HOST SELECTION AND SURVIVAL OF THE PARASITOID ENCARSIA-FORMOSA ON GREENHOUSE-WHITEFLY, TRIALEURODES-VAPORARIORUM, IN THE PRESENCE OF HOSTS INFECTED WITH THE FUNGUS ASCHERSONIA-ALEYRODIS

Successful control of greenhouse whitefly may be achieved by complementary activity of the parasitoid Encarsia formosa and the fungus Aschersonia aleyrodis. One way to obtain an additive mortality effect of both entomopathogen and parasitoid would be achieved by the selection of healthy hosts by the parasitoid and rejection of fungus-infected hosts. Third and fourth instar larvae of Trialeurodes vaporariorum which had been treated with a spore suspension of A. aleyrodis 0, 4, 7, 10 or 14 days beforehand, were presented to female parasitoids. The parasitoids adopted the oviposition posture on untreated hosts as well as on treated hosts, irrespective of the different stages of infection in the hosts. However, significantly more hosts were parasitized by E. formosa in the control treatment than in the fungal treatment. The parasitoids offered treated hosts, showed rejection behaviour after probing on hosts showing detectable signs of infection (containing hyphal bodies or mycelium in the haemolymph). For instance, when hosts were offered seven days after spore treatment, the parasitoids showed an oviposition posture on a total of 83 (95.4%) out of 87 infected larvae, but laid only 4 eggs (4.6%). In contrast, on 48 (94.1%) out of 51 noninfected (or showing no detectable signs of infection) hosts an oviposition posture was adopted and 40 eggs (78.4%) were found after dissection. When infected hosts were encountered the oviposition posture lasted less than 1'40'' while rejection of non-infected hosts occurred after more than 1'40''. Other experiments were carried out offering treated hosts for 24 h to the parasitoids. The hosts were dissected afterwards. Again, significantly more eggs were laid in the non-infected hosts. When hosts were parasitized shortly after fungal spore treatment they were colonized by the fungus and the parasitoids did not develop. Transmission of the entomopathogen after probing infected hosts was observed to a limited extent. In conclusion, A. aleyrodis and E. formosa can be used together in a glasshouse situation. The parasitoid will be most effective when introduced more than seven days after application of A. aleyrodis, because from that time onwards it is able to detect and reject fungus-infected hosts.