Spatial distribution of flower vs. honeydew resources in cereal fields may affect aphid parasitism

Abstract

The potential impact of nectar availability for biological pest control is of increasing relevance due to the disappearance of non-crop plants in intensified agricultural landscapes. However, little is known about how floral resources and their spatial distribution in and around crop fields influence cereal aphid parasitoid (Hymenoptera, Aphidiidae) survival and population dynamics. Cereal aphid parasitoids may be less dependent on flowers as they can use honeydew from their aphid hosts as an alternative food resource. However, honeydew appears to be less suitable as a food source for parasitoids when compared to nectar. Here the effects of flower distribution (nectar), aphid abundance (honeydew quantity) and honeydew quality on parasitoid life history parameters were explored using a spatially explicit, individual-based simulation model. In addition, effects of the availability of floral resources in field margins on parasitism levels in winter wheat were investigated in a field study. In the simulation study three flower distributions were compared: (i) flowers restricted to the field margin (most common in agricultural landscapes) or (ii) flowers in strips within the crop, and (iii) random flower distribution in the field. Simulations suggest that nectar feeding increases parasitoid longevity and parasitism at low aphid densities, but only in fields with randomly distributed flowers. In contrast, the spatial distribution of flowers did not affect the parasitoid longevity and parasitism rate at high aphid densities, where parasitoid feeding was restricted to the abundant honeydew resources. In simulations, parasitoid survival and the number of aphids parasitized were also affected by the nutritional quality of the honeydew, with low quality honeydew having a less positive effect on parasitoid longevity and parasitism rate. The field study did not indicate evidence of a significant relationship between flower cover in field margins and parasitism rates in wheat near the margins. This finding is compatible with results from simulations and suggests that honeydew feeding is more prevalent in cereal aphid parasitoids than nectar feeding. In conclusion, nectar may be an important factor for the potential biocontrol success of cereal aphid parasitoids, but most likely only at low aphid densities and especially when flowers are broadly distributed in the field.