Munching moths and parasitoid wasps

So far we’ve focused on specific objects housed in museum collections which use animal bodies. Last episode, when we were discussing the life and afterlife of the Oxford dodo, we talked a little bit about the processes of degradation which museum objects often go through in their museum lives. Despite our best efforts, objects, particularly those made from organic materials, are often susceptible to processes of decay. This got me thinking more about what factors might cause animal objects to change or degrade when placed within museums – in the case of the Oxford Dodo, the precise reasons for the loss of most of the dodo’s body was a bit unclear, but in my PhD research on featherwork a topic that often comes up is the control of “pests” on the objects – that is, insects which eat the objects and cause damage. 

The focus of my PhD isn’t really in the conservation of objects, so honestly I’ve never really given the exact identity of these so-called pests much thought. So today we are going to talk about one of the most common insects which can cause trouble for museum collections: the webbing clothes moth. In Animal Highlight fashion we are of course going to think beyond the very anthropocentric lens of the “pest” and instead look in detail at the ecological interactions this amazing animal takes part in within museum collections.

Webbing clothes moth (Tineola bisselliella) adult Image: Ben Sale

Webbing clothes moths

The webbing clothes moth or Tineola bisselliella is a pretty small and assuming insect. About 5mm long, and pale greyish yellow in colour, they probably aren’t the first species you would name when placed alongside the other more eye-catching members of the butterfly and moth family. Yet they are probably one of the moth species we have the most contact with. If you are like me, you may have some prejudices against them due to their annoying tendency to munch through your favourite knitted jumper, but in reading about these amazing animals more, and their relationship to museums and objects, I’ve really come to appreciate them and their incredible adaptability that enables them to be such destructive house guests. 

It isn’t actually the moths in their adult, winged form which cause a problem for our jumpers and our museum collections. The adult moths themselves don’t eat at all (they don’t even have mouths!), but in their larval form, webbing clothes moths can cause significant damage to anything made from keratinous materials. Whilst it can be extremely annoying to find a hole in your favourite garment, the fact that these moths can feed on keratin is pretty impressive and uncommon in the insect world. Keratin is a hard substance to digest and is resistant to most common digestive enzymes, but scientists believe that webbing clothes moths have adapted specialised enzymes to be able to thrive off a keratin only diet, even being able to metabolise their own water as a bi-product of digestion, so they can even thrive in relatively dry conditions compared to many other insects who have higher humidity requirements.

This proclivity for keratin-based materials, and a tolerance for low humidity of course makes the webbing clothes moth quite an impressive nuisance for anyone trying to preserve museum collections. They are one of the most common museum “pests” and will indiscriminately eat anything keratin-based in a collection. This includes textiles, especially woollen items, but also things made from materials like hair, fur or feathers. They will also happily munch on items made from cotton or linen if they are dirty with food, sweat or animal oils that might make them tasty. Museums with densely populated displays like the cases at the Pitt Rivers Museum where I am based are especially vulnerable and munchable objects are often unfortunately the most delicate.

Interestingly, they don’t seem to have been a problem for European museums or homes for all that long, only really becoming a proper concern in the 19^th century. Their numbers seem to be on the rise and they can be found in human dwellings all over the world. They are classed as a “synanthropic” species, which are organisms which live alongside humans and benefit from the environments we create. Places like our homes and our museums.  Unlike other related moth species, webbing clothes moths seem to pretty exclusively thrive in urban or at least domestic environments. They don’t seem to survive well in the winter conditions of Northern Europe, and they don’t seem to take up residence in places other moth species do, like bird’s nests, which are also rich in organic matter.

We still aren’t entirely sure how webbing clothes moths spread to human establishments across the world. They are certainly very successful cosmopolitan creatures.  One theory is that they derive from South or Central Africa and may have been transported to Europe and the rest of the world through the movement of trading ships sometime in the 19^th century. One naturalist aboard the HMS challenger in 1823 for example spends some time in his account of the voyage complaining about several of his garments being destroyed by moths. Given their taste for animal matter, it is also highly possible that the practice of rich travellers taking part in big game hunting in Africa may also have contributed to the spread, the transport of animal trophies bringing hidden munching passengers with them. 

However they got here, they are now thriving and seemingly increasing in number, posing a growing problem for museums wanting to preserve their collections and protect them from the damage that these munching caterpillars cause. 

Webbing clothes moth larvae feed on keratin materials. Image: Commonwealth Scientific and Industrial Research Organisation

Why are moth numbers increasing?

There probably isn’t a simple answer to why webbing cloths moth numbers are rising in museum collections. The changing climatic conditions in museums might be one reason – whereas previously museums often tended to be chilly places where you might need to walk around in a coat and scarf in order to stay comfortable, today most museums tend to be a bit better heated, which is great for your comfort as a visitor, but is also great for the moths who like warm, cosy, dark spaces. 

