Humidity-terminated diapause in the tropical braconid parasitoid Microplitis demolitor JAMIE E. SEYMOUR andRHONDDA E. JONES School of Tropical Biology, James Cook University, Australia Abstract. 1. Microplitis demolitor, a solitary braconid parasitoid of Helicoverpa armigera and Helicoverpa punctigera, uses an increase in relative humidity to signal the end of diapause and the re-establishment of direct development. 2. Photoperiod and temperature changes appear not to have a direct in¯uence on the duration or termination of diapause in this species. 3. Once diapause termination is signalled, the adult parasitoids emerge approximately 38 days later. 4. The cue for diapause termination in M. demolitor is different from that used by its hosts and ensures that the parasitoid emerges to ®nd a population of nonmigrating hosts. 5. It appears that the cocoon surrounding the diapausing parasitoid plays a role in diapause termination; parasitoids emerge earlier from diapause if the cocoon is removed but the weight of the intact cocoon does not appear to in¯uence the length of time the parasitoid is in diapause. Key words. Diapause, humidity termination, parasitoid, photoperiod, temperature. Introduction Changes in temperature and/or photoperiod are responsible for diapause termination in many temperate arthropods. Many researchers examining diapause duration have therefore varied photoperiods, temperatures, or combinations of both in attempts to isolate diapause termination factors (Danks, 1987). Only a few studies have suggested that moisture controls diapause development directly in arthropods (Tauber etal., 1998). Of these few cases, even fewer have been substantiated by experimental evidence (Tauber & Tauber, 1976). Water has, however, been shown to be essential in the post-diapause developmental phase of some arthropods (Tauber & Tauber, 1976; Danks, 1987). For example, the European corn borer Ostrinia nubilalis has a two-phase diapause process, whereby changes in photoperiod terminate diapause but water is required to activate the neuroendocrine system to allow post-diapause development to be completed (Beck, 1967). In other species of insects, for example some braconids, post-diapause development is hastened at high humidities (Laing & Heraty, 1987). The work reported here examined diapause termination in the solitary parasitoid Microplitis demolitor (Wilkinson), whose hosts include several economically important species of the noctuid genus Helicoverpa, namely H. armigera and H. punctigera. These two species exhibit a facultative diapause, often entering diapause at the end of summer (April) and terminating diapause over a protracted period during spring and early summer (Zalucki etal., 1986). Only a few studies have shown diapause to occur in M. demolitor and the environmental cues for diapause termination have not been isolated. Kay (1982) reported that M. demolitor collected from the Darling Downs region of Australia emerged from diapause in August/September in 1977/78 and coincided with diapause termination of H. armigera in the area. Cobb (1983) found a `diapause-like phenomenon' in a laboratory- based colony of M. demolitor imported into America from Southern Queensland, Australia, in 1981, which appeared to be terminated in some individuals when they were moved from 17 °C and 21 °C to 27 °C. The unpublished reports of Robertson (in Hafez, 1951) reported that overwintering in Queensland by M. demolitor lasted an average of 5 months, while Hafez (1951) found similar results for M. demolitor (originally from SE Queensland) in Giza (Egypt), where the parasitoids overwintered for 4 months. Diapause termination in two of the native hosts of M. demolitor, Helicoverpa (= Heliothis) armigera (Hu Èbner) and Correspondence: Jamie Seymour, School of Tropical Biology, James Cook University, McGregor Road, Cairns, 4878, Australia. E-mail: Jamie.Seymour@jcu.edu.au # 2000 Blackwell Science Ltd 481 Ecological Entomology (2000) 25, 481±485 Ecological Entomology (2000) 25, 481±485