Invasive Species Compendium

Detailed coverage of invasive species threatening livelihoods and the environment worldwide

Abstract

Host stage selection and sex allocation by Gyranusoidea tebygi (Hymenoptera: Encyrtidae), a parasitoid of the mango mealybug, Rastrococcus invadens (Homoptera: Pseudococcidae).

Abstract

Laboratory studies (choice and no-choice) were conducted to determine host stage selection and sex allocation by the encyrtid Gyranusoidea tebygi. The parasitoid reproduced on 1st-, 2nd- and 3rd-instar nymphs of the pseudococcid Rastrococcus invadens and avoided hosts that were already parasitized. Host feeding was occasionally observed. The sex ratios of offspring produced by individual wasps were highly biased in favour of females, whereas the sex ratio of groups of wasps foraging under crowded conditions varied from male-biased in smaller hosts to female-biased in larger hosts. Females had longer development times than males, developed faster in larger pseudococcids than in smaller ones and were always larger than males emerging from the same host instar. Their size increased with the instar of the host at oviposition. About 90% of all ovipositions in 2nd- and 3rd-instar nymphs resulted from an attack with multiple stings, starting usually with a sting in the head. The function of these head stings was either to assess the quality of the host or to subdue hosts prior to oviposition. Encounter rates, number of attacks and number of stings during one attack increased, while ovipositions decreased with host instar. Time investment per oviposition and time spent preening increased with increasing host age because older hosts defended themselves more vigorously than younger ones. Thus, while the fitness of the parasitoid increased with host size, fitness returns per time decreased. The implications of this host selection behaviour for the biological control of R. invadens are discussed.