Includes
Monodontomerus obscurus, M. montivagus, M. osmiae, and Pteromalus venustus
Pest description and damage Chalcid wasps (males: 2-3 mm, females: 3-4 mm) are metallic blue or green with red eyes. Female wasps pierce the upper surface of cells or cell caps to deposit 10 to 50 eggs on the surface of mature bee larvae or pupae within the bee cocoon. Wasp larvae will consume bee larvae and pupate within the host cocoon. Depending on the species, development of wasps from egg to mature adult takes about one month, at which time adults emerge through round holes chewed in the host body wall. Adult wasps emerge about 2 or more days before alfalfa leafcutting bees begin emergence and male wasps typically do not leave bee cells. Multiple generations can develop each season, so infestation levels may build up over the summer and cause serious economic loss.
Management-traps
The most important type of control for alfalfa leafcutting bee pests is the maintenance of clean bee stocks and nesting media. Nests made from drilled boards and polystyrene blocks are solid and minimize potential entry points particularly in the rear of the nest. Remove nests from field immediately after nest period to reduce wasp exposure. In the past, insecticide stripes with Dichlorvos were set in incubation rooms. However, users are no longer permitted to use this product in Canada after August 20, 2023. Caution should be taken if using Pyrethrin aerosol in the incubator, as it can be a risk to the bees.
To reduce wasp population levels, rear out parasitic wasps from bee nests in an incubation room. Place a few drops of liquid soap or detergent in water pan traps under ultraviolet lights on the floors of the incubation rooms. Wasps will be attracted to the lights, fall into water pans, and drown. Place bee board into field before adult bees emerge.
For more information:
Parker, F.D. and P.F. Torchi. 2016. Management of Wild Bees (http://www.beesource.com/resources/usda/management-of-wild-bees/)
Manitoba Agriculture. Parasites (Pteromalus venustus) of Alfalfa Leafcutting Bees. Available online (https://www.gov.mb.ca/agriculture/crops/crop-management/print,parasites-...)
Goerzen, D.W. 2001. Alfalfa leafcutting bee parasite control with pyrethrin aerosols. Saskatchewan Alfalfa Seed Producers Association Extension Publication 2001-03.
Eves, J.D., D.F. Mayer, and C.A. Johansen. 1980. "Parasites, predators and nest destroyers of the alfalfa leafcutting bee, Megachile rotundata." Washington State University, Agric. Exp. Stn. Pullman, WA, Western Regional Extension Publication No. 32. 1980.