Facultative endosymbionts of aphids on strawberry crops affect aphid-parasitoid interactions

Aphids are major agricultural pests and laboratory studies have shown that heritable bacterial endosymbionts can protect aphids against biocontrol parasitoids, but results from agricultural crop systems are lacking. Here we assess (1) which aphid species are present in Dutch strawberry crops, (2) which facultative endosymbionts they carry, and (3) how such facultative endosymbionts affect parasitism success of the biocontrol parasitoid wasps Aphidius ervi and Praon volucre on the most prevalent aphid species Acyrthosiphon malvae. By curing endosymbiont infections, we were able to determine the effects of endosymbionts and aphid genotypes on the parasitism success of both parasitoid species. We show that aphid species composition on strawberry crops is dynamic across the season and that facultative endosymbiont (co-)infections are common. Infection with facultative endosymbionts protected A. malvae against A. ervi, but not against P. volucre. Finally, we also found variation in levels of endogenous resistance among A. malvae lines against A. ervi. Our study is the first to show that a common aphid pest on agricultural farms can be protected against biocontrol parasitoids by both facultative endosymbionts and endogenous resistance.