The nutritional value of aphid honeydew for parasitoids of lepidopteran pests

Intake of sugar-rich foods by adult parasitoids is crucial for their reproductive success. This is an aspect of parasitoid biology that warrants special attention in the biological control of pests. Facilitating access to suitable foods might help optimize the efficacy of parasitoids as biological control agents. In situations where nectar is not readily available, homopteran honeydew can be a key alternative food source. We studied the impact of honeydew feeding on the longevity of the larval endoparasitoids Cotesia marginiventris, Campoletis sonorensis and Microplitis rufiventris, natural enemies of important lepidopteran pests. Females of these wasps lived longer when feeding on the honeydew produced by the aphid Rhopalosiphum maidis on barley compared to females that were provided only with water. However, they lived considerably shorter than females fed with a sucrose solution. Further investigations with C. marginiventris showed that honeydew also increases the number of offspring produced, but less so than the increase promoted by a sucrose solution. Moreover, females of this species need to feed several times throughout their life in order to reach optimal longevity and reproductive output. We analyzed the sugars in the honeydew produced by R. maidis on barley and found that it contains mainly plantderived sugars, but also several sugars produced by the aphid. The sugar composition of the honeydew changed over time and as a function of aphid infestation. In general, the higher the aphid infestation, the smaller the percentage of aphid synthesized sugars in the honeydew. Experiments with sugar mimics of the honeydew allowed us to reject the hypothesis that the relatively poor performance of the parasitoid when fed honeydew was due to the sugar composition. Instead, the results from feeding experiments strongly suggest that poor intake of honeydew due to high viscosity limits its value as food for the wasp. The possible consequences of these findings for the biological control of pests are discussed. Introduction To optimize their reproductive success, adult parasitoids not only need to find hosts for their offspring, but also food to extend their longevity and thus the time they have to find