Bottom-up regulation of a tritrophic system by Beet yellows virus infection: consequences for aphid-parasitoid foraging behaviour and development

Effects of plants on herbivores can cascade up the food web and modulate the abundance of higher trophic levels. In agro-ecosystems, plant viruses can affect the interactions between crops, crop pests, and natural enemies. Little is known, however, about the effects of viruses on higher trophic levels, including parasitoids and their ability for pest regulation. We tested the hypothesis that a plant virus affects parasitoid foraging behaviour through cascading effects on higher trophic levels. We predicted that the semi-persistent Beet yellows virus (BYV) would influence plant (Beta vulgaris) quality, as well as aphid host (Aphis fabae) quality for a parasitoid Lysiphlebus fabarum. We determined amino acid and sugar content in healthy and infected plants (first trophic level), lipid content and body size of aphids (second trophic level) fed on both plants, as well as foraging behaviour and body size of parasitoids (third trophic level) that developed on aphids fed on both plants. Our results showed that virus infection increased sugars and decreased total amino acid content in B. vulgaris. We further observed an increase in aphid size without modification in host aphid quality (i.e., lipid content), and a slight effect on parasitoid behaviour through an increased number of antennal contacts with host aphids. Although the BYV virus clearly affected the first two trophic levels, it did not affect development or emergence of parasitoids. As the parasitoid L. fabarum does not seem to be affected by the virus, we discuss the possibility of using it for the development of targeted biological control against aphids.