Efficacy of thymol in control of the fungus Nosema ceranae in Africanized Apis mellifera

Abstract

Nosema ceranae is an obligatory parasite of the honeybee midgut. It destroys the epithelial cells, negatively affecting food digestion and assimilation, and impacting bee development and colony survival. Antifungals such as fumagillin effectively control N. ceranae but can be toxic to humans who consume honey from treated hives, and are prohibited in many countries, including Mexico. Essential oils from plants are promising alternative antifungals. An evaluation was done of the efficacy of the essential oil thymol in controlling N. ceranae in Africanized Apis mellifera colonies over a four-week period. A total of 56 colonies were distributed in three experimental groups: G1) 18 colonies treated with fumagillin (25.2 mg fumagillin/week); G2) 19 colonies treated with thymol (66 mg thymol crystals/week); and G3) 19 untreated colonies (control). Infection levels (N. ceranae spores/bee) were estimated in 60 adult bees from each colony. Fumagillin (G1) reduced infection levels from 123,529 to 1,805 spores/bee (95.2 % efficacy). Thymol (G2) reduced infection levels from 133,438 to 28,099 spores/bee (31.1 % efficacy). Infection levels also declined in the control group (G3), from 119,306 to 36,447 spores/bee. The clearly higher efficacy with fumagillin compared to thymol highlights the need for further trials to test different thymol concentrations, and administration frequencies and times. Under the present study conditions thymol was not effective against N. ceranae, but the pressing need for non-toxic antifungals for use in Africanized A. mellifera colonies in the tropics makes research on thymol and other essential oils imperative.