A Taste for Paste: Beetle Larvae Fond of Museum Paintings’ Glue Lining
By Ed Ricciuti
The little beetle Stegobium paniceum has more aliases than Jesse James, due to the fact that it is said to eat everything but cast iron. Its taste for medicinal herbs and pharmaceuticals gives it the name drugstore beetle. That’s the name officially recognized in the Entomological Society of America’s Common Names of Insects list, but that’s just for starters. 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 for bread. Another of its names: cereal beetle. In fact, its love for cereals in any form makes it the bane of art museums worldwide. Given the chance, the tiny critter gorges on the flour-based glue paste that art conservators traditionally slather on the back of canvas to preserve oil paintings, a habit that Taiwanese scientists now have examined in hopes of decreasing the high-cost of damage and even loss of precious art that results from the beetle’s repast.
In the course of their research, described in a report published in December in the Journal of Economic Entomology, the Taiwanese scientific team not only quantified the damage but found that, when drugstore beetles butcher paintings, they create a mini-world that supports other insects. Some of these insects also attack the paste, while another actually may be a boon for museums because it seems to parasitize the beetles.
“We speculate that beetle larvae boring into paint linings not only caused direct damage but also provided a microenvironment for secondary pests, such as booklice (Liposcelis),” write the researchers, from the National Chung Hsing University in Taichung City, Taiwan, and the Chimei Museum in Tainan, Taiwan. They also found Pteromalid wasps, which they believe may parasitize the beetle, and Limothrips thrips, crop pests which may have strayed from farmland a few hundred feet from the Chimei Museum, where the paintings were stored.
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 homed in on three severely damaged paintings to confirm the identity of insects responsible and to assess the damage pattern for further development of control and monitoring methods. As is typical of painting maintenance and restoration by museums, each artwork had three support layers: the original canvas, a layer of glue paste reinforced with a gauze sheet, and the new lining canvas at the back of the painting. The back of the canvas had been lined with glue paste and the sides sealed with gummed brown paper tape.
The beetles attacked glue paste that adheres the supportive canvas lining to the back of the original painting. On the business side of the artworks, the paint layers were not damaged directly, but damage to the canvas from behind caused the paint to crumble off.
The team focused on minute details of the damage caused by the beetles, whose tiny but powerful mandibles can tear through even tinfoil. Two types of beetle damage were observed: feeding activities of larvae and exit holes of adults. The researchers measured the tunnels bored by the beetle larvae as they fed and the holes by which adult beetles exited the canvas after from pupae.
The work was extremely detailed, as the scientists measured and recorded the thickness and texture of the lining canvas, glue paste layer, and original canvas and the damaged area of these three layers. Furthermore, to understand the feeding preferences of the infesting insects, the damaged area of each layer was measured and compared.
“The quantification of the damage patterns—including losses of original canvas eaten by beetles, damaged direction in gauze, and distribution of damaged areas—provide us the basic model for developing targeted restoration treatment,” says Wen-Yuan Lee, a conservator at the museum, Ph.D. student in fine arts, and lead author on the study.
Adds fellow researcher Hou-Feng Li, Ph.D., of the National Chung Hsing University, “Correct identification of the major pest and biological control agent of glue paste-lined painting in Taiwan is crucial for planning the following integrated pest management strategies.”
The scientists found that the drugstore beetle larvae mainly bored into the glue paste layer and original canvas and required, on average, only 6 cubic millimeters (about 1.2 thousandths of a teaspoon) of feed to grow from egg to pupa. The larger the larvae grew, the more it burrowed from the glue paste into the original canvas. Their bores were not evenly distributed, and most were found in the shaded area covered by the stretcher and outer frame. When the researchers examined the exit holes, the team discovered that the inside of the support layer was hollowed out and filled with beige-colored powdery frass and feces.
Female beetles typically lay eggs on or near their food. Not even a millimeter long, on emerging from the egg the larva homes in on food and starts munching. It uses food materials to make a cell in which it pupates for at least nine days. The glue paste of one oil painting could provide sufficient food for many beetles, research indicted, suggesting that, once one painting is infested, it could become a major source of further infestation.
Even after pupation, drugstore beetles bored through the lining canvas or gummed tape, and only a few of them came out through the paint layer at the front. The researchers speculated that the inorganic nature of the grounding—a base layer of pigment applied to the face of the canvas before the actual painting is started—discouraged beetle feeding.
During the study, the scientists collected all insects found in the painting, of all stages, namely egg, larva, pupa, and adult. Insect species were identified based on both their morphological characteristics and gene sequences. “Correct insect identification and damage pattern quantification,” they write, “will contribute to the development of follow-up pest control tools, infestation prevention methods, monitoring strategies, as well as relevant painting restoration techniques.”
Journal of Economic Entomology
Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments 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 at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.