Beetle larvae love the glue lining of museum paintings


Entomologists and art restorers at a museum in Taiwan studied the infestation of more than 80 paintings by drugstore beetles (Stegobium panic), in hopes of developing better pest management and conservation practices. Shown here is an adult pharmacy beetle viewed from above (A) and below (B) and a larva (C). (Image originally published in Lee et al 2021, Journal of Economic Entomology)

By Ed Ricciuti

Ed Ricciuti

Ed Ricciuti

The little beetle Stegobium panic has more aliases than Jesse James, as he is said to eat anything but cast iron. Its taste for medicinal herbs and pharmaceutical products has earned it the name of pharmacy beetle. It is the officially recognized name in the Entomological Society of America’s Common names of insects list, but that’s just to start. It is also sometimes called the biscuit beetle as well as the bread beetle; indeed, its species epithet, or second part, of its scientific name, comes from the Latin word meaning bread. Another of its names: cereal beetle. In fact, his love of cereals in all their forms has made him the bane of art museums around the world. If given the chance, the tiny creature gorges itself on the flour-based glue paste that art restorers traditionally apply to the back of canvas to preserve oil paintings, a habit Taiwanese scientists have now examined in hopes of reducing the high cost of damage and even loss of valuable art that results from the beetle’s meal.

During their research, described in a report published in December in the Journal of Economic Entomology, the Taiwanese science team not only quantified the damage, but found that when drugstore beetles smash paints, they create a mini-world that supports other insects. Some of these insects also attack pulp, while another can be a boon to museums as it appears to parasitize beetles.

“We hypothesize that the beetle larvae burrowing into the paint coatings not only caused direct damage, but also provided a microenvironment for secondary pests, such as woodlice (Liposcele),” write the researchers from National Chung Hsing University in Taichung City, Taiwan, and the Chimei Museum in Tainan, Taiwan. They also found Pteromalid wasps, which they believe can parasitize the beetle, and Limothrips thrips, crop pests that may have strayed from farmland a few hundred meters from the Chimei Museum, where the paintings were stored.

Pharmacy Beetle Artwork Damage

When drugstore beetles (Stegobium panic) infest a work of art, they tend to attack the middle support layer, feeding on the glue paste that adheres to the support canvas liner on the back of the original painting. Damage visible from the outside may include exit holes where adult beetles emerge through the gummed brown paper tape that seals the sides of the artwork (A) and cracks in the paint layer (B ) due to damage to the canvas underneath. (Image originally published in Lee et al 2021, Journal of Economic Entomology)

In 2008, the museum was hit by a severe beetle infestation that damaged more than 80 of the 1,400 Western works in its art collection. The researchers focused on three badly damaged paintings to confirm the identity of the responsible insects and to assess the pattern of damage in order to further develop control and monitoring methods. As is typical of museum maintenance and restoration of paintings, each work had three layers of support: the original canvas, a layer of glue reinforced with a sheet of gauze, and the new lining canvas on the back. paint. The back of the canvas had been lined with glue paste and the sides sealed with gummed brown paper tape.

The beetles have attacked the glue paste that adheres to the backing canvas liner of the original painting. On the commercial side of the artwork, the layers of paint were not damaged directly, but the damage to the canvas from behind caused the paint to peel off.

The team zeroed in on the smallest details of the damage caused by the beetles, whose tiny but powerful mandibles can tear even foil. Two types of beetle damage were observed: larval feeding activities and adult exit holes. The researchers measured the tunnels dug by the beetle larvae as they fed and the holes through which the adult beetles emerged from the web after the pupae.

The work was extremely detailed, as the scientists measured and recorded the thickness and texture of the liner canvas, the glue layer and the original canvas and the damaged area of ​​these three layers. In addition, to understand the food preferences of infesting insects, the damaged area of ​​each layer was measured and compared.

“Quantification of damage patterns – including losses of original web eaten by beetles, direction of damage in the gauze, and distribution of damaged areas – provides us with the basic model for developing targeted restorative treatment,” says Wen-Yuan Lee, conservator at the museum, Ph.D. student in fine arts and lead author of the study.

Adds fellow researcher Hou-Feng Li, Ph.D., of National Chung Hsing University, “The correct identification of the main pest and biological control of glue-pasted paint in Taiwan is crucial for planning the following integrated pest management strategies. ”

Scientists have found that drugstore beetle larvae mostly burrow into the original glue layer and web and require, on average, only 6 cubic millimeters (about 1.2 thousandths of a teaspoon) of food to go from egg to nymph. The larger the larvae grew, the more glue paste they burrowed into the original web. Their bores were not evenly distributed and most were in the shaded area covered by the stretcher and outer frame. When the researchers examined the exit holes, the team found that the inside of the support layer was hollowed out and filled with feces and powdery beige-colored excrement.

Female beetles usually lay eggs on or near their food. Not even a millimeter long, emerging from the egg, the larva feeds and begins to nibble. It uses food materials to make a cell in which it pupates for at least nine days. The glue paste from an oil painting could provide enough food for many beetles, research has found, suggesting that once a painting becomes infested it could become a major source of further infestation.

Even after pupation, the drugstore beetles pierced the lining canvas or gummed tape, and only a few of them emerged through the front paint layer. The researchers speculated that the inorganic nature of the grounding — a base coat of pigment applied to the face of the canvas before painting began — discouraged beetle feeding.

During the study, the scientists collected all the insects found in the painting, of all stages, namely egg, larva, pupa and adult. Insect species were identified on the basis of their morphological characteristics and their genetic sequences. “Correctly identifying insects and quantifying damage patterns,” they write, “will aid in the development of pest tracking tools, infestation prevention methods, monitoring strategies, as well as restoration techniques. of relevant paint”.

Ed Ricciuti is a journalist, author and naturalist who has been writing for over half a century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His missions have taken him around the world. He specializes in nature, science, conservation issues and law enforcement. A former curator at the New York Zoological Society, and now the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.


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