Crops threatened by aphid plague 'of Biblical proportions'

GOVERNMENT scientists have issued warnings to sugarbeet farmers in England about a ''plague of Biblical proportions'' about to descend on them. A population explosion of aphids - greenfly and blackfly - has been brought about by the recent heatwave and the previous mild winter.

Their effect is likely to dwarf the insect plagues reported earlier last week, of sawfly larvae stripping trees bare on a Wolverhampton estate, and parasitic wasps causing cauliflowers to be withdrawn from supermarket shelves.

Aphids spread damaging plant viruses to valuable crops, and one estimate suggests that as much as 10 per cent of this year's sugarbeet crop will be lost to them.

Scientists are monitoring record numbers of the insect pests. One species in particular, the peach potato aphid, which attacks potatoes and sugarbeet, is more common than at any time since monitoring first began in the 1960s, said Ian Woiwood, a member of the insect survey at the Government's Rothamsted agricultural station in Hertfordshire.

''We've just never seen numbers like it. There are more than we've ever counted before, maybe tens or even hundreds of times the usual quantity,'' he said. ''It's a matter of synchronisation of crops being available and predators not being around. This year was probably just right for aphids.''

Rothamsted runs a national network of 15 traps that collect flying insects at a height of 40ft with the help of vertical ''chimneys'' attached to powerful electric pumps. The network monitors the numbers of about 300 different species of aphids.

It was a population explosion earlier this year of another species, the cabbage aphid, that caused the subsequent outbreak on cauliflowers of the parasitic wasp, which lives off it.

Mr Woiwood said the recent warm weather has been especially good news for aphids which have a prodigious capacity for reproduction. ''There are also reports of gardeners having problems with them.'' He predicted that the pea aphid will be a problem later this summer.

Ladybirds, the natural predators of aphids, suffered a setback when a cold spell earlier this year ''clobbered them'', Mr Woiwood said. ''I think they will make a comeback and exploit the amount of food there is around for them at present.''

John Maunder, director of the Medical Entomology Centre at Cambridge University, said that small, black ''thunderflies'', which swarm in millions on humid, still days, have been reported at hundreds of sites in the UK. Household fleas, he said, are also on the increase and will probably reach a peak in August.