“Candle fly?” That’s what the adult wax moth was called back when candles were made primarily of beeswax. There are several species of wax moth, but the most important is Galleria mellonella, the greater wax moth, responsible for much economic damage to bee colonies worldwide.
Adult female moths are nondescript, brown and do no damage. However, after mating, the female lays several hundred eggs, which hatch into wax-eating larvae, which can migrate from colony to colony. They bore into the comb, leaving silken galleries or tunnels in their wake, hence the name Galleria. Larvae cannot live on beeswax alone, but thrive on brood comb, which is full of impurities in the form of cast larval skins that provide a balanced diet. Thus, infestations in wax foundation are usually light. Those of dark brood comb, however, quickly result in a mass of webbing and cocoons which protect the developing wax moth larvae and pupae.
Because the wax moth is cold blooded, its development rate depends on the external or ambient temperature. In a temperate winter, most populations dwindle to small levels, resurging the next summer. The more southerly the latitude the more time the moth population is active and the less impact winter has on the insect. Indoor wintering may also result in larger populations the following spring.
Wax moth is not a disease. Infestations are the result of a colony that cannot keep the moths at bay. Honey bee colonies always have wax moth larvae present, but the population is controlled by workers that ruthlessly eject any they contact. Brood disease, queenlessness, and pesticide damage, however, can reduce a colony’s worker bee strength, and wax moth larvae are ever ready to exploit any such weakness in a colony’s defenses.
Chemical fumigation has traditionally been the best way to keep wax moths from infesting stored comb. Traditionally, several chemicals have been used to fumigate empty supers. These include ethylene dibromide (EDB), which now has no label and so cannot be employed; paradichlorobenzene (PDB), still labeled; aluminum phosphide, labeled in some states; and carbon dioxide (CO2). Only the latter can be used on comb that contains honey, but it is dangerous, difficult to handle, and costly.
Freezing can also protect comb honey (five degrees F for 24 hours will kill all stages of wax moth). Some beekeepers use cold rooms to store empty supers. The chemical of choice for fumigation at present continues to be PDB, but there remains concern that this material may be targeted for elimination by regulatory authorities.
Because of restrictions on pesticide use, there continues to be less and less the beekeeper can legally do to chemically fumigate empty supers for wax moth. Biotechnology has produced one organism, Bacillus thuringensis, which selectively controls wax moth larvae, but it has been considered too expensive to use on a large scale. However, a product called Certan continues to be on the market in some areas.
In the tropics, wax moth damage is minimized by storing empty combs outdoors on racks. This ventilates the combs and exposes the developing larvae to light, which affects them adversely. Ventilating supers by stacking them crosswise is also used by some beekeepers to reduce infestation. Heating or freezing combs and/or the use of gamma radiation might become more practical to kill the ever-present moth larvae in the future. The best storage for empty supers at the present time is on top of populous bee colonies.
Most beekeepers look at the wax moth as an arch villain. Ironically, however, it is a friend in need. Combs of colonies in the wild that have died of disease would be constant sources of infection, if the wax moth was not around to clean up the mess.