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