A STUDY conducted by British researchers concluded that with rising mercury levels, temperate farmlands face apotential threat in the form of insecticide-resistant aphids (insects of theHomopteran order, which live on plantjuices). The aphid population had tilldate been kept under check due to winter frost, but when the average wintertemperatures rise, their populationreceives 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 plantand spread viruses which can wipe outentire fields. These pests had been controlled till date, but researchers nowclaim that insecticide -resistant cloneshave developed. These 'super-aphids',are resistant to organophosphates,carbamates and pyrethoid groups ofagricultural insecticides.
The defence mechanism of thesuper-aphids is balanced by theirinability to survive in the cold. StephenFoster and his colleagues at theRothamstead Experimental Station inHertfordshire, UK, have conductedexperiments on insecticide - resistantaphids to show that they are more likelyto die during a cold spell. Clones withvarying degrees of resistance were introduced into experimental plots ofoil-seed rape. It was found that themost resistant aphids perished in severewintry conditions when temperaturesdipped below 2C.
These aphids use carboxylestrase E4enzyme to break down the insecticides."The more resistant forms producemore enzyme," explains Foster. This ishelped by the fact that they carry multiple copies of the genes which encode forthe enzyme. But this does not explainthe vulnerability of these 'super-aphids'to the cold. The researchers assume thatsince these duplicated genes appear on astretch of DNA which has moved fromone chromosome to another, they musthave done so by displacing other important genes. Experiments suggest thatthese genetic changes interfere witheither the plant's warning signals indicating danger or the plant, due to otherchanges, totally loses its ability to heedthese signals. The normal warning system comes in the form of chemicalchanges in the leaf sap.
The greatest fear dogging Britishresearchers is that by the end of nextcentury, average winter temperatures inBritain are likely to rise by 3C. With a25 per cent reduction in the number offrosty nights, the natural barrier againstthe growth of super-aphid populationsshall tumble and crop destructionwould inevitably follow.