Alfalfa leafcutting bee-Chalkbrood
(fungal disease)

Ascosphaera aggregata

Pest description and damage Chalkbrood is a severe fungal disease caused by several species of Ascosphaera, however, A. aggregata is the principle species that infects alfalfa leafcutting bees. It was first reported in the U.S. in 1973 and remains to be one of the greatest causes of alfalfa leafcutting bee deaths. Bee equipment can be contaminated by fungal spores facilitating the spread of the disease. Adult bees emerging from contaminated cells and bee larvae consuming contaminated pollen may become infected and continue to spread fungal spores. Spores germinate in the midgut and penetrate into the body cavity. Chalky white coloration occurs when the mycelium fills the body cavity and spores are released when the cuticle of infected bee cadavers rupture. Newly emerging adults become contaminated with spores as they chew through cells with spore-laden cadavers then spores are transferred to provisions of the new nest.


Replacing old used nest material can reduce or limit the spread of chalkbrood. Alternatively, decontaminate bee equipment and nesting materials using bleach or heat. Bee cells and nest materials can also be dipped in bleach or 3% solution of sodium hypochlorite for 1-2 minutes to reduce or prevent spreading of the infection. Dry bee cells away from direct sunlight or excessive heat. Bleaching and drying should be completed before incubation. Solid wood blocks, boards, and laminates can be heated in a kiln at 250°F to kill chalkbrood spores. Wood and polystyrene nesting boards can also be dipped in 5% solution of sodium hypochlorite. Disinfect boards in the spring and allow them to dry completely before use. Fumigation of cells and equipment using formaldehyde may be more effective than bleach or heat treatments. Formaldehyde, however, can be toxic and dangerous for the handler, therefore neutralization with ammonium bicarbonate is strongly suggested. Formaldehyde gas reacts with ammonia gas to produce hexamine which is relatively harmless to the handler, bee brood, and environment.

Loose cell management method where bee cells are contained loosely together in over-wintering bins or trays may improve ease of sanitation measures and reduce some pest problems. The loose cell method allows for the removal of chalkbrood cadavers and prevents newly emerging adult bees from having to chew their way through chalkbrood cadavers and becoming exposed. Note, however, that loose cell management may pose wide-spread risk of spore exposure.

Pollen balls

"Pollen ball" is a term used to describe cells provisioned with resources but contains no healthy egg or brood. Several reasons may contribute to pollen ball formation, such as the quality of floral resource, nectar to pollen ratio in the mass provision, pesticide contamination, and pathogens. Other factors may include position of the bee domiciles in the field, humidity, limited availability of the foraging resources in the fields, and overcrowding or high bee densities of bees nesting in similar areas.

For more information:

Baird, C.R. and R.M. Bitner. Loose Cell Management of Leafcutting Bees. Western Regional Extension Publication 12.

James, R.R. 2005. Impact of disinfecting nesting boards on chalkbrood control in the alfalfa leafcutting bee. J. Econ. Entomol 98(4): 1094-1100.

M.S. Goettel, G.M. Duke, and G.C. Kozub. 2014. Characterization of "pollen balls" in commercial populations of the alfalfa leafcutting bee, Megachile rotundata in Canada, Journal of Apicultural Research 53:4, 496-499