Factors involved in oviposition by the egg parasitoid Telenomus heliothidis (Hymenoptera : scelionidae)

Abstract

When a parasitoid forages for hosts, it must decide (1) where to search for and how to recognize hosts (2) which hosts to parasitize, (3) what sex ratio to produce and (4) how many eggs to lay. Various aspects of these oviposition decisions were examined using the egg parasitoid Telenomus heliothidis and its host, Heliothis virescens. T. heliothidis development, longevity, fecundity, and host acceptance behavior resembled strongly previous reports on other telenomines. Recognition of hosts involved both chemical and physical stimuli. H. virescens attached its eggs to the substrate with an adhesive produced in its accessory gland, but this material also acted as a host recognition kairomone for T. heliothidis. Host size, shape, and the recognition kairomone allowed T. heliothidis to assess host age and acceptability. T. heliothidis developed successfully in H virescens eggs by arresting embryogenesis and preorally digesting host tissue. Arrestment was due to a venom injected by the parasitoid at oviposition while host tissue digestion was due to T. heliothidis teratocytes. The parasitoid itself had no role in these alterations. Application of in vivo data on egg parasitoid development resulted in the successful in vitro culture of T. heliothidis, T. heliothidis teratocytes, and Trichogramma pretiosum. T. heliothidis displayed a variable sex ratio strategy when confronted with shifting local mate conditions. The parasitoid adjusted rapidly its sex ratio in response to varying host distributions and changes in conspecific rearing densities. However, neither the number of females ovipositing on a host patch nor the presence of already parasitized hosts influenced the sex ratios produced by T. heliothidis. Based on the assumption that egg parasitoid fitness is inversely related to host age, an optimal foraging model was developed and applied to qualitatively predict how solitary egg parasitoids are affected by host quality, oviposition frequency, and superparasitism.