Cotton 'topping' cuts bollworm infestations, study finds

Cutting the shoot tips of the tops of cotton plants may control key pests and potentially reduce cotton farmers' dependence on insecticides, according to a study by researchers from Mali.

The researchers say the practice — called topping — is already known to improve yields but the effect on pests in Sub-Saharan Africa have not been previously studied.

Cotton is mainly grown by poor small-scale farmers in Sub-Saharan Africa, where millions depend on it for their livelihood, according to researchers.

A combination of the high cost of insecticides and emerging pest resistance have motivated researchers to look at topping — which had been shown to reduce pest levels in a handful of studies in China, Egypt, and India — to examine its potential for pest control in Africa.

They therefore looked at infestation levels of three bollworm species, cotton's main pests in Sub-Saharan Africa, which account for significant seed cotton yield losses, in topped versus non-topped fields.

The researchers conducted 12 trials comparing manual topping and non-topping cotton plots in Mali over a six-year period beginning in 2002.

The results, published online in Crop Protection last month (2 July), showed that bollworm infestations were always lower on topped cotton and seven out of 12 trials had significantly lowered infestations on topped cotton.

Researchers recorded an average of 56 per cent fewer H. armigera larvae, 68 per cent fewer Earias spp. larvae, and 71 per cent fewer D. watersi, and the reduction in infestation was the greatest in years when the pests were the most numerous.

"We were amazed to see that non-topped cotton plants in plots near the topped plants also had reduced bollworm infestations," said Mamoutou Togola, co-author of the research and an entomologist at Mali's Institute of Rural Economy.

They did not investigate why topping reduces pest numbers, or whether it could save money by significantly reducing insecticide use — both topics left for future research.

But, according to Togola, topping may be relevant to African countries whose economies largely depend on cotton, such as Benin, Burkina Faso, Chad and Mali where about 10 million people depend on cotton farming.

Rafiq Chaudhry, head of technical information at the International Cotton Advisory Committee, in United States, said many cotton-producing countries, such as Argentina, India and Pakistan, do not employ topping, although it "may control some pests". 

"We always recommend that insecticides be used minimally and other non-chemical control practices be encouraged," Chaudhry said.

Ibrahim Sourabié, the country coordinator of the West African Cotton Improvement Program in Burkina Faso, said that cotton farmers there use genetically modified Bt cotton or oil extracted from the seeds of neem tree to control pests. But, he added, topping could help conventional cotton farmers control bollworm infestations.

Source: SciDev.Net