Parasitism of non-target butterflies by Trichogramma brassicae Bezdenko (Hymenoptera: Trichogrammatidae) under field cage and field conditions

The egg parasitoid Trichogramma brassicae Bezdenko was introduced from Moldavia to western Europe some 30 years ago and since then has been inundatively released to control the European corn borer, Ostrinia nubilalis Hübner, in maize. We investigated parasitism of non-target butterfly eggs by T. brassicae in field cages and under field conditions in Switzerland. Parasitism of eggs (exposed for 24 h) of six non-target species (Papilio machaon L., Clossiana selene Denis and Schiffermüller, Maniola jurtina L., Melanargia galathea L., Hipparchia alcyone Denis and Schiffermüller, Cyaniris semiargus Rottemburg) in field cages of 2×2×2 m ranged between 2.5 and 18.7% when based on a relative high number of released parasitoid females (780±273 SD/cage). This number is 16-fold more compared to agricultural practices (120,000/ha). As a control, Ephestia kuehniella Zeller eggs were glued on the same plant. A difference in parasitism rates between the non-target species and E. kuehniella was only observed for M. galathea. During inundative releases of T. brassicae in maize, sentinel eggs of two suitable non-target hosts and of E. kuehniella were exposed for 72 h. Parasitism rates of E. kuehniella egg masses inside maize fields averaged 40% compared to significantly lower parasitism rates of 26.2 and 12.6% for eggs of H. alcyone and M. galathea. At 2 m distance from the maize field, parasitism rates decreased to 2.3% for M. galathea, 6.1% for H. alcyone, and 9.8% for E. kuehniella while no single egg was found parasitized at 20 m distance from the release fields. We conclude that, although the tested non-target butterflies were all attacked under semi-field and field conditions, effects on non-target butterflies due to mass released T. brassicae are minimal.