Résumé des résultats (Abstract)
(Anglais)
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The overall objective of this project is to facilitate the development of sustainable, biologically-based production systems, in line with the commitments of the Swiss and many EU governments to reduce use of chemical pesticides in Agriculture. More specifically, it is aimed to investigate potential risks for the environment due to the use of exotic natural enemies which have been introduced in order to control agricultural pests. In Switzerland it is investigated whether large scale releases of the generalist egg parasitoid Trichogramma brassicae (Hymenoptera: Trichogrammatidae) to control the European Corn Borer (ECB) in maize may pose a risk for non-target species including beneficial insects and rare butterflies.
Since earlier results within this project have clearly demonstrated that T. brassicae is searching effciently inside the target crop (maize), we investigated whether mass releases of T. brassicae will have detrimental effects on populations of other beneficial insects in maize. In a tiered approach we carried out parasitization experiments under laboratory, semi-natural and field conditions including two coccinellids (Coccinella septempunctata and Adalia bipunctata) the hoverfly Episyrphus balteatus and the lacewing Chrysoperla carnea. Eggs of the flour moth, Ephestia kuehniella, were used as control. Under laboratory conditions, high parasitism rates by T. brassicae females were observed on eggs of C. carnea and E. balteatus while egg masses of A. bipunctata and C. septempunctata were either rejected or failed to support development of parasitoid offspring. Under semi-natural conditions in cages in the greenhouse, the parasitism rate on C. carnea eggs (7 %) and E. balteatus eggs (0.4 %) were significantly lower than on E. kuehniella eggs (21 and 27 %, resp.). In the final tier, 3.1 % of C. carnea eggs were parasitized by T. brassicae under field conditions, significantly less than what was observed on E. kuehniella egg clusters. We conclude that ecologically significant effects of mass released T. brassicae on populations of beneficial insects in maize are unlikely to occur.
We furthermore carried out an experiment to see whether the released T. brassicae will form a major part of the overall Trichogramma population during the whole season. Prior to inundative releases we found significantly more native Trichogramma wasps in non-target habitats compared to T. brassicae. Native Trichogramma were distributed throughout maize, wildflower strips and reeds while T. brassicae was virtually absent. Following the two inundative releases of T. brassicae in maize, T. brassicae temporally outnumbered the native Trichogramma species in maize and surrounding non-target habitats. However, three weeks after the second release, the T. brassicae population was strongly decreased both in maize and the non-target habitats, indicating that native trichogrammatids do not suffer from competition or only for a limited period of time.
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