10 April 1999

SCENT OF SEX

IN THE AIR…

Natures way may be best, but sometimes it

needs a boost as Lucy Stephenson reports.

SPEED is of the essence when it comes to keeping any pest or disease problem under control. But natural enemies such as parasitic wasps which attack aphids are not known for their prompt action, often arriving in crops later than their hosts.

So how can this be improved? Scientists speaking at the Society of Chemical Industry conference identified location as the key. For natural predators to have a controlling effect on any aphid invader they must be present at the same time – and same place – as the pest.

One way to do this is harness the aphids own sexual allure system – pheromones. These chemical attractants are produced by female aphids to lure males during their sexual phase in the autumn. Yet these very chemicals attract parasitic wasps too. They can therefore be used to trap the natural predators where and when they are needed, explained Lester Wadhams and Wilf Powell of IACR Rothamsted.

This idea of being able to synchronise the presence of natural enemies with the pest all started when Dr Powell began to look at how parasitic wasps overwintered.

By chance, two mild winters were followed by a hard winter in his investigation. "We expected aphids to be a big problem after the mild winters – but the opposite happened. When I looked at the aphids I found over 50% were carrying parasitic larvae."

The presence of the parasitic wasps gave a better understanding of the predators life cycle and is the key to putting the science into a practical context.

Since the parasitic wasps live inside the aphids, spraying in the autumn to control the pest on winter crops wipes out the natural predator too, pointed out Dr Powell.

However, by diverting the predators into field margins where they are not affected by autumn aphid sprays, they can survive and be ready to attack aphids in the summer.

Parasitic wasps that attack cereal aphids commonly move into grassland. If they can be lured into field margins or conservation headlands which are next to the crop, using the aphid pheromone, their populations can be encouraged – making them ready to take on the aphid invaders later in the season.

A key finding in the research was that parasitoids respond to pheromones throughout the year, and they can be lured to an infested crop at any time.

If aphid numbers can be checked early by the waiting parasitic wasp population, later arriving natural predators such as the ladybird will be able to help mop up, explained Dr Powell.

A chemical similar to the aphid sex pheromone has been isolated in catmint (Nepeta cataria). Extracting this chemical is 100 times cheaper than artificially synthesising the aphid pheromone. So Dr Wadhams is now investigating how to grow catmint as a commercial crop.

How would the catmint extract be used? Although precise application rates and timing are still being worked on, Dr Wadhams predicted that a capsule containing the pheromone could be applied at generous spacing in the field margin. The capsule would be effective for six weeks.

Such a method of control would be attractive for growers in countryside stewardship schemes which have stricter controls on pesticide use.

Field trials are now being planned to put the theory into practice, but the small scale trial results have been encouraging. "Aphid parasitisation was increased five to 10 times when potted wheat plants were tagged with the pheromone," said Dr Powell.