Heatwave helps the plant pests to flourish High cost of controlling balsam and knotweed

While the hot weather continues, Himalayan balsam and Japanese knotweed are largely defying efforts to control or eliminate them.

Conservation organisations and local authorities are faced with vast and rising bills as the aliens colonise river banks and marshy ground.

Eliminating the more pernicious of the two, Japanese knotweed, can cost (pounds) 100,000 a hectare and a recent government estimate suggested a minimum price tag of (pounds) 1.56bn to control knotweed in the short term.

However, not everyone is unhappy about the aliens. Eric McArthur, editor of Scottish Beekeeper, said the Himalayan balsam, which is hugely successful along Glasgow's Clyde, Kelvin, and Cart rivers and the many urban streams and marshy places, had proved a boon to the UK's bees and beekeepers, both of which are under threat.

Mr McArthur rejected fears expressed by some conservationists that the Himalayan balsam has such an attraction for bees, both domesticated honey bees and their wild cousins, that the insects will ignore nearby native plants, reducing their success.

''I suggest that those who want to eliminate these species should stop and think again. The knotweed attracts bees and other insects but it is so damaging to bankside habitats, eliminating other species, that efforts to control it are probably justified,'' he said.

''The balsam, however, spreads by seeding annually, so can be much more easily controlled. It flowers from the beginning of July right through to the end of September and provides an invaluable nectar source. It now plays a very important role as a food source for bees, and any beekeeper who has a stand nearby is lucky indeed.''

Mr McArthur believes that bees and other insects are now seriously threatened because of generations of reliance by farmers and gardeners on insecticides and pesticides. With many nectar sources gone and species such as bumble bees now officially threatened, the Himalayan balsam's elimination should be reassessed, he said.

He pointed to other invaders which had become vital spring food sources for bees and insects, including the lime tree - brought back to the UK by the Crusaders - and the sycamore, another alien from the south.

''Bumble bees are a threatened species which need all the help they can get. We require to protect the natural environment but we should recognise there are alien species which are helpful as well,'' he said.

A Scottish Wildlife Trust spokesman said yesterday that dealing with Japanese knotweed, which spreads by means of rhizomes - underground stems that can be up to seven metres in length - was the ''weeding job from hell''.

Duncan Clark, SWT assistant site manager at Cumbernauld, said: ''It is on the rampage throughout most of Scotland, so the key to managing it is to be constantly vigilant and to have a planned and co-ordinated approach to tracking and eradicating it.''

Keith Geddes, conservation officer for Glasgow City Council, said: ''The fact that these aliens thrive on our river banks reveals an underlying problem of the high nutrient levels in our waters.

''Reducing the nutrients we put into our watercourses would reduce the problems of invaders.''

Yesterday, meteorologists predicted even higher temperatures for the weekend.

Temperatures should reach 26C in Glasgow, 34C in London, 31C in Birmingham and 24C in Belfast.

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growing concern

Himalayan balsam, impatiens glandulifera, is the most widespread and abundant of three impatiens in the British Isles, although it has a restricted habitat in the Himalayas.

It was introduced to Britain in 1839 as a hothouse annual, but gardeners discovered it could thrive outdoors.

An attractive plant which can reach two metres, its ''policeman's helmet'' flowers produce explosive pods which spread down rivers with ease.