Plant characteristics mediated by growing conditions can impact parasitoid's ability to attack host aphids in winter canola

Winter canola (Brassica napus) is an important crop in various parts of the world. It is planted in fall, vernalized during winter and offers higher yields than spring canola. One of the key aphid pests of this crop is Myzus persicae Sulzer (Homoptera: Aphididae). Recent surveys in Europe showed that M. persicae in winter canola is parasitized by Aphidius matricariae and Diaeretiella rapae, but not by other aphidiine parasitoid species known to attack M. persicae in other crops. During vernalization, winter canola plants acclimatize to survive specific climatic conditions of winter, e.g. through the formation of epicuticular waxes on leaves. These waxes could affect efficiency on natural enemies. In this study, we characterized (1) the ability of aphidiine parasitoids to locate and attack M. persicae on winter canola, and (2) how plant vernalization influences parasitoid ability to attack suitable hosts. In the laboratory, the ability of A. ervi, A. matricariae and D. rapae to orient toward odors of the M. persicae-winter canola complex was studied in an olfactometer, as well as their ability to attack M. persicae on winter canola. In addition, the impact of plant species and plant vernalization on the ability of A. colemani to attack and locate M. persicae was evaluated. The results show that A. matricariae and D. rapae readily locate and attack M. persicae on winter canola as does A. ervi, albeit to lesser extent. Aphidius colemani shows low ability to parasitize M. persicae on this plant species. Vernalization of canola is an important factor mediating the ability of A. colemani to attack M. persicae and it likely results from the presence of Brassica's waxes that reduce parasitoid mobility and impact foraging behavior. These results show that aphid parasitoids differ in their ability to locate and attack M. persicae on winter canola, the latter being a function of parasitoids' capacity to cope with plant characteristics that are mediated by growing conditions.