Diaeretiella rapae (McIntosh)
Geographical distribution: Cosmopolitan.
Life history: Diaeretiella rapae is a cosmopolitan solitary endoparasitoid of many aphids, and has several biotypes in different parts of the world. It usually attacks all stages of aphids that infest brassicas and cereal crops, the most prevalent host being Brevicoryne brassicae. Developmental time is usually around 2 weeks and fecundity comes to several hundred eggs/female, depending on the host and its size. Parasitized aphids are mummified, the parasite pupating therein and emerging as an adult. In the Middle East D. rapae is active mostly in the autumn through spring, rarely in summer. Among several aphid species assayed, Myzus persicae, due to its large body, was the most suitable host for mass-rearing D. rapae.
Economic importance: Diaeretiella rapae attacks about 100 aphid species infesting almost 200 plants in 43 plant families. Its main hosts, which it may control, are (besides B. brassicae) Diuraphis noxia, Myzus persicae, and Lipaphis erysimi. The parasitoid has been introduced into North America in order to control D. noxia infesting cereals. One method of releasing the parasitoid in a Brassicae plot is by placing cabbage plants that bear mummies in that plot.
Host plant effect: Cultivars of Brassica affected the percent parasitism of D. rapae on B. brassicae. Highest rates (40%) were on cabbage, as against 33% on turnips. Diaeretiella rapae, emerging from parasitized D. noxia that had infested resistant wheat cultivars were smaller, took longer to develop, and had fewer progeny, as compared with those from susceptible cultivars.
Effect of pesticides: Pyrethroids and a carbamate, when applied (against aphids), at slightly reduced rates, had only minimal adverse effects on parasitoid emergence from its hosts and on their subsequent survival, longevity and fecundity.
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