Host-parasitoid network in extreme conditions: the case of cereal aphids in wheat crops in Saskatchewan, Canada
Résumé
In agricultural areas, the climatic conditions at the northern limit of crop cultures could impact pest regulation. In this paper, we study the aphid-parasitoid-hyperparasitoid network in cereal fields around Saskatoon (SK, Canada), one of the most harsh climatic conditions for cereal crops in the world, with an extremely short growing season. We have investigated the aphid-parasitoid-hyperparasitoid species relative abundances over two consecutive years. In 2014, a cold spring year, aphids were only found late in the growing season, and their arrival probably corresponded to late Southern winds. In 2015, aphids, parasitoids and hyperparasitoids arrived in the cereal fields in July and August, whereas the cereal harvest is typically complete by the end of the month of August. Three species of primary parasitoids were found for a total parasitism rate of 7.62%, and four genera of hyperparasitoids, with a hyperparasitism rate of 20.1%. There were 87.4% apterous aphids, and the proportion of apterous and winged (alate) aphids did not differ among the 3 aphid species, or among use by parasitoid species and hyperparasitoid species. The sex ratio of parasitoids in the complex was biased towards females for most of the species. In areas with extreme weather conditions and short growing seasons, effective biocontrol through parasitism is limited, although the synchronization level of parasitoids to their aphid hosts is high. We discuss the consequences of the limited length of the growing season on the provision of ecosystem services and on the seasonal ecology of insects of agronomic interest. We also provide information about morphological criteria for identification of parasitoids from the genus Aphidius.