Essential oils show promise in killing the parasitic Varroa mites that devastate crop-pollinating honeybees, but a successful delivery system for them has yet to be found, according to USDA scientists at the Carl Hayden Bee Research Center in Tucson.

Losses caused by the invasive, blood-feeding mites to honey bee colonies is a serious challenge to California almonds, as well as other crops in the state and around the world that depend on the bees for pollination.

Researchers say a single Varroa mite can shorten the lifespan of its bee host by one-third, and two can shorten it by one-half. The bees' overwintering stage is the most vulnerable to the mites. Annual economic losses to managed bees from Varroa range from 25 to 80 percent, and feral bees are also impacted.

One of the USDA researchers, Gordon Wardell, gave a progress report at the recent 33rd annual almond industry conference recently in Modesto. The Almond Board of California hosted the conference and contributes funding for the projects discussed.

In detailing his work, Wardell said “beekeepers need immediate relief from Varroa infestation and the development of new control methods is crucial.”

While essential oils, or extracts from plant parts, are known to control Varroa, the barrier has been finding an effective delivery system. For example, Apivar, a commercial product based on thymol and camphor, produces vapor that kills Varroa on adult bees but its effects vary with environmental conditions, making results unpredictable.

Contact phase

Wardell said they wanted to take a step further in reaching the mites within the bees' systems. “We are now looking at a contact phase, which is a little more difficult. We need something that the bee larvae will pick up and carry to the mites,” he said. The idea is to get the essential oil into the blood of the bees and repel the mites from feeding.

They tried a syrup containing essential oils, but it was unsuccessful because the bees stored it instead of consuming it.

The project continues with a protein liquid diet carrying the essential oils in suspension. Wardell and others will use additional gas chromatograph studies to trace movement of the oils in the bees.

Wardell reported on a separate project on delivering essential oils of thymol, origanum, clove, and cinnamon microencapsulated in cornstarch. It is being carried out by Fabiana Ahumdada-Segura of the University of Arizona-USDA, who was unable to attend the conference.

The process for microencapsulation turns food-grade cornstarch into a microcellular gel that the essential oils are then incorporated into. The product is ground to the fineness of pollen and mixed with powdered sugar.

Wardell said this form was chosen to approximate the materials, such as powdered sugar with antibiotic, commonly used in “dusting” by beekeepers.

The research continues with trials on various concentrations of oils, and Ahumada-Segura has reported that she anticipates one or more of the oils will reduce brood cell invasion or mite reproduction.

Bee nutrition

On another front against Varroa, Allen C. Cohen, also a scientist at the Hayden laboratory, is a member of a group observing nutritional supplements to improve vigor in honey bees as a way to resist mites and microbial infestations they carry.

“A healthier insect is less susceptible to disease than one that is malnourished,” he said.

The project has four objectives, the first being to find nutritional deficiencies in almond pollen. They are analyzing the composition of the pollen and adding protein, lipid, mineral and antioxidant compounds to correct deficiencies.

A second objective is to determine the uptake of iron, calcium, and other nutrients and how they are taken up and move in bees.

The third objective is to chart the bees' blood chemistry to see what makes up a healthy colony, including examination of the structure that filters bacteria, viruses, and fungi that may enter the insects' stomachs.

The fourth objective is testing nutritional supplements in almond orchards to optimize delivery of the supplements.

Gloria DeGrandi-Hoffman, research leader at the Hayden laboratory, said improved bee nutrition is key in dealing with the Varroa problem and they hope to use it in developing a sustainable solution.

DeGrandi-Hoffman also stressed that time is critical in finding the solution in a very few years to protect the beekeeping industry and the crops the bees pollinate.

Compound studied

She reported on her project, funded by both the Almond Board of California and the California Beekeepers Association, on research with microencapsulated 2-Heptanone, the compound most effective against Varroa. It occurs in various spices, cheese, fruits, and breads.

However, she said, “2-Heptanone evaporates quite rapidly, and that is both a good thing and a bad thing,” she said. Its rapid volatilization is good because it disappears and leaves no residue to cause resistance in the mites, but the drawback is it vanishes from colonies before the month of exposure needed for effect on the mites.

Her effort was to develop a delivery system for the compound in a biodegradable form that would be placed in the colony once, instead a treated plastic strip that would have to be placed and later removed by beekeepers.

The first prototype was the encapsulated substance in a mixture of cornstarch and paraffin pressed into a tablet or a strip coated with beeswax or paraffin. “As the bees chew on the outside of the delivery system, they slowly release the 2-Heptanone,” she said.

The next prototype combined the active ingredient in powdered form with powdered sugar. Current tests are evaluating rates of the substance for the greatest control with powdered cornstarch, soy flour, and other materials.

The trials thus far suggest that although the bees might be grooming the powder from their bodies, enough 2-Heptanone may remain in the colony to contribute to mite mortality.