Results of Research: Using Essential Oils for Honey Bee Mite Control

West Virginia Unniversity

Extension Service

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Jim Amrine, Bob Noel, Harry Mallow, Terry Stasny, Robert Skidmore
(Last Updated: December 30, 1996)

We have found that several essential oils can either kill, or adversely affect varroa mites.

Essential Oils have Two Modes of Action:

1) Toxicity by direct contact:
When varroa mites contact essential oils such as wintergreen, patchouli, tea tree oil, et al., mixed into oil or grease, they are killed on contact--usually within a few minutes.

2) Impaired reproduction via feeding syrups containing essential oils:
When varroa mites feed on larvae that contain essential oils, their reproduction is interrupted. If the oil is strong enough, the females are unable to lay eggs. If the oils are in lower concentration, eggs are layed, but development of immature mites is delayed; young mites do not reach maturity before the bees emerge from the cell; consequently, the immature mites die.

Involvement of Essential Oils in Impaired Reproduction of Varroa Mites: Syrup containing the essential oils is fed at the hive entrance or in the broodnest. Many bees feed on the syrup and pass the essential oils around by trophalaxis (adult bees sharing their food reserves). The syrup and essential oil is ingested by nurse bees and enters the communal food in the crop and passes into the milk glands. When the nurse bees feed larvae, the essential oils are in the bee milk and communal food and are ingested by the larvae. Thus, when female varroa mites feed on treated larvae or larval food at the bottom of the cell, they ingest the essential oils which adversely affect their reproduction. The probable mechanism is interference with enzymes in the complex gestation (especially in the production of nutrients and new proteins) of the oocyte and embryo-larva of the varroa mite. Research needs to be conducted to verify this hypothesis and to verify the presence of the essential oils in bee larvae and ultimately, in the female varroa mites.

Impaired reproduction is not observed when canola oil, mineral oil, or shortening (eg., Crisco, a vegetable lard) containing essential oils are delivered to the hives. The fats and greases do not enter the food chain as readily as syrups, and the amounts of essential oils ending up in larval food or in the larvae themselves are inconsequential. Thus, there is no interruption of the development of mite eggs or of immature varroa mites. The mites that directly contact these materials rapidly die; but others are able to escape the essential oils in grease or canola oil by entering cells of mature larvae that are about to be capped, or by moving onto displaced nurse bees (see below, "Recent Findings") near the top of the colony, where the grease patties and tracking strips are not placed. We found that putting paper towels soaked in canola + essential oils in the tops of colonies from July to September, kills the varroa mites residing on the displaced nurse bees which congregate in the upper supers of large colonies.

Feeding of sugar syrup with essential oils at the entrance, or in the brood nest, places the essential oils into the food chain and prevents oviposition by female mites or retards the development of immature mites in capped larval/pupal cells.

We had several colonies that were treated with tracking strips and grease patties only, and we saw resurgence of varroa mites, especially when bee populations were at their peak, lots of brood was present, and when the bees occupied many supers as well as two brood chambers. However, we also had several colonies that were treated with the tracking strips and grease patties, and were continually fed syrup + essential oils at the entrance; in these colonies very few varroa mites were found. Those few that were found appeared to have come into the colonies on drifting bees.