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Dr Hugh Evans, Dr David Wainhouse and Mr Nick Fielding.
Forest Research.

Owing to our island situation, Britain's forests are relatively free from the serious pests that are found in the forests of continental Europe. When foreign pests do establish themselves in Britain they have the potential to cause very extensive damage. The spruce bark beetle, Dendroctonus micans, is one such pest that is now widely established in the west and south of Britain. Because it attacks spruce, the most important commercial tree grown in the UK today, this pest could pose a major economic problem for the UK forestry industry as well as being a threat to the environment.

'The Dendroctonus beetle has probably been in the UK since the early 1970s', says Hugh Evans. 'It went undetected for a few years as it spends most of its life ""in hiding"", burrowing into the living bark of trees and destroying them from the inside. However, it soon became apparent that a long-term control strategy was needed.'

Fortunately help was available in the form of another exotic beetle, this time a predator, Rhizophagus grandis. This remarkable beetle has proven to be an invaluable biological weapon in the covert 'spruce wars' that have been ongoing in our forests for over 20 years. Dendroctonus has no natural enemies in the UK, so R. grandis had to be obtained from Belgium before a rearing and release programme could be instigated.

The successful development of this biocontrol strategy - the first in the UK using a non-indigenous predator - depended on the highly specific nature of the predator.

R. grandis effectively sniffs out its prey by detecting chemicals in the excrement - usually called frass - of the bark beetle larvae! Wind tunnel experiments show that R. grandis can detect very small quantities of the chemical 'bouquet' of the frass. As the predator cruise missile homes in on its prey it uses a combination of smell and vision to locate infested trees and the tube-like entry to the bark beetles' tunnels deep within the bark.

Once in the prey's tunnel the predator will eat some of the bark beetle larvae and wound others before laying its own eggs and departing to find yet more prey.