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.