Aphid honeydew may be the predominant sugar source for Aphidius parasitoids even in nectar-providing intercrops

Abstract

The nectar provision hypothesis predicts that the introduction of nectar-producing plants in agroecosystems benefits parasitoid populations in the field and enhances biological control. Intercropping is a common crop diversification scheme that may bring complementary nectar sources for parasitoids and increase herbivore pest control. For instance, intercropping cereals with faba beans introduces nectar sources in usually sugar-devoid systems (i.e. cereal single crops). However, the nectar provision hypothesis has never been evaluated at the field scale in such intercropping systems. To test this hypothesis, we evaluated if sugar is a limiting factor for Aphidius parasitoids in single triticale crops and if their nectar feeding activity increases in faba bean-triticale intercrops. Aphidius feeding patterns were evaluated from their sugar profiles, using high-performance liquid chromatography (HPLC). In parallel, aphid density and parasitism rates were estimated at the edge and in the centre of single crops and intercrops. Sugar analyses revealed that honeydew was always the main sugar source for parasitoids, and although a significant proportion of parasitoid populations were recorded to feed on nectar, this proportion did not increase in intercrops. Besides, parasitism rates did not increase in intercrops, nor were aphid populations reduced. Thus, our results do not support the nectar provision hypothesis, but rather suggest that although nectar provision benefits parasitoid populations in some systems, its effects on biological control are highly context-dependent. They also confirm that honeydew can be a major food source for parasitoids, which may not necessarily be sugar limited at the field scale.