Overwintering potential in California of two Russian wheat aphid parasitoids (Hymenoptera : Aphelinidae et Aphidiidae) imported from Central Asia
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Aphelinus albipodus Hayat and Fatima and Diaeretiella rapae (M'Intosh) (Hymenoptera: Aphelinidae and Aphidiidae) were imported to the USA for biological control of Russian wheat aphid (Diuraphis noxia Mordwilko) in 1992. Early laboratory studies identified potential limitations for their successful establishment in northern California where these parasitoids were extensively colonized and evaluated against Russian wheat aphid. Specifically, those studies showed that A. albipodus was particularly susceptible to moderately low temperatures (10 C). These results for A. albipodus were in contradiction with seasonal field temperatures prevalent in its collection site (Tahcheng, People's Republic of China) where field temperatures are below freezing for several months each year. This study sought to reconcile these apparently contradictory results. We examined whether short daylength-induced diapause, i.e. winter diapause, occurs in A. albipodus and D. rapae, and thus could be used as an overwintering strategy by these parasitoids. Under laboratory conditions, > 2/3 of A. albipodus individuals entered diapause when daylength was reduced from 14 h to 12 h. In contrast, diapause was not detected in D. rapae under the same conditions. However, we suggested that diapause may occur in D. rapae under shorter daylengths because (i) this parasitoid's development and survivorship are not affected by temperatures associated with 12 h daylengths at its collection site (Wuqia, PRC), and (ii) other studies have demonstrated diapause in this parasitoid at daylengths < 12 h. We discuss our results in light of A. albipodus' rapid establishment in northern California, and the use of our analytical procedure as a component of a process for screening natural enemies in importation biological control programs.
