Overwintering of the egg parasitoid Trichogramma brassicae in Northern Switzerland

As part of a general risk assessment study we investigated the overwintering abilities of Trichogramma brassicae Bezd. (Hym.: Trichogrammatidae) in Northern Switzerland. Eggs of six host species parasitized by T. brassicae were exposed under outdoor conditions every two weeks between 26 September and 7 November 1998. We found that T. brassicae is able to overwinter successfully on six lepidopteran species in the families Tortricidae, Noctuidae, Plutellidae, Pyralidae and Crambidae. Between 75 and 100% emergence was observed in the following spring for all of the six tested host species exposed on 26 September. On later exposure dates, spring emergence decreased significantly and no development of T. brassicae offspring occurred from host eggs parasitized on 7 November. Emergence of T. brassicae from eggs of Ephestia kuehniella Zeller parasitized at weekly intervals in autumn 1999 occurred in the same year until the end of October if the eggs were parasitized by 10 September. All offspring from eggs that were parasitized from 17 September onwards went into diapause. Spring emergence of T. brassicae adults was observed between end of April and beginning of May both in 1999 and 2000. Fecundity of T. brassicae females that overwintered on E. kuehniella was not significantly different from the fecundity of control females that were reared without diapause under optimal conditions at 25degreesC. Our results demonstrate that the egg parasitoid T. brassicae is able to overwinter successfully in Northern Switzerland and has the potential to establish in Switzerland. As a result, potential non-target effects are not locally restricted but may occur on a large geographical scale.