With 750 million acres of forests in the United States, the scale of the problem is massive. Since 1999, the country has lost, on average, 1 per cent of its tree cover per year. This means these small insects have killed about 10 per cent of all US forests in 10 years.
Two of the bugs, says the government, have the potential to destroy $700bn (£429bn) worth of forests.
Already, one beetle – the emerald ash borer – has invaded 13 US states and two Canadian provinces. In those places, all movements of firewood are illegal and contractors who have moved logs have been fined by the courts and banned from working in the quarantined areas.
Last month, the emerald ash borer (or EAB) was identified in New York State, home to 700 million ash trees which sustain a profitable furniture industry and even provide the raw material for baseball bats. Ironically, the trees were replacements for elms killed off by Dutch elm disease. America’s chestnut trees have also suffered catastrophic damage from blight.
The borer, which comes from China, first entered America in the wood of crates shipped to Detroit in the early 1990s, but it was 2002 before it was formally identified. The tiny, creamy-white larvae bore through the bark and adults start emerging in mid-June. The larvae damage causes general yellowing and thinning of the foliage, followed by crown dieback and the eventual death of the tree. The borer has killed around 50 million trees in Michigan and tens of millions in 12 surrounding states and in Canada’s Ontario and Quebec.
Therese Poland, of the US Forest Service, said that in a bid to attract, trap, identify and monitor the insect, the service has researched the odours, or kairomones, produced by the ash; these allow the insects to identify the trees. Ms Poland said the service is hoping to find the “ultimate attractant”, and it is hoped this will prove more successful than the sticky traps hoisted in tens of thousands of ash trees across the Midwest. Another theory – not Ms Poland’s – is that purple attracts the male insect because it replicates the colour of a female’s backside.