Submitted to: Journal of Invertebrate Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 15, 2011
Publication Date: July 15, 2011
Citation: James, R.R. 2011. Chalkbrood Transmission in the Alfalfa Leafcutting Bee: The Impact of Disinfecting Bee Cocoons in Loose Cell Management Systems. Journal of Invertebrate Pathology. 40(4):782-787. Interpretive Summary: A good understanding of how a disease spreads in nature should bring to light new methods for prevention and control. A case in point for this is the alfalfa leafcutting bee, a bee that is used to pollinate alfalfa grown for seed. The health of this bee is severely hampered in the U.S. by chalkbrood, a disease of the larvae caused by a fungus. Hundreds of thousands of pounds of the bees are imported into the U.S. from Canada every year for crop pollination. This leafcutting bee is a solitary bee, and that means that every female bee is her own queen. It is thought that the pathogen is passed on by mother bees, who become contaminated with spores when they emerge from hibernation in the spring. The leafcutting bees nest in holes or tunnels in wood in the summer, and then overwinter in the nest where they first started life. In the spring they must emerge to mate and start a new nest, but their way out might be blocked by siblings that have died from chalkbrood. Frequently, a new bee must chew her way through the dead, sporulating siblings, and in this way, become contaminated with spores. The spores do not infect adult bees, but a mother bee will accidentally mix the fungal spores with pollen as she collects it to feed to her young, which do become infected. Farmers try to reduce disease spread by removing the overwintering bees from the nesting boards, and this method is called loose cell management. However, ARS has previously shown that bees emerging from winter hibernation in a loose cell management system are still contaminated with the fungal spores, although to a lesser degree, and this may explain why chalkbrood is still so common in the U.S. To test this hypothesis, loose cells were treated with a fungicide to kill the fungal spores before the adult bees emerged from winter hibernation, then the bees were allowed to nest in a field, and the offspring were evaluated for the disease. Chalkbrood levels were reduced in the offspring, but not by very much. It was clear in these experiments that the disease is spread by another unknown source, as well.
Technical Abstract: A good understanding of pathogen transmission in a host population should illuminate methods for disease prevention and control. A case in point for this is the alfalfa leafcutting bee (Megachile rotundata), a solitary bee which is used extensively for pollination of alfalfa grown for seed. Propagation of this bee is severely hampered in the U.S. by chalkbrood, a disease of the larvae caused by Ascomycetes in the genus Ascosphaera. It is thought that the pathogen is transmitted by the mother bee, who picks up spores during spring emergence. These annual bees nest in holes or tunnels in wood, and the offspring overwinter in their natal nest. When new adult bees emerge in the spring, their way may be blocked by siblings that have died from chalkbrood. Healthy adults must frequently chew through dead, sporulating siblings during emergence, and thus become contaminated. The spores do not infect adult bees, but a contaminated nesting female will inadvertently mix the spores into the pollen provisions she prepares for her own offspring. The spores infect larvae through the gut. Removing overwintering cocoons from the nesting boards is used to reduce this transmission, and is referred to as loose cell management. Bees emerging in a loose cell management system have previously been shown to also be contaminated with pathogen spores, and this may explain the continuation of high chalkbrood prevalence in alfalfa seed fields. To test whether this is a mechanism by which the disease is transmitted in commercial fields, nest cells were removed from boards and treated with a sporocidal fungicide before adult emergence, and chalkbrood levels in the progeny were quantified. Chalkbrood reduction was obtained, but it was also clear that disease transmission is also originating from another unidentified source, particularly in farm fields.