The wax moth

A nightmare for some, an opportunity for others, an incredible hope for visionaries, the wax moth is an insect that intrigues all those interested in its evolution and its perfect adaptation to the hive and its superorganism. There are therefore three ways of viewing the wax moth: the beekeeper fears the damage it causes in hives; the entomologist marvels at the undertaker role played by the insect; researchers, for their part, focus on its highly ecological ability to digest highly polluting plastic. So what is this curious moth?

Wax moth

Adult wax moth (Galleria mellonella) : nocturnal moth

Galleria mellonella is also called “gallérie” in some regions of France, “wax moth”, or “wax moth of the comb”. People often simply speak of “moths”, thereby grouping together the greater wax moth caused by Galleria mellonella and the lesser wax moth caused by Achroia grisella.

The (greater) wax moth (Galleria mellonella) is one of the lepidopteran species in the Pyralidae family living throughout Europe. Its larval stage is called a caterpillar, whereas the nymphal stage is called a chrysalis. This nocturnal moth, which flies from May to October, has a wingspan of 30 to 40 mm.

The lesser wax moth (Achroia grisella) is about half the size of the first species. Its small size makes it difficult to detect, but the damage it causes in the hive is identical to that caused by Galleria mellonella.

Adult Achroia grisella (lesser wax moth)

The wax moth is a formidable pest strictly specific to hives. Over the course of evolution, it has become so specialised that it is now unable to develop outside hives. To such an extent that a certain number of wax moth caterpillars can practically always be found in the sealed brood of all hives, from the strongest to the weakest…

The 4 developmental stages of the wax moth : the egg, the caterpillar, the pupa or chrysalis, and the moth.

After mating, which takes place during a nocturnal nuptial flight in the forest near the apiary, the adult female is attracted by the odour of the colony and enters the hive thanks to the speed of its movement once it has landed at the entrance.

All beekeepers have noticed its jerky run, constantly changing direction and alternating with short flights in every direction when they try to crush it on a frame. The imago is not particularly pursued by the worker bees because it secretes pheromones simulating the presence of a queen. The adaptation of Galleria mellonella through elaborate olfactory mimicry is perfect at all stages…

Biological cycle of the wax moth according to S. Boucher

Once inside the hive, the fertilised female begins laying between 300 and 1'000 whitish eggs, 0.1 mm in size, grouped in clusters. Very small caterpillars emerge from these eggs in 5-15 days if the temperature exceeds ~9°C. Depending on thermal conditions, the eggs or caterpillars at the pre-nymphal stage may either overwinter for several weeks or several months, or pupate and produce moths within a few days. The adult females are fertilised by the males outside the hive, and the cycle can begin again.

The imago (adult moth) does not feed, and its mandibles are reduced to a rudimentary state. The adult insect stage is very short (~2 weeks) and is intended solely for mating and then reproduction through intensive egg laying.

Although the adult moth does not feed, the same is absolutely not true of the caterpillar, whose voracity is striking from a biological point of view. Thanks to its sharp mandibles, the larva devours everything it finds in its path : residues at the bottom of brood cells, pollen, wax, honey, larvae, wood, the polystyrene of mating nucs… The caterpillar’s rapid growth enables it to reach a length of several cm, doubling its weight every day during the first 10 days after hatching!

Laying of hundreds of eggs grouped in a cluster

This incredible growth rate explains why the wax moth can destroy all the combs of a weakened hive in 10 to 15 days.

Large caterpillar shortly before the metamorphosis phase

Although worker bees attack the adult insect little or not at all, as it emits pheromones similar to those of a queen, the caterpillars, by contrast, are subjected to vigorous attacks and are killed by repeated bites and stings. The young caterpillars concentrate on brood reared at 34°C because they are attracted to it by thermotropism.

They protect themselves by digging furrows at the depth of the cells beneath the food (royal jelly, then a honey/pollen mixture) intended for worker/drone brood. Once the cell is capped, they are completely sheltered and can feast in complete safety.

They devour the brood food, the wax, the cocoon residues that remain at the bottom of the cells after several generations of bees have emerged, and the brood itself. This explains the presence of “bald brood” in straight lines, with the capping missing and revealing the head of the larva whose body has been partly eaten away deeper down.

The caterpillar digs a network of galleries to find its food. It lines these tunnels with silk, which serves as protection against pursuit by the worker bees. The damaged cells are beyond repair, and the entire frame eventually becomes unusable. The larva then moves on to neighbouring frames, building silky bridges that obstruct the bees’ passage.

“Bald brood” forming straight lines

If it has a choice, the caterpillar will settle preferably on a brood frame lined with pollen stores. Otherwise, any wax comb will do. For its development, the wax moth can establish itself both in occupied hives and on frames stored for winter (super frames after extraction or brood chamber frames loaded with food reserves). When conditions (temperature, humidity, light, etc.) are not suitable for proper development, the eggs or larvae are able to delay their development for several weeks. This explains why a frame taken out of a hive may at first sight appear free of wax moth. In reality, if it contains even a few eggs—and it almost always does—these may hatch several weeks after the frame has been stored and contaminate all neighbouring frames during the bad season.

As it passes, the moth leaves a large quantity of wax moth frass in the form of small black striated pellets, for example on varroa inserts and on frames, betraying its presence in the hive.

Network of galleries lined with protective silk

Once the caterpillars have reached their maximum size, at the pre-nymphal stage about 15 days after hatching from the egg, they are less dependent on temperature and leave the brood nest. The bees then become even more aggressive and kill large numbers of these big, less agile caterpillars.

