Parasitism of non-target lepidoptera by mass released <i>Trichogramma brassicae</i> and its implication for the larval parasitoid <i>Lydella thompsoni</i>

The release of high numbers of the eggparasitoid <i>Trichogramma brassicae</i> Bezd.(Hym. Trichogrammatidae) to control the European corn borer (ECB), <i>Ostrinianubilalis</i> Hb. (Lep.: Crambidae) in maize hasraised concerns about potential negativeeffects on native natural enemies. The nativelarval parasitoid <i>Lydella thompsoni</i> Herting (Dipt.: Tachinidae) is the most frequent and important ECB parasitoid insouthern Switzerland and can achieve highparasitism rates. Its first generation emerges too early to find ECB larvae and must rely onalternative hosts living in natural habitats close to maize fields. Inundative releases of <i>T. brassicae</i> coincide with the oviposition period of the alternative hosts ofthe tachinid. <i>T. brassicae</i> moving out of release fields may attack and diminish the population of these hosts, creating abottleneck situation for <i>L. thompsoni</i> in the subsequent spring. Laboratory hostspecificity tests showed that the tachinid's two most abundant spring hosts <i>Archanarageminipuncta</i> Haworth (1809) (Lep.: Noctuidae)and <i>Chilo phragmitellus</i> Hübner (1805)(Lep.: Crambidae) are successfully parasitisedby <i>T. brassicae</i> females in no-choice situations. Our extensive field surveys, however, showed that the two tested springhosts escape parasitism since their eggs are well hidden or not attractive. Negative effects of inundative releases of <i>T.brassicae</i> on the native tachinid fly <i>L. thompsoni</i>, such as population density reduction, displacement, or local extinction, are very unlikely.