Two successive winter field trials were performed in the cereal fields of Brittany in north-western France in 2019/20 and 2020/21. During field trials, the abundance of aphid pests and their parasitoids wasps, as well as pollinators in cereal fields was studied in relation to winter flowering crops. No effect of winter flowering crops on parasitism rate was determined. Furthermore, there was no effect of pollinator abundance on the parasitism rate of aphids. Results thus suggest that, at least at present, there is no competition between bees (pollination) and parasitoids (biological pest control) over floral resources in winter in the temperate cereal fields of north-western France.
To further investigate potential competition between pollinators and biological control agents over floral resources, small-scale cage experiments were performed to investigate the effect of bee presence (Bombus terrestris) on the provision of biological control services of cereal aphid pests by Aphidius wasps. Cages were manipulated to vary the composition of winter flowering species available to the bees and parasitoids. Irrespective of floral mixture, the parasitism rate of aphids was lower in the presence of bees, suggesting the possibility of competitive interactions between parasitoids and bees over floral resources, although the result was not statistically significant. Furthermore, parasitism rate appeared higher in the presence of buckwheat (Fagopyrum esculentum), although the result was not statistically significant. Further work is thus required, although results hint at the potential for buckwheat to reduce competitive interactions between bees and parasitoids and, in doing so, enhance the natural biological control services provided by parasitoids.
Since many beneficial insects such as Aphidius parasitoids are becoming increasingly winter active as a consequence of warming winters, the project further investigated the potential for supplementary feeding of specific floral nectars to enhance the cold tolerance of parasitoid wasps. Preliminary results have suggested that supplementary feeding with faba bean (Vicia faba) nectar acts to increase the cold tolerance of Aphidius wasps. For this reason, the use of faba bean in winter flower strips and winter cover crops could aid the winter survival of this beneficial insect. Any measure to aid the winter survival of Aphidius wasps would act to boost the resultant spring population and thus contribute to enhancing the ecosystem service of biological control.