Equally, changing uses in pesticides have likely contributed to increased numbers. In the past, museums would use some really harmful chemicals to try and prevent pest damage to objects. Substances like arsenic or mercury for example, which may have had the desired effect on killing the insects eating the objects, but also unfortunately probably impacted the health of anyone working with the collections. Nowadays, there is a much greater awareness of the risk posed to museum workers by using such harsh and dangerous chemicals. Care  and harm works both ways in museums, people and objects can mutually cause a risk to each other and past practices give many objects the potential to harm people handling objects today. When I handle most featherworks, especially older objects, I will be wearing protective gloves and often a mask in order to protect myself from things like mercury contamination. While good for our health as museum workers, the reduced use in carcinogenic pesticides also has the impact of increasing insect presence in the museum. 

What to do without pesticides?

So, if mercury and arsenic poisoning are out, how do you manage moths eating their way through your precious collections? This was the part of this research I found the most exciting honestly. 

Most museums today acknowledge that complete eradication of “pest species” is not a realistic endeavour, and instead implement an Integrated Pest Management System. This is based on measures that reduce the desirability of the environment for species which might damage collections, and trying to limit the opportunities animals have to feed, breed and grow. In general terms, this means monitoring population levels and implementing good cleaning practices which reduces the amount of organic material rich dust hanging about that the moths find so tasty. 

This also means using some other more specific methods which range from the romantic comedy to the horror show in theme:

Pest confusion 

One way museums attempt to limit the moths from breeding is by using a “pest confusion” strategy. This is a non-toxic treatment which seeks to purposefully confuse the moths when they try to seek out breeding partners, using a treatment called “Exosex”. This works by luring the male moths into a trap using female moth pheromones. As they leave the trap, the male moths are coated in the female pheromone themselves. The male moths are then not only completely overwhelmed by the female pheromones that they can no longer detect genuine female moths themselves, but in turn they also attract other males to them, meaning males are less available to meet with genuinely viable females, resulting in fewer fertilised eggs and therefore hopefully fewer larvae to munch on your feather collections. The adult moths only live a few weeks, so the hope in doing this is that you break up the breeding cycle and overtime reduce moth populations without using toxic pesticides. 

Parasitoid wasps

Parasitoid wasp eggs can disrupt the breeding cycles of other species Image: Dr Victor Fursov

If the pest confusion treatment was the romantic comedy side of moth control, the other technique recently being developed perhaps falls more under the horror category. More recently, museums have been experimenting using a different form of biological pest control, by perhaps somewhat counter-intuitively introducing more insects to the museum through the use of parasitoid wasps. 

Parasitoid wasps honestly deserve their own full episode, they are such interesting insects. Nearly all orders of insect and arachnid in the world will have a corresponding species of parasitoid wasp, and the eggs and larvae of the Lepidoptera family (the moths and butterflies) seem to be especially vulnerable to them. Parasitoid wasps are super important for us as humans, as they play a really important role in damaging insect populations within our agroecological system. 

The tiny wasps work by depositing their own eggs into other insects. The strategy varies between wasp species, some will deposit eggs into the larvae or adult stages of insects, others, like the wasp which targets webbing clothes moths, will oviposit into the moth eggs. Then, instead of a moth larvae hatching, the wasp larvae will emerge, eat the moth egg and eventually develop into an adult wasp, before hopefully breeding to continue the cycle and then dying. 

It might be worth saying that this whole alien-esque sequence occurs at a completely microscopic level. The 0.5mm wasp lays its eggs into the moth eggs which are only about 0.3mm in length. The wasps are therefore completely unnoticeable to the visitors in the museum, and provide an effective and fascinating form of moth population control which avoids using harmful toxic chemicals. The technique of purposefully introducing parasitoid wasps is growing in usage within the heritage sector, and big organisations like the National Trust have been implementing them as part of their Integrated Pest Management strategies. 

"Think like a pest"

I’ve really enjoyed delving deeper into the world of webbing clothes moths in museums. Something which I’ve previously only thought about in terms of “pests” which cause a headache to museum workers and damage objects are actually amazingly impressive animals which have adapted some really clever mechanisms in order to do the very damage we resent so much. 

I sort of love the idea that as you walk around a museum a really complex world of pheromones, sexual confusion and parasitism might be taking place at a microscopic level within the cases and store rooms around you.

One conservator once said to me that in order to get on top of insect population control in museums you need to “think like a pest”. While of course, “pest” is a really anthropocentric and limited view of the lives of webbing clothes moths, I like this idea of thinking like a moth, and that the solution to managing object damage in museums without toxic chemicals has been to take an approach which fits quite well with Animal Studies, in that conservators have had to think creatively and enter into the complex lives of the moths to find more creative solutions for managing moth populations. 

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Rosa Dyer is a Collaborative Doctoral Project PhD candidate at Birkbeck College University of London and the Pitt Rivers Museum, University of Oxford. Her practice-based project at the Pitt Rivers Museum focuses on featherwork collections made by South American Indigenous peoples. Her work aims to reveal the dynamic relations that exist between birds, people and environments by working with Indigenous collaborators to reimagine how the feathered objects are represented in the museum. You can connect with Rosa via Twitter (@rosajdyer).