The wax moth: a forest cleaner in the disguise of a pest

You open a box of stored combs and there it is: webbing, cocoons, tunnels and writhing larvae. Wax the bees worked on for weeks turned into a matted, spun-over mass. The first thought is a curse aimed at the wax moth. And fairly so, because it has just eaten your frames. But right after that it is worth asking a question few people ask: since the wax moth has always been part of nature and bees somehow endured it for millions of years, maybe it is not only a pest. Maybe it plays a role we simply do not see when we take the colony out of a tree cavity and put it into a box.

This article is about that double face of the wax moth: how to actually protect withdrawn frames from it, but also why, in spite of everything, nature needs it, and what its presence in a living hive really tells you.

What it actually is

Under the name wax moth hide two species of moth from the family Pyralidae. The greater wax moth (Galleria mellonella) reaches up to three centimetres of wingspan and does the most damage. The lesser wax moth (Achroia grisella) is half the size and less destructive, but can also wreck stored combs.

And here the first important thing: the damage is done not by the moth but by the larva. The adult wax moth does not feed, its only task is to lay eggs in cracks or on the comb. The hatched larva tunnels through the wax, leaving behind silk cocoons and frass, and in worse cases even bores into the frame wood to pupate. Worse still, it does not spare polystyrene and polyurethane hives: the larva can chew a whole network of channels and holes in the box wall, through which wasps and other intruders then get in. That is why storing frames in plastic or polyurethane boxes outdoors, unprotected, can end not only in ruined wax but in perforated boxes.

The second important thing, which explains all the rest: the wax moth larva does not feed on pure wax. It needs protein, and it finds that in what accumulates in old, dark comb, that is in the remains of brood cocoons, in pollen, in the crumbs of chewed pupae. All that residue in a black brood comb is a feast for the wax moth, while fresh, white foundation or clean honey comb barely interests it. Remember this, because in a moment it turns out to be no accident but the heart of its role, and at the same time the key to how you store combs.

What it is for in nature: forest disinfection

Now the most interesting part, the answer to why nature keeps the wax moth at all. Picture a tree cavity in which a colony lived for a few years and finally died, or swarmed and did not return. It leaves behind old combs loaded with everything that piled up over the years: foulbrood spores that survive in wax for decades, nosema spores, remains of diseased brood. If that cavity stayed in such a state, every next swarm that moved in would inherit the whole diseased burden.

And here the wax moth steps in. The larvae break the old combs down to dust, eat the cocoons and the remains, clean the cavity interior right down to bare wood. After their work there remains an empty, scrubbed hollow, ready for a healthy swarm to settle on a clean slate. The wax moth is therefore a forest cleaner and a natural means of disinfection: it breaks the chain of disease transmission between successive generations of bees occupying the same place. This is exactly the problem we fight with comb replacement and retiring black combs. The wax moth does in the forest what you do in the apiary with your black frames, only for free and without asking, and the effect is the same, provided you are not burning the old wax but handing it back to nature.

When you look at it this way, it stops being pure evil. It is the last link in the cycle: bees build, live, sicken and depart, and the wax moth closes the loop so that new, healthy life can begin in the same place. The trouble is that in our apiary this mechanism, useful in the forest, strikes the thing we value most, that is our ready-drawn frames, especially those brooded in.

The wax moth in a living hive is an alarm signal

Since the wax moth cleans up dead and abandoned nests, its appearance in a living, covered hive is an alarm signal. And this is the knowledge that separates the beekeeper who looks from the one who merely sees.

A strong, healthy colony deals with the wax moth on its own. The bees patrol every comb, chew out eggs and larvae, throw them from the hive, and the moth has no chance to settle. The wax moth pushes in only where the colony has lost control of its wax. That is why cocoons and tunnels in a living hive are almost never the primary cause of trouble, but its most visible symptom. The wax moth does not kill strong colonies, it scavenges on those already collapsing.

Find it in the nest? Do not only ask how to fight it, but above all why the bees stopped fighting it for you. There are usually a few reasons: the colony is too weak and too small to cover and patrol all the combs; it lost its queen or has a problem with her; it is sick and weakened; or, and this is a very common case, it sits on a nest too big for its strength. That last one ties directly to the rule of nest assembly: the colony should sit on as many combs as it actually covers, because a comb no one guards is an invitation to the wax moth. In this sense the wax moth is a free test of whether your colony is matched to the space you gave it.

Protecting withdrawn frames: the specifics

After the last harvest you withdraw frames, and here the real battle begins, because outside the colony, in a warm box, the wax is defenceless. Before you reach for any method, though, remember the golden rule everything rests on: the wax moth lives off protein. Clean, white honey comb barely appeals to it, while old, dark brood comb with residue is its canteen. So old, black combs do not go into storage at all, but straight to the melter. The ones brooded once or twice, light brown, are the ones to watch: keep them apart from clean, light combs and keep an eye on them, because if a larva settles in such a comb it will move onto the light one next to it. Safest of all is to store clean, light combs, on which the wax moth will not run riot anyway.

