Biological activity of grapefruit leaf burning residue extract and isolated compounds on Varroa jacobsoni

The varroa mite, Varroa jacobsoni Oud. (Acarina: Varroidae), is the most se rious parasite of honey bees, Apis mellifera L. (Hymenoptera: Apidae) in th e U.S. Varroa immatures and adults feed on bee hemolymph, causing morpholog ical abnormalities and transmitting debilitating viruses. Without control e fforts by man, varroa eventually destroy the colony within ca. 2 years. App lication of smoke from burning of grapefruit leaves over varroa-infested ho ney bees has been shown to cause biological activity on the varroa: after e xposure, varroa detach and fall from their honey bee hosts. Such detachment of varroa and falling to the hive floor during cooler weather could provid e substantial control of the varroa, since many varroa are unable to crawl back to the cluster of bees. We investigated the compounds involved in prod ucing this biological activity by grapefruit leaves. Extracts mere made of whole leaves, smoke collections, and residue from the burning chamber. Bioa ssay of these extracts indicated that the;residue from the burning chamber contained the compounds most active in varroa detachment. Analysis of this residue by gas chromatography/mass spectrometry revealed a number of compou nd classes, particularly phenolic compounds. Bioassay of these individual c ompounds showed that the phenolic 3-methylphenol was most active in causing varroa to detach from bees in the laboratory in both indirect and direct e xposure bioassays. Discussion is given on the role of other phenolic compou nds in the control of arthropod pests.