Compound Safely Quells Bee-Killing Chalkbrood

From rabbits to horses to cows, many animals love alfalfa. America's premier pollinator of that crop, the alfalfa leafcutting bee (Megachile rotundata), is vulnerable to a deadly fungal disease called chalkbrood. But the bees might be best protected from chalkbrood if their leafy nests are sprayed with an iprodione fungicide, according to Agricultural Research Service (ARS) entomologist Rosalind R. James.

Caused by the Ascosphaera aggregata fungus, chalkbrood kills bees while they're larvae—wormlike young that hatch from eggs laid in nests by female bees.

Healthy larvae spin cocoons within those nests, and later emerge as young bees.

But chalkbrood-infected larvae may die before cocooning, according to James. She leads the ARS Pollinating Insect Biology, Management and Systematics Research Unit in Logan, Utah.

Microscopic spheres, called fungal spores, on dead larvae serve as potent reservoirs of the disease. A healthy female alfalfa leafcutting bee may—after emerging from her cocoon and nest in spring—inadvertently pick up some of those spores. If she spreads them to nests that she makes for her eggs, she may doom her young.

James worked with alfalfa seedgrowers in Washington to determine how to best protect alfalfa leafcutting bees from chalkbrood. The disease is so pervasive in the United States that these seedgrowers buy at least 50 percent of their alfalfa leafcutting bees each year from Canada, where chalkbrood is less prevalent.

In experiments, James sprayed an iprodione fungicide on the leafcutting bees' nests in spring, shortly before the adult bees left their cocoons and nests. The treatment reduced the incidence of chalkbrood in the bees' next generation by up to 50 percent, with no measurable loss of young, James reported.

Now, James and her colleagues are looking for fungicides that may be even more effective.

The research is part of ongoing studies to discover more ways to safeguard wild bees, so they can help America's harried honey bees with pollination chores.

Read more about this research in the February 2008 issue of Agricultural Research magazine.

ARS is the U.S. Department of Agriculture's chief scientific research agency.