Well-known as a key ingredient of beer, hops are used to balance the sweetness of the malt ingredient and provide aroma. Thanks in part to scientists at the Agricultural Research Service (ARS) in Tucson, Arizona, hops are also protecting the nation's honey bees from their most dangerous foe.

Varroa mites, tiny parasites that feed on the bees' blood-like hemolymph, wipe out nearly one-third of the nation's private and commercial bee colonies each year. The danger to American agriculture is significant. Honey bees pollinate more than $15 billion worth of crops annually across the country, according to the U.S. Department of Agriculture (USDA).

Although synthetic miticides have been in widespread use, Varroa populations have become resistant to the chemicals, prompting beekeepers and scientists alike to seek alternative controls.

As part of a partnership with BetaTec Hop Products, Gloria DeGrandi-Hoffman and her colleagues at the ARS Carl Hayden Bee Research Center in Tucson demonstrated that hop beta acids (HBAs) can be used safely and effectively as a miticide - now commercially available under the brand name HopGuard.

Their studies began in 2005, when BetaTec approached the lab looking for a use for HBAs, a byproduct of processing hops for beer. BetaTec knew that HBAs can kill mites on plants, and wanted to determine whether the acids might also work for Varroa mites.

Initial petri dish experiments were promising, so the lab entered into a CRADA with BetaTec to expand the studies, conducting colony trials in Arizona and California.

In the process, the researchers tried different application methods, ultimately settling on HBA-coated cardboard strips hung from a frame in the hive's center. Bees walk on the strips, pick up the HBAs, and spread them among the adult population of the hive through bee-to-bee contact. Mites on bees carrying the HBAs die and drop off.

The trials showed that, while lethal to Varroa mites, the HBAs had no adverse effect on worker bees, their queens or brood, and are environmentally benign. 

HopGuard has been approved by the EPA for use in selected states, and BetaTec has started a registration process that will enable HopGuard to be used to control Varroa mites in honey bee colonies throughout the U.S.