Dispatches from the killing fields

 作者:舜莲     |      日期:2019-03-08 06:02:00
By David Concar PERHAPS the most serious charge levelled against crops engineered to produce insecticidal toxins is that they will poison beneficial insects as well as wiping out pests. The latest findings will fuel the debate over the environmental safety of these crops by giving both sides more ammunition. One unpublished study, which looks at the impact on insects of a bacterial toxin engineered into maize, suggests the toxin’s effects mysteriously increase as it passes along the food chain. But another team is disputing earlier claims that ladybirds are harmed by plants that produce a protein toxic to aphids and other sap-sucking pests. Experts can’t agree on exactly what the two studies mean—particularly since any harmful effects of insecticidal GM crops must be balanced against the probable benefits of reduced pesticide use in fields where the plants grow. Everybody accepts that plants producing insecticidal toxins will reduce the number and nutritional value of the pests that beneficial insects feed on, but confusion surrounds the extent to which the toxins poison predatory insects directly. At the Swiss Federal Research Station for Agroecology and Agriculture near Zürich, Angelika Hilbeck and her colleagues say they have found evidence confirming that lacewings, which eat caterpillars and aphids, can be poisoned by an insecticidal toxin engineered into maize. The gene for this toxin, called Bt, comes from the bacterium Bacillus thuringiensis. Hilbeck raised a red flag about the effects of Bt toxin on lacewings last year (This Week, 2 May 1998, p 21). In later experiments, her team fed identical quantities of purified Bt toxin directly to lace-wing larvae or via caterpillars that had consumed the toxin. Fifty per cent more lacewings died after eating the caterpillars. Hilbeck believes that the toxin became more potent, perhaps because its chemical structure was altered. Her results may require changes in the way that biotech firms test for any “collateral damage” their crops might cause. They tend to feed the engineered toxins directly to beneficial predators, rather than through their prey. “You need to use a realistic route of exposure,” says Hilbeck. Meanwhile, researchers led by John and Angharad Gatehouse at the University of Durham have studied what happens to ladybird larvae fed aphids that had eaten a purified lectin protein from a snowdrop. This is the same protein that was engineered into the potatoes that sparked Britain’s current GM food scare. Fears for the safety of beneficial insects surfaced in 1997, when a team led by Nick Birch of the Scottish Crop Research Institute found evidence that eating aphids reared on transgenic potatoes reduced the lifespans and egg production of ladybirds (This Week, 1 November 1997, p 4). Now the Durham team suggests the lectin is not acutely toxic to ladybirds. The lectin stunted the growth of aphids, but when the ladybird larvae were given more aphids to compensate for the aphids’ small size, they developed normally to the pupal stage. More on these topics: