The museum whose insect collection is being eaten ... by insects

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To the human eye, the display cases of iridescent butterflies and delicate blooms in the archives of the Natural History Museum are objects of interest and beauty. To Anthrenus sarnicus, a tiny beetle just two millimetres in length, they represent something else - dinner.

The curators of Britain's leading repository of plant and insect specimens revealed yesterday that the diminutive bug is the voracious vanguard of a creepy-crawly army that is threatening the nation's botanical and zoological treasures.

Staff are fighting a daily battle to halt the progress of Anthrenus sarnicus - one of a species aptly called museum beetles - and a cast of other hungry creatures hell-bent on munching their way through cases of rare vegetation and delicately preserved creatures.

Experts at the museum estimate that up to 12,000 plant specimens and several thousand insects have been eaten or damaged over the past 20 years, despite stringent efforts to stamp out the invaders.

The collection of 34 million plants and insects urgently needs to be moved from its home in the spectacular but outmoded grade I-listed museum, which opened in 1881. If all goes to plan, it will be transferred to a £68.5m glass and concrete "cocoon" that will form the second phase of the museum's Darwin Centre, a state-of-the-art and bug-proof display and research facility.

But with the project not expected to open until 2007 and £36.5m of funding still to be raised, the managers of the vast botanical and entomological collection are having to cope with unwelcome lodgers capable of wriggling and burrowing their way through a creaking infrastructure.

Graham Pellow, the director of the Darwin Centre scheme, said: "Most museums have an insect or insects which are eating its insects. In nature, these animals are looking for dead and dried material that forms their favourite food. Of course, that is exactly what we have in our collections. We have to manage it as best we can with a regimen to monitor the specimens, but it is a mammoth task - you cannot individually check 28 million insects and six million plants."

It is the store-rooms and research areas of the museum - far away from the public displays of dinosaur bones and stuffed animals - that form the front line in the war against pests. Measures such as freezing specimens at minus 30C for three days to curtail any infestation have been introduced as part of a system called in museum jargon IPM, or integrated pest management. But for the museum beetle, along with its specimen-nibbling confrères such as the cigarette beetle, the common silverfish and even a colony of mice, the Victorian building still provides plenty of hiding places and opportunities for a good meal.

Despite efforts to protect the collection further by placing it in sealed cabinets, the size and determination of creatures such as Anthrenus sarnicus mean that they can crawl through the tiniest gap and settle down to a banquet.

Others, such as the cigarette beetle, which has a taste for dead plants, paper and even the glue used to secure items to paper, make their way into the folios of plants in the museum's herbarium.

Mike Fitton, head of the entomology collections, housed on three floors of a 1930s building reeking of mothballs (despite the fact that they have not been used for 15 years), held up a glass-fronted wooden box as an example of the damage that can be caused. Inside, the pins and tiny labels remained undisturbed but the collection of a dozen or so moths that it once held had been reduced to a dusty jumble of wing remnants and bits of antennae.

Dr Fitton said: "This is as bad as it gets. Fortunately, it is an extreme case, but in a building like this it can happen. We have no segregation between the collection and where we work and if it gets hot we have to open the windows, so the pests can fly in."

The problems are exacerbated by the fact that the museum is banned from using the pesticides that once helped to control the bugs, after a European Union ruling that hundreds of common chemicals, now out of patent, should be tested. No commercial producer is willing to go through the expense. Even then, Anthrenus sarnicus, which was first discovered in the museum in 1963, has been found to be resistant to the active ingredient in mothballs.

Senior managers at the museum point out that the estimated attrition rate to the botany and entomology collections of 0.2 per cent over 20 years means that they are not in any general danger.

But as the guardian of thousands of "type specimens", the original samples of a species collected by naturalists including Charles Darwin and Sir Hans Sloane, the 18th-century collector whose specimens started the museum, there is a risk of significant damage. More than 300 scientists work in the museum, relying on its archives for vital samples to develop new medical treatments.

Rob Huxley, a senior botanist at the Natural History Museum's herbarium, said: "We have samples from the first voyages of Cook and Darwin. All it takes is for an infestation in a 400-year-old flower and a unique type specimen will have been lost. Thankfully, that has yet to happen."