Activity of various essential oils against Bacillus larvae White in vitro and in apiary trials (Activité in vitro sur bacillus larvae White de plusieurs huiles essentielles et essai d'utilisation au rucher)

The action of 21 essential oils against six strains of Bacillus larvae White, the causal agent of American foulbrood of honeybees, was studied in vitro. In a preliminary trial (table II) the effectiveness of either pure or diluted (1:5) oils was assessed by the diffusion agar method (Carta et al, 1989). The more effective ones were then tested in nutrient broth, at doses ranging from 50-400 mg/kg, to determine the minimal bactericide and sporicide concentrations. The most significant results were obtained with the oils of Citrus sinensis, Cinnamomun sp, Cuminum cyminum, Eugenia spp, Thymus vulgaris and reconstituted oil of Verbena (tables III and IV, fig 1). The same oils proved to be non-toxic for adult honeybees at the dose of 400 mg/kg. Cinnamomun oil was the most effective, with minimal bactericide and sporicide concentrations of 50 and 100 mg/kg respectively. This oil also gave positive results in controlling the American foulbrood in field trials, particularly in autumn and winter, when administered in semi-solid food (honey + easter sugar, 1:1.5) at a concentration of 400 mg/kg. In particular, the action of Cinnamomun oil on disease growth was evaluated in two distinct trials using four and five nuclei, respectively, obtained from the same colony (Apis mellifera ligustica Spin) so that each of them contained an approximately equal amount of adult bees, brood, honey and pollen beside a young queen (Liendenfelser, 1968). In both experimental trials, the worker brood was also carefully examined periodically by laboratory tests (Lloyd, 1986). The results showed that during autumn and winter, when the bees' consumption of the extra food is favoured by the environmental conditions (weather, scarcity or absence of flowers), Cinnamomun oil acts also in vivo on B larvae. However, its use in the control of American foulbrood poses several practical problems including the time and the method of administration to take full advantage of the bactericidal properties found in vitro. Ii essential oils can be used against American foulbrood, the toxicological risks and the establishment of undesirable resistance factors associated with use of chemicals would be avoided.