Thermosensibilité des lignées germinales de Trichogramma brassicae Bezdenko (Hymenoptera). Conséquences sur l'efficacité du parasitoïde

Trichogramma brassicae, a parasitoid used in the biological control of the European corn borer, Ostrinia nubilalis, becomes less effective if certain thermal conditions prevail at the time of its release. A laboratory study demonstrated that the generation liberated in the field had reduced fecundity and that the proportion of females was lower in the following generation. These results prompted us to examine the germ lines of the parasitoid. Exposure to a temperature of 35 °C at the white nymph stage does not affect ovarian function, but exposure to 44 °C does: the number of mature ovocytes at emergence is lower than that observed in controls; certain ovaries are abnormal or remain juvenile. At the time of emergence, the seminal vesicles of male controls contain large numbers of typical sperm and a few atypical sperm, the latter becoming more numerous in males exposed to 35 °C. After exposure to 44 °C, vesicles contain only a few sperm or are completely empty. Since T. brassicae is an arrhenotokous parthenogenetic species (unfertilized eggs become males), these observations explain the reduced number of females in the second generation.[Journal translation]