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?

The wax moth

Adult wax moth (Galleria mellonella): nocturnal moth

Galleria mellonella is also referred to in some regions of France as “galerie”, as well as “wax moth” or “greater wax moth”. The term “moths” is often used to encompass both the greater wax moth, Galleria mellonella, and the lesser wax moth, Achroia grisella.

The greater wax moth (Galleria mellonella) is a lepidopteran species of the family Pyralidae, found throughout Europe. Its larva is referred to as a caterpillar, while the pupal stage is called a chrysalis. This nocturnal moth flies from May to October and has a wingspan of 30 to 40 mm.

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

Adult Achroia grisella (lesser wax moth)

The wax moth is a formidable pest strictly specific to beehives. It has specialised over the course of evolution and can no longer develop outside hives. As a result, a certain number of wax moth caterpillars are almost always found in the capped brood of all colonies, from the strongest to the weakest.

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

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

All beekeepers have observed her jerky run, constantly changing direction and interspersed with short flights in all directions when they attempt to crush her on a frame. The imago is not actively pursued by worker bees, as it secretes pheromones that mimic the presence of a queen. The adaptation of Galleria mellonella through sophisticated olfactory mimicry is remarkable at all stages.

Biological cycle of the wax moth after S. Boucher

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

The imago (adult moth) does not feed, and its mandibles are reduced to rudimentary structures. The adult phase is very short (around two weeks) and is devoted solely to fertilisation and reproduction through intensive egg laying.

While the adult moth does not feed, the caterpillar is extremely voracious. Using its sharp mandibles, the larva consumes everything in its path: residues at the bottom of brood cells, pollen, wax, honey, larvae, wood, and even polystyrene from mating nuclei. Its rapid growth allows it to reach several centimetres in length, doubling its weight each day during the first ten days after hatching.

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

Although worker bees rarely attack the adult insect, which emits queen-like pheromones, the caterpillars are subject to vigorous attacks and are killed by repeated bites and stings. Young caterpillars concentrate on brood maintained at 34 °C, attracted by thermotropism.

They protect themselves by burrowing grooves at the depth of the cells, beneath the food intended for the brood (royal jelly, then a honey/pollen mixture). Once the cell is capped, they are completely sheltered and can feed undisturbed.

They consume brood food, wax, cocoon residues left at the bottom of cells after several generations of bees, and the brood itself. This explains the appearance of so-called “bald brood” in straight lines, where the capping is missing and the larval head is visible, with the body partly eaten away.

When caterpillars reach their maximum size at the pre-nymphal stage, about 15 days after hatching, they become less dependent on temperature and leave the brood nest. Bees then increase their aggressiveness and kill many of these larger, less agile caterpillars.

Those that survive spin elongated, very robust silk cocoons fixed in grooves, corners or cracks, often aligned side by side, sometimes under the hive floor or even in the ground at the base of the hive. The chrysalis eventually metamorphoses into an imago more or less rapidly (1–9 weeks), depending on climatic and temperature conditions. The wax moth does not merely consume everything it encounters; it can also spread highly contagious and serious diseases, such as American foulbrood, throughout an apiary. A hive weakened by wax moth infestation is very often robbed by workers from neighbouring colonies, leading to the transmission of contagious diseases through robbing and drifting, which poses a serious apicultural health problem.

From an entomological perspective, the wax moth is considered a cleaning or scavenger insect of dying or abandoned hives. While a strong colony can effectively defend itself against wax moths, a weakened colony cannot. Areas of the hive little or not visited by workers are ideal for wax moths. Abandoned combs are quickly cleaned down to the metal wires. If a swarm leaves its parent colony and is not recovered by the beekeeper, it will build combs in the wild and revert to a feral state. Without varroa treatment, such a colony will disappear within two years, leaving ample time for the wax moth to parasitise and completely destroy the combs as the colony declines. Through remarkable evolutionary adaptation, the wax moth has thus become something of the “waste collector” of hives, penalising any negligence or lack of attention on the part of the beekeeper.