Those that survive spin elongated silk cocoons, very strong and fixed in grooves, corners or cracks, often aligned side by side, sometimes under the insert or even in the ground at the foot of the hive. The chrysalis ultimately metamorphoses into an imago more or less rapidly (1-9 weeks), depending on climatic and temperature conditions. The wax moth does not merely devour everything that comes within reach of its mandibles. It can also spread very contagious and very serious diseases, such as American foulbrood, throughout an apiary. A hive weakened by wax moth is very often robbed by worker bees from surrounding colonies. The transmission of contagious diseases through robbing and drifting then becomes a real beekeeping health problem.

The caterpillars have devastated an entire frame before the chrysalides metamorphose sheltered in their silk cocoons

The entomologist considers the wax moth to be a cleaning insect or grave-digger of dying or abandoned hives. Indeed, while a populous colony defends itself effectively against the wax moth, this is not the case for a colony weakened for any reason. Corners of the hive little or not at all visited by worker bees are ideal for wax moths. Abandoned combs are quickly cleaned down to the metal wires. If a swarm leaves the parent colony and is not recovered by the beekeeper, it will build its combs in nature and revert to a wild state as a feral colony. This colony will not benefit from varroa treatments and will disappear within 2 years. That gives the wax moth ample time to parasitise the combs and destroy them completely when the colony declines. Thus, through remarkable adaptation over the course of evolution, the wax moth has become something of the garbage collector of hives. In this sense, it punishes every mistake made by a beekeeper who is not conscientious or not sufficiently present in the apiary.

In 2017, an international team of researchers reported in the scientific journal Current Biology that the caterpillar of the wax moth could also degrade polyethylene (annual production 100 million tonnes, i.e. 1/2 of all plastic packaging produced in 2019) thanks to the catalytic properties of its digestive proteins. Over the course of its evolution and adaptation, the wax moth has developed a remarkable capacity to digest wax, the chemical components of which are hydrocarbons close to the structure of plastic. This ability to consume plastic opens a new avenue for scientific research into the biodegradation of the worrying accumulation of plastic waste, notably in the oceans.

The wax moth thus joins the bacterium Flavobacterium sp. KI72 (a nylon eater), Ideonella sakaiensis, an aerobic bacterium discovered in 2016 that enables the degradation of PET, Pestalotiopsis microspora, a fungal species capable of decomposing polyurethane, and the mealworm beetle (Tenebrio molitor), a coleopteran insect fond of cereal flours whose larva can eat expanded polystyrene…

Polyethylene…

And now, in practical terms, what should be done to prevent wax moth in beekeeping ?

Regarding the hive :

Because the bee is an effective enemy of caterpillars, the beekeeper must maintain particularly strong colonies whose worker bees occupy all the available space in the hive. Special attention should be paid to mating nucs and nuclei, which must be fed regularly.
Never leave combs or wax in an unoccupied hive, because eggs that are always present may hatch as soon as temperature conditions become favourable.
Clean the crown boards and the upper bars of the frames (presence of a groove for the passage of the metal wires).
Clean the varroa inserts regularly and remove any caterpillars found there; also check the open mesh floor, especially its edges, where pupae frequently lodge. Crush all moths flying nearby.
Renew the combs of the frames regularly, because the wax moth is attracted by the residues present at the bottom of the cells after several brood cycles. After 3 years of service, a frame will certainly have turned black and may even be deformed by drone comb construction.
Melt down these old combs quickly, especially if they contain pollen, as well as more recent ones that are heavily infested.
In the event of a massive invasion, the eggs present in the wood must be destroyed, either with the flame of a blowtorch, by sulphuring, or by freezing (-20 degrees for several hours).

Overwintering of supers

Regarding stored frames :

Sort the frames by separating the old high-risk frames from freshly drawn frames that have neither contained brood nor been used as a food reserve (pollen).
Examine the super frames carefully and meticulously remove any pollen that the bees may inadvertently have stored in certain cells.
Treating brood chamber or super frames by freezing them at -20°C for 48 hours eliminates all stages of the wax moth (eggs, caterpillars, chrysalis, imago).
Store the frames stacked in columns in cool, bright and ventilated places that provide very unfavourable conditions for caterpillar development. Brood chamber frames are more difficult to store in a “chimney” arrangement. After freezing, they can be kept in airtight boxes or cupboards.
Heat treatment at 46 degrees is also effective, but the risk of wax melting is an obstacle for foundation or drawn combs.
A biological treatment by spraying Bacillus thuringiensis on the frames is also conceivable. The expiry date must be strictly respected, because this Gram+ bacterium with insecticidal properties has a limited lifespan. Its action against caterpillars extends over several months. The side effects of this treatment on the beekeeper have been little analysed, and its possible impact on the ecosystem is currently poorly described. Because of a tightening of the conditions for the authorisation of biocides, the sale of Mellonex was suspended in September 2014. Reintroduction of the product is under way…
Chemical treatment by vaporising acetic acid (100 ml per 50 l of storage volume) or formic acid (40 ml at 85%/50 l) is applied above the storage “chimneys” of the frames. Inhaling the vapours of these products can cause severe intoxication of the pulmonary system and requires the wearing of a mask. Moreover, these treatments must be applied several times for optimum effectiveness. Naphthalene (paradichlorobenzene) and sulphur must be completely avoided, both for brood chamber frames and for super frames. Sulphur is usually reserved for the elimination of colonies.