Sowing of margin strips rich in floral resources improves herbivore control in adjacent crop fields

Field margin vegetation provides resources for natural enemies of crop herbivores. Thus, the design of plant mixtures improving resource provisioning is being discussed increasingly with respect to improving herbivore control. We set up a field experiment to assess the effect of (i) a wildflower strip optimized for nectar provisioning; (ii) a grass strip and (iii) spontaneous vegetation on herbivore regulation in oilseed rape and wheat. We also analyzed the attractiveness of plant species to natural enemies. The cover of flowering entomophilous plant species as a proxy of floral resource provisioning was twice as high in wildflower strips compared with the other strip treatments. Natural enemy densities were higher within and close to wildflower strips in the field. A corresponding effect on aphid predation and aphid infestation was observed. Significant negative correlations between natural enemy abundance in the margin and aphid infestation in the fields supported a causal relationship. The sown wildflower strip species were more attractive to natural enemies than spontaneous vegetation. The results of the present study demonstrate that improved nectar provisioning increased the predation and parasitism of crop herbivores. Further research on the spatio-temporal dynamics of interactions is needed to determine why not all crop herbivores respond to an increase of natural enemies in field margins.