VARIATION BETWEEN LABORATORY POPULATIONS IN THE PERFORMANCE OF THE PARASITOID ENCARSIA-FORMOSA ON 2 HOST SPECIES, BEMISIA-TABACI AND TRIALEURODES-VAPORARIORUM

We tested the hypothesis that populations of the parthenogenetic paras itic wasp Encarsia formosa Gahan (Hymenoptera: Aphelinidae) differed i n their ability to use two different host species, Bemisia tabaci Genn adius (Homoptera: Aleyrodidae) and Trialeurodes vaporariorum Westwood (Homoptera: Aleyrodidae). Of the three wasp populations tested, two po pulations had been reared for many generations on B. tabaci and one po pulation had been reared for many years on T. vaporariorum. Performanc e was measured by the number of whitefly nymphs that were successfully parasitized by individual wasps, and performance on either host was m easured in separate experiments. There was variation between wasp popu lations in their performance on the host B. tabaci, with one wasp popu lation reared for many years on this host performing considerably bett er than the other two populations. There were no significant differenc es between populations in their use of the preferred host, T. vaporari orum. The experiments were conducted in such a way that we could disti nguish heritable differences between populations from environmentally- induced conditioning differences due to the immediate host from which an individual wasp enclosed. In either experiment there were no signif icant effects of conditioning, although there was a trend within each population for wasps conditioned on T. vaporariorum to have higher per formance than those conditioned on B. tabaci. Thirdly, we conducted a selection experiment, initiated with wasps from a single population hi storically reared on T. vaporariorum, to measure the effect of laborat ory rearing on different hosts for 17 generations. We did not see any difference in the performance of wasps on B. tabaci after this period of rearing on either of the two hosts. In summary, populations of E. f ormosa do differ in their relative performance on B. tabaci. The one p opulation that was tested further did not show any response to selecti on by rearing, but the ability to respond to selection on performance may not be equal for all populations. The possibility that wasp popula tions have differential performance on particular hosts may affect the use of this species as a biological control agent.