Genetic cotton linked to rise in voracious pest

In an ironical twist, the genetically engineered cotton varieties that Kenya is about to embrace for cultivation by farmers, due to their ability to ward off caterpillars by producing their own insecticide, have now been linked to higher numbers of aphid another equally destructive pest.

Cotton is generally a pesticide intensive crop with a survey carried out by KARI indicating that 35 per cent of farmers cited the cost of pesticides as the biggest constraint to growing cotton. Biotechnology cotton, BT cotton, which industry players have been pushing for due to its superior anti pest qualities, produces an insecticidal protein from the naturally occurring soil bacterium Bacillus thuringiensis (Bt) making it insect resistant.

Non-Bt plants respond to caterpillar infestation by producing defence compounds, which also protect the plant against other pests. But scientists now tie the suppressed production of natural defence compounds in Bt cotton, caused by the lack of caterpillar attacks, to a surge in non-target pests such as the cotton aphid.

Scientists were studying the correlation between Bt and non-Bt cotton plants, defence compound levels and aphid populations in both glasshouse and field conditions. In the glasshouse, they artificially infested Bt and non-Bt cotton plants with caterpillars and monitored the levels of various defence compounds known as terpenoids that are released in response to caterpillar damage. The plants were then artificially infested with aphids.

In the field, one set of Bt and non-Bt cotton plants were artificially infested with caterpillars and another was left to natural infestation by caterpillars. Terpenoid levels were measured and both sets were then exposed to natural aphid infestations.

In the glasshouse, caterpillars on the Bt cotton plants died. As a result, the plants were less damaged and therefore contained less defence compounds than their non-Bt counterparts. The researchers thus attribute the resulting increase in aphid populations on these plants, compared with non-Bt plants, to lowered terpenoid production.

Although a relative rise in aphid numbers was noted on a few occasions on Bt cotton plants in the field, the scientists found no correlation between aphid populations and terpenoid levels in this part of the experiment. "Our study shows that a technology like Bt cotton should not be used in isolation. To control other herbivores we have to use the technology as part of integrated pest management approach," said Jörg Romeis, one of the researchers. The study is published in Proceedings of the Royal Society journal.

The discovery is a blow to Kenyan government and proponents of the BM cotton having actively advocated for the growing of a superior cotton variety that increases yields in small acreages and fights off stubborn pests. The full commercialization of the BT cotton is set to be in 2014 with farmers in Embu already having been trained how to grow it. “These are new developments which we would factor in as we further develop the cotton variety to the needs of our farmers. We wouldnt want a situation where the same problem we are shielding them from to haunt them,”said Dr Wellington Odima a scientist.

KARI which has been involved in the development of the BT variety in Kenya says Kenya has untapped potential to grow 350,000 hectares of cotton which would produce 260,000 bales or 52 million kilos. Currently only about 30,450 hectares is under cotton.