Facing- rough weather

Scientists are making gloomy predictions that rising temperatures shall jeopardise not just human health but our crops as well

Published: Thursday 29 February 1996

A STUDY conducted by British researchers concluded that with rising mercury levels, temperate farmlands face a potential threat in the form of insecticide-resistant aphids (insects of the Homopteran order, which live on plant juices). The aphid population had till date been kept under check due to winter frost, but when the average winter temperatures rise, their population receives a boost (New Scientist, Vol 148, No 2009/2010).

The peach potato aphid (Myzus persicae) which reproduces asexually, except when on a peach tree, is an agricultural pest feeding mainly on potatoes, oil-seed rape and sugar beet plants. These pests feed on the sap of the plant and spread viruses which can wipe out entire fields. These pests had been controlled till date, but researchers now claim that insecticide -resistant clones have developed. These 'super-aphids', are resistant to organophosphates, carbamates and pyrethoid groups of agricultural insecticides.

The defence mechanism of the super-aphids is balanced by their inability to survive in the cold. Stephen Foster and his colleagues at the Rothamstead Experimental Station in Hertfordshire, UK, have conducted experiments on insecticide - resistant aphids to show that they are more likely to die during a cold spell. Clones with varying degrees of resistance were introduced into experimental plots of oil-seed rape. It was found that the most resistant aphids perished in severe wintry conditions when temperatures dipped below 2C.

These aphids use carboxylestrase E4 enzyme to break down the insecticides. "The more resistant forms produce more enzyme," explains Foster. This is helped by the fact that they carry multiple copies of the genes which encode for the enzyme. But this does not explain the vulnerability of these 'super-aphids' to the cold. The researchers assume that since these duplicated genes appear on a stretch of DNA which has moved from one chromosome to another, they must have done so by displacing other important genes. Experiments suggest that these genetic changes interfere with either the plant's warning signals indicating danger or the plant, due to other changes, totally loses its ability to heed these signals. The normal warning system comes in the form of chemical changes in the leaf sap.

The greatest fear dogging British researchers is that by the end of next century, average winter temperatures in Britain are likely to rise by 3C. With a 25 per cent reduction in the number of frosty nights, the natural barrier against the growth of super-aphid populations shall tumble and crop destruction would inevitably follow.

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