Now the methods themselves, from the cleanest.

Freezing. The simplest and surest method for a smaller apiary. You put the frames in the freezer for at least a day or two, at around minus fifteen to minus eighteen degrees. The frost kills all stages at once: eggs, larvae, pupae and moths. After taking them out you pack the combs tightly so new moths do not lay eggs afresh. The only limit is freezer capacity.

Storing in boxes in the frost. Freezing on a large scale, for those who will not fit the apiary into a freezer. You simply stack empty boxes with frames into columns and leave them outdoors or in an unheated space for winter. A few days of hard frost does exactly what the freezer does: it kills larvae, pupae and moths. You only have to remember two things. First, it works provided solid frosts of a dozen or so degrees below zero actually come, which with today's mild winters is not always guaranteed. Second, in early autumn, before the cold hits, the wax has to be got through somehow, because in the warmth the larvae will do their damage. So many beekeepers combine methods: in autumn they knock out the first generation with sulphur or acid, and then hand the boxes over to winter.

Sulphur fumigation. The oldest and still one of the most effective methods for stored wax. You burn a sulphur wick or strip in a tightly sealed column of boxes, and the sulphur dioxide given off kills larvae and moths. Sulphur is volatile, not fat-soluble and barely soaks into wax, so it poses only a slight risk to bees and honey, which makes it relatively safe. It has one important weakness, though: it does not reliably kill eggs. So the treatment has to be repeated after about ten to fourteen days, to finish off the larvae hatched after the first fumigation. Mind fire safety and do not breathe the fumes.

Acetic acid. The classic of comprehensive disinfection. Evaporating acetic acid in a tightly sealed stack instantly kills eggs and moths, and along the way destroys nosema spores and a range of other pathogens, so it disinfects broadly. The larva, especially in the cocoon, is more resistant and needs longer exposure to the vapours, which is why the combs are treated right after coming off the colonies. There are two drawbacks: the acid is corrosive and attacks metal parts, and its vapours are dangerous to the airways. Work in gloves and a mask, in the open, and air the combs thoroughly before use.

Formic acid. Familiar to you from varroa, it works on the wax moth too, and without residue, because it simply evaporates. You place a pad soaked in acid on top of the column of boxes, and the vapours descend and kill the larvae. Professionals use it successfully precisely for stored wax. You have to watch the concentration, though, and remember that, as with varroa, formic acid used wrongly can leave a trace in wax and honey, so use it with sense.

Bacillus thuringiensis preparations. The biological finesse. This is a bacterium in the aizawai subspecies, sold under names such as Certan, Mellonex or B401 and B402, which you spray onto the comb before a problem appears. The wax moth larva, biting into the treated wax, eats the bacterial protein, which destroys its digestive system, and dies. To bees, people and honey it is harmless, it leaves no residue, and one application protects the comb for the whole storage season. There is one caveat: it is a preventive method, it works on larvae, not on eggs or moths, so you spray the comb before infestation, not after. And the second thing, honestly: the availability of these preparations varies by country and regulations, so check what is legally available where you are.

Storage conditions. The cheapest weapon, because free, and the base on which all the others work. The wax moth loves darkness, warmth, damp and stillness, that is a tightly wrapped, warm stack in the corner of a shed. Turn that against it: store combs cool, light and airy. Set the boxes so light and air pass between them, not in a closed, dark column. A tip from practice: wrapping frames in foil, against intuition, favours the wax moth, because it creates a warm, damp, dark pocket, so the more ventilation the worse for it. Below a dozen or so degrees the development of larvae practically stops.

Traps for adult moths. A simple, cheap addition for monitoring and thinning. In a bottle with a small opening you make a fermenting mix of water, sugar, vinegar and a piece of fruit or a banana peel. The smell of fermentation lures adult wax moths, which fall in and do not get out. This gives no full control and will not replace freezing or fumigation, but hung in the store or by the apiary it catches moths before they lay eggs, and tells you at once whether you have a problem at all. That, incidentally, is the only sensible use of sugar in fighting the wax moth, because dusting combs with icing sugar, contrary to what you sometimes hear, does nothing here, it is a method for varroa, not the wax moth.

Diatomaceous earth. A powder of crushed diatoms, sharp as glass on a microscale, dehydrates and kills insects that cross it. Scattered around the stack of boxes and in the cracks of the store it forms a barrier for wandering larvae and moths. It is perimeter support, not something you dust onto the comb itself, but in a dry store it works and is non-toxic to people and bees.

Monitoring and the right order. A thing that has to be said plainly: withdrawn frames almost certainly already carry wax moth eggs. So merely shutting them in a sealed box or bag protects nothing, because you shut the problem in with the comb. The order is the reverse: first you kill what sits on the comb, by freezing or fumigation, and only then seal the stack so new moths cannot get in. On top of that it is worth keeping a trap or a plain luring lamp in the store, to catch in time that something is starting.

