Honeydew as a food source for natural enemies: Making the best of a bad meal?

The majority of predators and parasitoids require non-prey food as part of their diet. The availability of suitable non-prey food can impact predator–prey and parasitoid–host dynamics and may be critical to the efficacy of biological pest control. A growing body of work addresses the role of floral nectar in conservation biological control programs. In comparison, we know relatively little about the role of honeydew as a sugar source for predators and parasitoids, in spite of the fact that honeydew is often the predominant sugar source in agroecosystems.Here we test the hypothesis that honeydew is a less suitable food source when compared with other sugar sources, using data from parasitoid literature. We also explore whether parasitoids of honeydew-producing insects show particular adaptations allowing them to optimize the exploitation of this suboptimal food source. A review of available literature supports the general hypothesis that honeydew is inferior to other sugar sources, although there is considerable variation in the quality of honeydew depending on the honeydew producer and its host plant. The accrued dataset is not consistent with our second hypothesis that parasitoids of honeydew-producing hosts fare better on this nutritionally inferior food source as compared with parasitoid species which exploit hosts that do not produce honeydew. Finally we review evidence on the use of honeydew by predators and parasitoids under field conditions and explore implications for biological pest control programs.