Farmers battling against crop pests will soon be helped by a new device which can predict pest outbreaks.

The Tempest pest predictor has been developed by Peter Langley, the manager of Insect Investigations Ltd, a subsidiary company of the University of Wales' College of Cardiff.

It will enable farmers to spray their crops with pesticide at exactly the right time.

"Spraying at the correct time cuts chemical usage and increases effectiveness," said Professor Langley.

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He added: "If a farmer delays spraying by just a few days he may find that there are suddenly too many insects to avoid crop damage, as some species breed very quickly."

The technology is based on the knowledge that insects will only develop after a certain number of days above a particular temperature.

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The new device, voted the best farm-related product at the International Innovations and Inventions Fair at Birmingham's National Exhibition Centre, enables farmers to measure time and temperature against the pest development threshold, and therefore to predict pest outbreaks.

The Tempest is placed in a field where the time and temperature for a particular pest is recorded by the passage of a small bubble within a silver thread. When the bubble reaches the end of the scale, it is then time to spray.

"At the moment farmers can use a thermometer, which only offers a very crude estimate, or they can buy a computer system which costs several thousands," said Professor Langley.

"The Tempest should be available by next spring and will retail at about Pounds 50."

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Available this year will be a device that will indicate when pea-moths are ready to lay their eggs. A pheromone trap, hung in a pea-field, will catch male moths looking for a mate.