What to avoid. Do not reach for naphthalene or mothballs. Naphthalene is toxic to bees, soaks into wax and honey and stays there, so such a comb is fit only for the melter. The same goes for paradichlorobenzene, the once-popular Para-Moth, which in the EU is withdrawn from use with bees.

Curiosities and controversies

The wax moth eats plastic. One of the most surprising discoveries of recent years. In 2017 Bombelli's team described in Current Biology that the larvae of the greater wax moth can break down polyethylene, the plastic ordinary shopping bags are made of. It is probably down to enzymes and microorganisms in their gut, the same ones that handle the long carbon chains of wax. Science is now studying the wax moth as a possible tool against plastic waste. The insect you curse over a box of frames may turn out to be an ally in cleaning up the planet, and along the way it explains why it chews through polystyrene hives so easily.

The wax moth larva works in laboratories. Galleria mellonella has become a valued model organism in microbiology and medicine. The larvae are infected with bacteria and fungi to test drugs and the virulence of pathogens, because their immune response predicts surprisingly well what happens in mammals. Thanks to this, in many studies the wax moth replaces mice, which is cheaper and more ethical. A humble apiary pest makes a quiet career in science.

The wax moth is not a cause but an effect. A controversy that divides the beekeepers who look from those who panic. The sight of silk galleries in a hive is taken as an enemy attack, whereas in a living colony it is almost always a secondary symptom. The wax moth moves in where the colony has already lost to something else: queenlessness, disease, weakness. Whoever fights the wax moth and does not look for the cause treats the symptom and comes back to the same problem in a month.

Icing sugar is a weapon against mould, not the wax moth. Worth separating, because it is easy to confuse. Some dust stored combs with icing sugar, and yes, that makes sense, but against damp and mould, not against the moth. The sugar dries the surface of the wax and hinders mould from developing on stored frames. On the wax moth it has no effect whatsoever, so if you reach for icing sugar, do it with a dry, clean comb in mind, and leave the fight with the wax moth to freezing, fumigation and the other methods in this article.

The wax moth tincture. In folk medicine, especially in the east, wax moth larvae have long been steeped in alcohol and used as a tincture, credited with various health effects. It is a hot topic, wrapped in a mix of tradition, enthusiasm and thin evidence, so it deserves its own article in which I will take apart what is real and what is legend. Here I only mention that yes, the same pest has this second, apothecary face too.

Two myths

Myth one: the wax moth attacks and kills healthy colonies. Untrue. The wax moth is an opportunist that moves in only where the colony has lost the ability to patrol its combs. A strong colony matched to its nest deals with it on its own and you will not even notice it. If the wax moth took your colony apart, it means the colony was already in trouble, and it merely finished the job.

Myth two: the wax moth is pure evil to be wiped out to zero. Also no. In nature the wax moth plays the useful role of forest cleaner and natural disinfection of abandoned nests. Your enemy is not its existence, but its presence in your frames and in weak colonies. Fight it where it does harm, that is in the store with your wax, and do not try to eradicate it from nature, because there it does work you too indirectly depend on.

Most common mistakes

Confusing effect with cause. The wax moth in a living hive is a signal that the colony is weak, sick or queenless. Fighting the wax moth alone without finding the cause is treating the symptom.

A nest too big for colony strength. A comb the colony does not cover and patrol is an invitation to the wax moth. The colony should sit on as many combs as it actually covers.

Storing old, dark combs together with light ones. A dark brooded comb is the wax moth's canteen, from which the larva moves onto the light ones. Withdraw and melt down old, black combs, and store only clean, light ones.

Wrapping frames in foil. A sealed, warm, damp pocket is a paradise for the wax moth. Store cool, light and airy, not in a closed wrap.

Sealing frames in a bag without first killing the eggs. A withdrawn comb almost certainly already has wax moth eggs, so a sealed box or bag shuts the problem inside. Freezing or fumigation first, sealing only after.

Reaching for naphthalene. Mothballs poison bees and contaminate wax and honey for good. Such a comb is fit only for the melter.

Delaying protecting the wax. The wax moth works fastest in warmth, so frames withdrawn in summer have to be protected at once, not put off, because in a week they may be gone.

Sources and further reading

• Textbook accounts of the biology of Galleria mellonella and Achroia grisella in works on bee diseases and pests. Basis for the description of the life cycle and damage.

• Protection of honey combs from wax moth damage. A review of comb protection methods: sulphur, acetic acid, formic acid, freezing, with their advantages and limits.

• NC State Extension. Wax Moth Control in Stored Equipment. On the wax moth's role in breaking down old nests and on storage conditions.

• Bombelli P., Howe C. J., Bertocchini F. (2017). Polyethylene bio-degradation by caterpillars of the wax moth Galleria mellonella. Current Biology 27(8): R292-R293. On the breakdown of polyethylene by the larvae.

• Work on Galleria mellonella as an infection model in microbiology, and manufacturers' materials for Bacillus thuringiensis aizawai preparations (Certan, Mellonex, B401, B402).