© 2003 The Netherlands Entomological Society Entomologia Experimentalis et Applicata 108: 115 – 124, 2003 115 Blackwell Publishing Ltd. Effects of mass releases of Trichogramma brassicae on predatory insects in maize D. Babendreier*, M. Rostas, M. C. J. Höfte, S. Kuske & F. Bigler Swiss Federal Research Station for Agroecology and Agriculture, Reckenholzstr. 191, 8046 Zurich, Switzerland Accepted: 3 June 2003 Key words: intraguild predation, inundative release, risk assessment, non-target effects, biological control, Hymenoptera, Trichogrammatidae Abstract We investigated whether mass releases of Trichogramma brassicae against the European corn borer will have detrimental effects on populations of other natural enemies in maize. In a tiered approach, experiments investigated the host acceptance of T. brassicae towards eggs of Chrysoperla carnea , Episyrphus balteatus , Adalia bipunctata , and Coccinella septempunctata under laboratory, caged, and field conditions. The factitious host, Ephestia kuehniella , and in one case also Mamestra brassicae were used as control. Under laboratory conditions, 53% of the T. brassicae females parasitised C. carnea eggs, 70% parasitised E. balteatus eggs, while 91% of the females parasitised E. kuehniella eggs, with T. brassicae emerging from C. carnea, E. balteatus , and E. kuehniella at rates higher than 81%. No parasitoid offspring emerged from eggs of A. bipunctata and C. septempunctata. However, we observed significantly increased mortality on A. bipunctata eggs, compared to the control, and also found young instars of T. brassicae inside A. bipunctata eggs. In a second experiment where the host acceptance behaviour of the parasitoid female was directly observed for 10 min, 10% of T. brassicae females were found to oviposit in eggs of A. bipunctata , but the development of parasitoid offspring failed. No C. septempunctata eggs, 13% of E. balteatus eggs, 23% of C. carnea eggs, 33% of E. kuehniella egg clusters, and 83% of M. brassicae eggs were accepted for oviposition. In the green- house under caged conditions, the parasitism rate of C. carnea eggs (7%) and E. balteatus eggs (0.4%) were significantly lower than of E. kuehniella eggs (21 and 27%, respectively). In the final tier, 3.1% of C. carnea eggs were parasitised by T. brassicae under field conditions. This was significantly less than the observed parasitism rate of E. kuehniella egg clusters (64%). From direct observations of the parasitoids’ host acceptance behaviour and the low parasitism rates observed under caged and field conditions, we conclude that detrimental effects of the mass release of T. brassicae on populations of natural enemies in maize are unlikely to occur. Introduction Non-target effects have become important in insect biologi- cal control projects over the past decade, and several review papers have addressed this issue (Howarth, 1991; Simberloff & Stiling, 1996; Stiling & Simberloff, 2000; Lynch et al., 2001). Non-target effects may arise if either non-target herbivores or natural enemies present in the habitat are attacked by the biological control agent. Natural enemies may be affected in two ways: they may be directly attacked by the biological control agent or may suffer from interspecific competition by the biological control agent. There is a general agreement that the potential for non-target effects to occur is higher in polyphagous than in specialist parasitoids (Howarth, 1991). Egg para- sitoids of the genus Trichogramma are known to parasitise mainly species within the order Lepidoptera, but were also reared from hosts of other insect orders. As furthermore Trichogramma is one of the most widely used biological control agents (van Lenteren, 2000), the potential for non- target effects due to the release of trichogrammatids has been recognised whereby special emphasis has been given to populations of butterflies that might be at risk by wasps emigrating from the release fields (Andow et al., 1995; Orr et al., 2000; Babendreier et al., 2003a,b). *Correspondence: Dirk Babendreier, Swiss Federal Research Station for Agroecology and Agriculture, Reckenholzstr. 191, CH-8046 Zürich, Switzerland. Tel.: +41 1 377 72 17; Fax: +41 1 377 72 01; E-mail: dirk.babendreier@fal.admin.ch