In 2017, an international team of researchers reported in the scientific journal Current Biology that wax moth caterpillars are also capable of degrading polyethylene (annual production of 100 million tonnes, about half of all plastic packaging produced in 2019), thanks to the catalytic properties of their digestive proteins. Through its evolution and adaptation, the wax moth has developed an astonishing ability to digest wax, whose chemical components are hydrocarbons similar in structure to plastics. This capacity to consume plastic opens new avenues of research into the biodegradation of the worrying accumulation of plastic waste, particularly in the oceans.

From a practical point of view: what can be done in beekeeping to avoid wax moth infestation?

Regarding the hive:

As bees are effective enemies of caterpillars, beekeepers should maintain particularly strong colonies whose workers occupy all available space in the hive. Special attention should be paid to mating nuclei and small colonies, which require regular feeding.
Never leave combs or wax in an uninhabited hive, as eggs that are always present may hatch as soon as temperature conditions become favourable.
Clean inner covers and top bars of frames (grooves for metal wires).
Regularly clean varroa drawers and remove any caterpillars found there; also inspect screened bottom boards, especially their edges, where pupae often lodge. Crush all moths flying nearby.
Regularly renew comb wax, as wax moths are attracted by residues left at the bottom of cells after several brood cycles. After three years of good service, a frame is generally darkened and often deformed by drone comb.
Melt down these old combs quickly, especially if they contain pollen, as well as even newer ones that are heavily infested.
In the event of massive infestation, eggs present in the wood must be destroyed, either with a blowtorch, by sulphur fumigation, or by freezing (–20 °C for several hours).

Regarding stored frames:

Sort frames, separating old, high-risk frames from freshly built ones that have not housed brood or served as food reserves (pollen).
Carefully inspect honey supers and meticulously remove any pollen that bees may have inadvertently stored in some cells.
Treating brood or honey frames by freezing at –20 °C for 48 hours eliminates all stages of the wax moth (eggs, caterpillars, pupae, imago).
Store frames stacked in columns in cool, bright and well-ventilated places that present very unfavourable conditions for caterpillar development. Brood frames are more difficult to store in a “chimney” configuration; they can be kept in boxes or airtight cabinets after freezing.
Heat treatment at 46 °C is also effective, but the risk of wax melting is a constraint for foundation or built combs.
A biological treatment by spraying Bacillus thuringiensis on frames is also possible. The expiry date must be strictly observed, as this Gram-positive bacterium with insecticidal properties has a limited lifespan. Its action against caterpillars lasts several months. Potential side effects for beekeepers and impacts on ecosystems are still poorly documented. Due to stricter authorisation conditions for biocides, the sale of Mellonex was suspended in September 2014; reintroduction is currently under way.
Chemical treatment by vaporisation of acetic acid (100 ml per 50 l of storage volume) or formic acid (40 ml at 85 % per 50 l) is applied above stacked frame “chimneys”. Inhalation of vapours can cause serious pulmonary intoxication and requires the use of a respirator. These treatments must be applied repeatedly for optimal effectiveness. Naphthalene (paradichlorobenzene) and sulphur must be strictly avoided for both brood and honey frames; sulphur is generally reserved for colony elimination.

► Read also:

Fact sheet “Wax moth”

Article: Protection of frames against wax moth

Bibliography

https://www.2imanagement.ch/fr/divers/liens/wwwapisavoirch/la-fausse-teigne-et-ses-ravages-

https://www.apiservices.biz/fr/articles/87-le-pas-a-pas-la-fausse-teigne-ou-papillon-de-la-ruche

https://www.agrireseau.net/apiculture/documents/PR%C3%89DATEURS.pdf

http://gdsa27.free.fr/spip.php?article104

http://rucherecole68.thann.free.fr/Echo/themes/la_fausse_teigne.pdf

https://fr.wikipedia.org/wiki/Bacillus_thuringiensis

https://fr.wikipedia.org/wiki/Poly%C3%A9thyl%C3%A8ne

Maurice Mathis, Vie et mœurs des abeilles, Chapter XII: “The Wax Moth and Its Ravages”, Payot, Paris, 1951 (OCLC 6456072).