8 March 2012 | EN
Triple-layer bags deprive weevils of oxygen
Millions of farmers in West and Central Africa could benefit from a method of storing cowpea, an African staple, after a study provided an important insight into how the method works.
Cowpea is a drought-tolerant legume that grows well in poor soils with a high sand content. However the harvested beans are vulnerable to cowpea weevils, Callosobruchus maculatus, which can multiply extremely rapidly. Even a small infestation can destroy an entire stored crop in a few months.
In August last year SciDev.Net reported that the Bean/Cowpea Collaborative Research Support Program had surveyed farmers to collect data on better storage methods and learnt that a long-standing method of killing cowpea weevils involved storing the harvest inside three layered and individually sealed plastic bags.
It was assumed the pests died of suffocation. Now researchers in Niger and the US have published a study of the method in the Journal of Stored Products Research, saying while that the lack of oxygen reduces the weevils' feeding activity, what actually kills them is a lack of water — and that the drier the harvest at the time of storage the harder it will be for the weevils to survive and multiply.
"The weevils use oxygen to produce water and so are deprived of their main water source. But some of their water, maybe 15–20 per cent, comes from the seed," said Larry Murdock, an entomologist at US-based Purdue University and lead author of the study.
Millions of triple plastic bags have already been distributed across West and Central Africa through the Purdue Improved Cowpea Storage (PICS) bags project.
"The simple recommendation is to store — in a PICS bag or via other methods — your grain when it is drier, if you can, because you're reducing the supply of water and placing [the weevils] under even greater water stress," Murdock told SciDev.Net.
The team are now investigating whether the dry, airtight environment inhibits other cowpea pests in the same way.
Tahirou Abdoulaye, an agricultural economist at the International Institute of Tropical Agriculture in Nigeria, welcomed the study.
"For cowpea, for example, we know that average prices can increase 60–100 per cent between harvest and the next planting season. So any improvement in storage has the potential to increase revenue by that much," he said.
Journal of Stored Products Research doi 10.1016/j.jspr.2012.01.002 (2012)
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