The Wax Moth and Its Damage

Although the wax moth has a very bad reputation, beekeeping manuals often underestimate the damage caused by the larvae of the greater wax moth Galleria mellonella and the lesser wax moth Achroea grisella, which consume everything that comes within reach of their mandibles (wax, food reserves, pollen stores, wood, polystyrene—not to mention the brood!). It is commonly assumed that strong colonies can defend themselves very easily against wax moths … but this is not always the case. In fact, wax moths reproduce very rapidly, and the larvae of the greater wax moth feed on capped brood, sheltered from counter-attacks by worker bees. As a result, the bee population may decline, as may the colony’s defences against this formidable pest. Selection for overly gentle bees and the pheromonal mimicry of the wax moth, which imitates queen pheromones, can ultimately lead to colony collapse.

Despite these harmful effects, it should be emphasised that wax moths fulfil an important hygienic function, especially in the context of “poor” beekeeping practices. They destroy abandoned bee nests and return the recovered materials to the natural cycle. In this way, pathogens responsible for diseases such as American foulbrood and European foulbrood are also eliminated.

Dr Maurice Mathis (1907–1982) was a physician who took a long-standing interest in viruses and malaria. He was also an entomologist regarded as a reference in apiculture. As laboratory director at the Institut Pasteur in Tunis, he devoted his life to the study of bees. His book Vie et mœurs des abeilles is not outdated—quite the contrary: its content and the observations on which it is based make it more relevant than ever.

All beekeepers are familiar with the wax moth through the damage it causes to combs taken out of the hive and not exposed to the vapours of a toxic gas: sulphur fumes, carbon tetrachloride, chloropicrin. All beekeepers have had to lament the loss of weak colonies almost entirely destroyed by the wax moth, yet everyone believes that the damage caused by this parasite is non-existent in populous colonies that defend themselves well. This is an error, and we shall demonstrate the considerable role played by this insect in the destruction of bees.

This parasite was very well observed by Réaumur himself, who gave it the name “wax moth” to distinguish it from mites or true moths. Modern taxonomy designates this insect as Galleria mellonella, family Pyralidae (also called “gallérie” in French).

T. L. Smith of Arkansas College, studying the rearing of the wax moth, begins his article as follows: “The wax moth Galleria mellonella is found wherever bees are kept.” This is a crucial point. The wax moth is a parasite strictly specific to hives; outside hives it is impossible for it to survive. We immediately recognise this characteristic of strict specificity common to all parasites. It follows that, if the wax moth cannot live outside hives, it has adapted in a rigorous way to its host.

Some apicultural authors of the last century recognised in the wax moth a formidable enemy, without, however, understanding it precisely. Lombard wrote in 1812: “For 15 to 16 years, this vermin has caused me to lose annually about one fifteenth of my hives.” He left us observations that are entirely accurate on the biology of this parasite. “Its moth,” he says, “enters all strong and weak hives, probably aided by the movement of its wings or by the speed of its run, for it runs rather than walks. The moth of this vermin appears around the hives as early as April, and it continues to be seen up to and including October.”

In the course of our research on the biology of bees, we had the opportunity to observe this parasite under many circumstances, but at first its role seemed to us rather insignificant. This was a serious mistake.

One “always” finds a certain number of wax moth caterpillars in the capped brood of all hives, from the strongest to the weakest.

Anyone can verify this in a very simple way. It is enough to remove a comb of capped brood from a hive after driving off all the bees, and to place it in a wire-mesh cage kept at a mild temperature: 25 to 30°C. Within a few days, one can collect dozens of caterpillars aged 10 to 15 days.

In Paris in 1941, we isolated, on 6, 13, 25 September and 9 October, frames of capped brood from different hives. We placed them in an incubator, protected from any moth; one week after their removal from the hive, all these frames were ravaged by large wax moth caterpillars.

In Tunis, a François Huber frame isolated on 19 November 1943 yielded: 31, 68, 27, 10, 3, 6, 2, 2, 1 wax moth caterpillars, i.e. 150 in total.

Another frame removed on 25 November 1943 yielded, under the same conditions, 91 caterpillars.

A very small piece of capped brood comb of 10 cm2, which we removed from a nucleus hive on 24 June 1946, released 35 wax moth caterpillars within a few minutes.

Admittedly, under natural conditions bees continually fight against attacks by wax moth caterpillars, but to what extent is this fight effective?

It can be stated with certainty that the wax moth is present in almost all phases of development (eggs, young caterpillars, caterpillars at the pre-nymphal stage inside their cocoon) throughout the year.

Another observation had always astonished us: the small size and stunted appearance of wax moth caterpillars that one tries to rear in the laboratory by giving them old combs, compared with those found in certain colonies completely destroyed by these caterpillars, which are then large, fat, flourishing to perfection, and which produce superb moths. We then undertook a methodical study of this parasite, hoping that its biology would indirectly, but with certainty, shed light on that of bees.

