Effects of Sugar Spray and Aphid Honeydew on Field Populations of the Parasitoid Bathyplectes curculionis (Hymenoptera: Ichneumonidae)

Abstract

Sugar spray applied to crop foliage may serve as a surrogate for homopteran honeydew in meeting the nutritional needs for self-maintenance in adult parasitoids of crop pests. We tested the importance of timing of application of sugar spray and the interaction between sugar spray and naturally occurring aphid honeydew in influencing the local abundance of the alfalfa weevil parasitoid Bathyplectes curculionis (Thomson). Plots of hay alfalfa in northern Utah were sprayed with sucrose dissolved in water (sugar plots) or with water only (control plots) early, midway, or late during the preflowering period in the 5th yr of production. When sampled 2 d later, numbers of adult parasitoids were consistently higher in sugar plots than in control plots. Increased wasp density in response to local pea aphid density also occurred, but only in plots that did not receive sugar spray. Weevil parasitism was elevated in sugar plots compared with control plots only when sprays were applied late in the 1st crop. Lack of enhanced rates of wasp oviposition in sugar plots early and midway during the 1st crop may have resulted from poor weather, mutual interference among the wasps, or wasp feeding. Our study was conducted in a field harboring relatively low numbers of aphids. In general, sugar sprays may be redundant for adult parasitoids in a crop when naturally occurring homopteran honeydew is abundant. But the positive responses of B. curculionis to sugar sprays recorded in this study suggest application of this material may be beneficial to the wasp throughout the 1st crop of alfalfa if aphid densities (and associated availability of natural honeydew) remain low.