Hops Protect Honey Bees From the Scourge of Mites

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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 Cooperative Research and Development Agreement (CRADA) with an industry partner, 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.

Their studies began in 2005, when BetaTec Hop Products, Inc., a subsidiary of J.I. Haas, 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. Because the HBAs are water-soluble, they do not bind to the bees' wax or leave a residue in the comb. This means the strips can even be used during honey flows without affecting flavor or quality. The bees eventually chew up the strips, removing them from the hive. That prevents residual amounts of the miticide from remaining in the hive, which could hasten mite resistance to the compound.

DeGrandi-Hoffman was involved in all phases of the research. She constructed a mathematical model of the population dynamics of Varroa mites in honey bee colonies and used it to determine the best time of year to apply the HBAs so the mites don't overwhelm the colony.

While it is difficult to completely eliminate mites from a colony, all that is needed to protect the hives is to keep the level of mites below a certain threshold, according to DeGrandi-Hoffman. "If you can keep the mites from going into exponential growth, you're golden."

The lab also conducted experiments to determine the optimal amount of HBAs that could be used without harming the bees, as well as the right concentrations to put on the strips. BetaTec provided various formulations that were used in the process.

The lab's work guided the design of the product, which DeGrandi-Hoffman said is an effective miticide. "We need to keep coming up with new ways to control Varroa mites, and this is a tool in our arsenal."

As a result of work completed under the CRADA, BetaTec began commercializing the miticide under the brand name HopGuard. The product has been approved by the EPA for use under the Section 18 emergency use exemption 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.