae reared on D. reticulatum in culture following trials also experienced the greatest survivorship to the start of pupariation. While these results suggest that D. reticulatum may remain the ideal prey for third instar T. elata larvae, they also demonstrate the ability of larvae to survive on alternative species. The implications of these findings in the context of using T. elata as a biocontrol agent are discussed.