Can low release numbers lead to establishment and spread of an exotic parasitoid: The case of the diamondback moth parasitoid, Diadegma semiclausum (Hellén), in East Africa
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From 2001 onwards, Diadegma semiclausum, an exotic parasitoid of the diamondback moth, was introduced and released in Kenya, Tanzania and Uganda. Contrary to common practice where thousands of parasitoids are released, we released very low numbers, 125 females in Kenya, 160 in Uganda and 350 in Tanzania. About 2 years after this single release, the establishment and natural spread of the parasitoids was assessed in all release areas. Two methods were employed: in Kenya, a grid with equidistant points in the four cardinal directions (2–50 km) with the release area in the centre was used and collections were made 27 months after release at the predetermined points. The parasitoid was found up to a distance of 30 km from the release site. In Tanzania and Uganda, surveys were made starting from the release area following major roads. At regular intervals, fields were inspected and their position recorded with a Geographic Positioning System (GPS). The results of a field survey conducted 24 months after release indicate that in Tanzania, the parasitoid had spread >20 km from the release site while in Uganda, the spread was >30 km. Wherever D. semiclausum was collected, it was the major parasitoid species. Indigenous parasitoids collected were Oomyzus sokolowskii (Hym.: Eulophidae), Diadegma mollipla (Hym.: Ichneumonidae) and Apanteles sp. (Hym.: Braconidae). Overall parasitism and the contribution of the introduced parasitoid to the control of diamondback moth population tended to decrease with increasing distance from the release point. The introduced parasitoid had displaced the indigenous species wherever it was well established.