Biology of the wax moth

From eggs laid either inside the hive or outside, tiny caterpillars hatch, endowed with very great agility. These caterpillars actively penetrate the colony and, attracted by positive thermotropism, move into the brood-nest zone. Most of them are slaughtered by the bees, but many escape thanks to their small size. They then excavate tiny galleries between the walls of the hexagonal cells and feed at the expense of the jelly on which the small brood larvae rest. When the bee larvae are capped (eight days after the queen bee lays the egg), the wax moth caterpillars are completely sheltered from attack by the bees. The wolf is, so to speak, locked inside the sheepfold. This is an absolutely astonishing adaptation, like all those we observe in a parasite to its host. The wax moth caterpillar calmly devours the bee nymphs and, thanks to its mandibles, can easily pass from one cell to another by piercing the thin wax partition wall. Since the bee’s nymphal stage lasts 13 days, the wax moth caterpillar has ample time to complete its growth. At the pre-nymphal stage, its positive thermotropism decreases and it moves away from the brood nest. It then pierces the capping and tries to leave the hive. This stage is critical for it, because the ever-attentive bees very often slaughter it. These are the large wax moth caterpillars that one sees being expelled from the hive, carried far away by the bees.

The wax moth caterpillar then spins its cocoon, either in the hive—if the bees are few in number and leave it in peace—or between the wooden walls of the hive, or in the soil. At this stage the caterpillar has very powerful mandibles and can hollow out, even in the hardest wood, a true protective chamber.

Depending on temperature, the wax moth caterpillar may either remain for several weeks or several months at the pre-nymphal stage, or it may pupate and produce a moth within a few days. From birth, the moths mate, and the female, which needs no food, begins laying 300 to 1 000 eggs. This female is attracted by the sourdough-like odour given off by any hive in full activity and actively seeks it out, flying almost exclusively at night.

For a long time we wondered how this moth could enter with impunity a bee colony whose entrance is always guarded with great vigilance. We then thought that the moth had to lay outside the hive; this happens very often, but this mode is unusual. The moth normally enters the hive and is less repelled than we imagined. Réaumur had noted the fact and was astonished that bees made so little of an enemy that they actively pursue in the caterpillar stage. One might think that bees do not know how dangerous this moth is. The reality is quite different. One day, noticing a moth in a hive, we crushed it on the spot. How great was our astonishment to see the nearest bees rush onto this shapeless mass, beating their wings as if in the presence of a queen.

There would therefore be a phenomenon comparable to mimicry, but affecting the sense of smell—hence the name olfacto-mimicry. The adaptation of Galleria mellonella, a parasite of bees, is perfect at all stages.

It can be stated that a colony attacked by the wax moth will always be defeated and destroyed within a shorter or longer period of time. How does this destruction occur? Can the damage caused by the wax moth be assessed? How can it be controlled?

The damage caused by the wax moth

This damage varies greatly from one year to the next and depends to a large extent on the strength of the attacked colony and on the abundance of honey flows. In our view, Lombard’s estimates are far below reality.

All beekeepers have noticed, without attaching much importance to it, that a comb of capped brood always shows a certain number of holes or gaps. They thought these holes could easily be explained, either because the queen had forgotten to lay in a cell, or because the nymph had emerged. These two explanations are not valid: 1° because the queen lays in every cell without exception; it is enough to examine a freshly laid frame; 2° because a bee cannot be born in isolation since it comes from an egg laid only moments apart from those of its neighbours.

These holes or gaps are the work of the wax moth. On a frame one can count 200 to 1000 gaps on a single face of a comb, i.e. 400 to 2,000 for both faces; i.e. for 10 frames, 4,000 to 20,000. Since these gaps are renewed at each 21-day laying period, one can estimate, at the very least, between 40,000 and 100,000 as the number of bees that the wax moth can destroy in a hive without the beekeeper’s knowledge. Of course, we leave aside the colonies entirely destroyed, the fifteenth of Lombard’s stock.

This rate of destruction may seem considerable, and many beekeepers will doubt it. Let them nevertheless reflect and search their memory. Have they not sometimes been surprised by a drop in production in their apiary despite a season that promised to be particularly favourable?

A beginner beekeeper installs a swarm in a hive; he does not have much experience, yet for some years he has fine harvests and increases his stock; then progressively, despite his experience, harvests decline, bees produce fewer swarms, nothing works as before. What happened?

The first established swarm was not parasitised, then the wax moth was introduced into the apiary through a new colony in a hive. It developed little by little, increasing its damage. Of course one can fight by keeping only very strong colonies and helping them with stimulating feeding, but we know very well that sooner or later the parasite will prevail. What then is to be done?

