Do sown wildflower strips enhance the parasitism of lepidopteran pests in cabbage crops?

Summary

We investigated the parasitism rates of cabbage lepidopteran pest in relation to presence or absence of adjacent sown, species rich wildflower strips at two sites on farms in the region Grosses Moos, Switzerland. Three treatments have been tested: (1) cabbage adjacent to a wildflower strip, (2) cabbage near to a wildflower strip (>10 m up to 90 m), and (3 = control) cabbage without wildflower strip or any other semi-natural habitats nearby. Caterpillars and eggs of Mamestra brassicae, Pieris rapae, and Plutella xylostella (only larvae) have been sampled weekly in the unsprayed cabbage crops at 6 dates (cauliflower) and 9 dates (red cabbage). The parasitism rate of each pest species and its stage of development was recorded.
Caterpillars of M. brassicae and P. rapae were parasitised at a higher rate in cabbage fields near to the wildflower strip. In contrast, caterpillars of P. xylostella were significantly more parasitised in the field without strip. The parasitism rate of eggs was relatively low in general (14-35%), but increased in the proximity of the strips. In addition, the diversity and abundance of parasitic wasp families (Hymenoptera parasitica) recorded by sweep net catches were much higher in wildflower strips (up to 3 fold more specimens than in cauliflower; 14-16 families) than in the cabbage crops (7-11 families). In conclusion, data of this on-farm trial indicate that wildflower strips enhance the diversity and abundance of parasitic wasps, and thus may improve parasitism of certain lepidopteran cabbage pests in adjacent fields.