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Chalkbrood, A Disease of Bees

Chalkbrood is the common name of a fungal disease that kills the developing brood. (Immature bees are called "bee brood" or bee larvae.) Unlike molds and other fungi that may grow opportunistically on dead and decaying insects, or on pollen, the fungi that cause chalkbrood (Ascosphaera sp.) are pathogens that cause fatal infections in bee brood. But these fungi only affect bees; they do not infect any other animal.

The alfalfa leafcutting bee (Megachile rotundata), blue orchard bee (Osmia lignaria) and honey bee (Apis mellifera) are all susceptible to chalkbrood.

Healthy brood                                                       Chalkbrood-infected brood

Chalkbrood and the Alfalfa Leafcutter Bee

Most of the chalkbrood research conducted at the Pollinating-Insect Research Unit in Logan relates to the alfalfa leafcutting bee. This bee, at left, is used extensively as a pollinator for alfalfa seed production in North America.

However, American producers face 5-26% loss in bee populations each year due to chalkbrood. Many seed producers must purchase new bees annually from low-disease areas.

Pollination costs for alfalfa seed have been estimated to be about 7% of the total production costs (Hinman, H. and Kugler, J. 2006 Cost of Producing Alfalfa Seed in the Columbia Basin of Washington State.  WSU Extension Bulletin EB2013E).

Alfalfa seed field in Idaho with shelters for the alfalfa leafcutting bee (above).

Here is a cross-section of an alfalfa leafcutting bee nesting board. You can see the cells that the bees construct from alfalfa leaf pieces. Each female bee constructs a series of cells inside the holes of the nesting board (which look like grooves in the cross-section). Each cell contains a ball of pollen and nectar and one bee brood. The mother bee provisions each cell with enough pollen and nectar for the brood to complete it's development, and so she never comes back to give her brood more food.

In the alfalfa leafcutting bee, spores of the fungus are transferred to brood by adult bees during nest building.  As eggs hatch, the larvae consume spore-infested pollen, and the spores then germinate in the gut and infect the insect. Once a larva dies, the fungus grows throughout the insect and produces large quantities of spores.

The next summer, emerging adults will pick up spores released from these dead larvae and transfer the spores to their own young.

The adult bees are not affected by the disease.


Spores of chalkbrood come tightly packed into balls, as shown above. These balls form inside a sack (to the left) called the cyst, or ascoma. The spores pictured above are of Ascosphaeraaggregata, and the cyst is of Ascosphaeraproliperda.  Both these fungi infect the alfalfa leafcutting bee.

The spores are microscopic but are so abundant that they are easily seen and look like a black powder covering the dead brood.

Here is a diagram of one way that emerging adult bees pick-up chalkbrood spores. A brood in the middle of a nest has died from chalkbrood and is full of spores. The live bees in the same nest will continue to develop to adulthood, but any bee behind the cadaver in the nest is stuck and cannot emerge unless she chews her way out through the cadaver. Females that emerging in this way become contaminated with spores.

Below is another way that the bees can become contaminated with spores.

Alfalfa seed growers often remove leafcutting bee cells from nesting boards at the end of the season. The bees are stored as "loose cells" overwinter in bins. In the early summer the loose cells are transferred to trays, such as the one pictured above, and covered with a screen. These trays are incubated to allow the bees to complete development to adults for release into the field. Adult bees emerging in these trays inadvertently pick up large numbers of chalkbrood spores that have contaminated the loose cell material.

We are currently working on methods for treating the loose cells to kill the spores that reside there.  In Canada, it is common practice to treat the loose cells with formaldehyde gas.  However, this treatment is uncommon in the U.S., where most uses for formaldehyde gas are banned.