Natural defence of bees against the wax moth.

How did bees defend themselves against the wax moth during the millennia preceding their exploitation by humans? Quite simply: by swarming.

Let us suppose a swarm that has travelled several kilometres and settles in a rock cavity or in a tree trunk. This swarm brings with it neither eggs, nor caterpillars, nor moths of the wax moth; it develops without any parasite and each year swarms once or several times depending on favourable seasons or not. At a given moment, sooner or later, a wax moth attracted by the colony will come and lay its eggs. From that moment, the colony is destined for certain destruction; its resistance may last for years, but it will ultimately succumb, the number of parasites increasing without cease.

Once the colony is destroyed and the combs completely consumed, the parasite will in turn disappear. One year, two years, or several years will pass before the rock cavity or tree trunk is again populated by a new swarm coming from far away without parasites, and the cycle begins again.

This is an entirely general phenomenon. An animal species settles in a favourable area; it develops in geometric progression; a parasite appears, it develops at the expense of the first species faster than the latter can sustain it (a new geometric progression); the parasitised species drags the parasite into its disappearance. It may happen that the parasite itself is parasitised and the cycle becomes more complex. With all these different factors coming into play, a state of equilibrium can be reached, satisfactory for the set of species as a whole. This is what always happens for the flora and fauna of a territory or a country presenting the same climate. The introduction of a new species can again break this equilibrium, which will then again tend to re-establish itself, and so on. All these biological considerations do not take us away from our subject; contrary to what one might think, they provide us with the solution to the problem.

Control of the wax moth

As we have just seen, the wax moth is the most terrible enemy of bees, but it is so well adapted to them that it is futile to believe that one will be able to destroy it in the hive. What solution should be adopted? That of the bees themselves:

1° Never establish an apiary near an existing apiary;

2° Create an apiary isolated from all hives—one must reckon 500 to 1000 m, the flight capability of the wax moth—in which only naked swarms or swarms on new foundation wax are installed;

3° Create, from this first apiary, a second apiary, still distant, again with naked swarms;

4° Completely destroy the first apiary at harvest time two, three, or four years after its establishment, depending on the degree of its infestation by the wax moth.

The technique we recommend is the best and the only one, even if it may seem to some to be impracticable.

If we examine the old practices of beekeeping, we find that two methods effectively fight the wax moth without knowing it: suffocation and transhumance.

Suffocation.

In certain beekeeping regions, such as Brittany and the Landes, where beekeeping is widespread and all farms have about ten colonies housed in baskets that are populated each year with natural swarms, it is customary to suffocate with sulphur fumes a certain number of colonies and harvest them entirely. This very old practice, which gave excellent results both from the point of view of harvest and of maintaining the stock, was prohibited in France in 1942. We shall not discuss here the appropriateness of this law, perhaps taken a little hastily and without sufficient consideration of the biology of bees, but we shall point out that this suffocation had as its immediate consequence a massive and annual destruction of a great quantity of wax moth caterpillars. We shall see whether the future will prove us right and whether the prohibition of suffocation will increase the winged stock or reduce it.

Transhumance.

We saw in the previous chapter what transhumance consists of. One of the major advantages of this technique, which had gone unnoticed, was the reduction in the number of wax moth caterpillars.

1° All caterpillars buried in the soil could no longer infest the hives in the form of moths because the latter were several kilometres away;

2° During transport, the caterpillars present in the hives were strongly excited by the vibrations of transport and tended to leave the colonies. The colonies that arrived at their new location were therefore, individually, very strongly deparasitised.

The lesser wax moth — Achroia grisella

Everything we have just said also applies to the lesser wax moth. It causes the same damage, but its small size makes its discovery much more difficult. We shall therefore not dwell on it unduly.

In summary, the wax moth is the most terrible enemy of bees; it does not attack wax as was believed and is still believed, but the brood itself in all its forms: eggs, larvae, nymphs. The nymph corpses that one sees being expelled by bees were killed and more or less gnawed by wax moth caterpillars. Very often one finds young bees without wings or with mere stumps, queens in their cell with the same lesions; all these disturbances are the work of destruction by wax moths.

Of course, a strong colony defends itself and repairs the damage, but it nevertheless suffers a major weakening; any colony whose population happens to drop as a result of untimely swarming and which cannot fight effectively is destroyed within a few days. This is the usual way of dying for all colonies in Tunis, where the wax moth causes even greater damage than in mainland France, because of its incessant activity: winter diapause does not exist.

We believe that this study of the wax moth and its damage, which we are the first to have developed, will render the greatest services to practical beekeeping.

Source: Maurice Mathis, Vie et mœurs des abeilles, chapter XII : « La fausse teigne et ses Ravages [archive] », Payot, Paris, 1951 (OCLC 6456072).

Ask your questions here