Effect of parasitoid host choice on apparent competition between pest and non-pest aphids

Background/Question/Methods

Apparent competition is an important mechanism structuring herbivore communities in natural and managed ecosystems.  If one host species population increases, the shared parasitoid population may also increase, leading to a higher mortality for one or both hosts. We are investigating how aphids living on non-crop plants affect pest aphid suppression by the parasitoid wasp Aphidius colemani.  Understanding factors such as parasitoid choice that affect the outcome of apparent competition can help improve the outcome of biocontrol.

The goal of this study was to determine 1) A. colemani preference for green peach aphids (Myzus persicae) on pepper plants (pest) versus bird-cherry oat aphids (Rhopalosiphum padi) on barley (non-pest); 2) how natal host affects A. colemani preference; and 3) whether apparent competition increases pest suppression compared to a single aphid-parasitoid system.   To achieve this we compared preference for, and parasitism of, each aphid by wasps reared on each host in factorial petri dish, caged and greenhouse experiments. To help explain this preference, parasitoid were reared from each host to compare life history traits related to parasitoid fitness.

Results/Conclusions

A. colemani reared from R. padi on barley showed equal preference for M. persicae and R. padi. However, when A. colemani was reared from M. persicae on pepper, it chose M. persicae nearly 90% of the time. This suggests that 50% of parasitoids reared on non-pests on barley will switch to parasitize pest aphids on pepper.  Further, we expect 90% of parasitoids emerging from the pest aphid to parasitize more pest aphids rather than switch to non-pests.  When M. persicae was the natal host, parasitoid survival to adult was 3 times higher on M. persicae than on R. padi, which may influence preference. Due to the positive feedback of stronger choice and greater fitness on the pest aphid, apparent competition favors the survival of the non-pest aphid, and exerts strong pressure on the pest aphid. This work has important implications for designing and implementing conservation biological control strategies, such as conservation strips and banker plant systems.