Effect of the bee glue (propolis) on the calorimetrically measured metabolic rate and metamorphosis of the greater wax moth Galleria mellonella

Among the moth pests of the honeybee, the greater wax moth Galleria mellonella (Lepidoptera: Pyralidae) causes the greatest damage, unless controlled at an early stage, because it feeds on wax, pollen, and cocoon of the bee larvae. This leads to the destruction of honeycomb and subsequent deterioration of weakened colonies. For controlling the pest, natural products are second to none, not least because the use of synthetic substances carries with it the problem of residues, which remain in the beehive to affect the bee products. This paper reports the results of calorimetric investigations on the effects of the bee natural insecticidal glue, propolis, on pupal metamorphosis and the metabolic rate of different larval instars. Experiments were performed by batch calorimetry to record the heat flow rate of individual larvae/pupae before and after the treatment, which consisted of dipping L5, L6, and L7 instars in a graded series of different concentrations of ethanol-dissolved propolis for 30 s before blotting them. The heat production rates were then recorded for 6-7 h (short period experiment) or during the entire pupal metamorphosis (long period experiment). The fifth larval instar (L5) showed higher sensitivity to propolis treatment than L6 and L7 whereby total mortality was obtained by 4% propolis for L5 and 8-10% for the latter. The higher sensitivity of L5 can be accounted for by the very high mass-specific metabolic rate and the thinner and more fragile cuticle, typical of early larval stages, allowing the free transit of nonpolar toxic substances from the surroundings after being easily disrupted by components of propolis. The treatment of the late L7 stage with nonlethal doses of propolis shortened the duration of pupal metamorphosis significantly. An untreated larva required 6.8+/-0.8 days (mean+/-S.E., n = 5) between larval-pupal and pupal-adult eedysis, whereas this time was shortened to 5.4+/-0.9 and 4.8+/-0.5 days after treatment with 1 and 2% propolis, respectively. Though all treated larvae went through larval-pupal ecdysis, 40 and 100% of those treated with 2 and 4% propolis, respectively, displayed abortion of pupal metamorphosis and died. These results indicate that propolis is toxic at higher concentrations and an insect growth regulator at lower ones. The use of propolis in the control of G. mellonella and its subsequent occurrence in honeybee products such as honey and wax may not cause the problem of a toxic residue, as it is the natural component in the beehive. (C) 2003 Elsevier B.V. All rights reserved.