Effects of mass releases of Trichogramma brassicae on predatory insects in maize

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 greenhouse 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.