Walling out weevils ups potato yields

Simple sheets of plastic are providing a cheap alternative to expensive and harmful insecticides in fighting potato weevils, delivering higher yields, and paying extra dividends in environmental and biodiversity gains.

Potato weevils pose a major problem for farmers in potato areas such as Kinangop, where potato is an important staple and cash crop. The weevils, very common at such high altitudes, crawl into potato fields at the beginning of the growing season to breed. When the larvae hatch they move into the soil, feeding underground on tubers. To date, it has been very difficult to protect crops from the weevil. Farmers tend to use 2-4 applications per season of insecticides, but these are expensive, highly toxic, and often ineffective, because the dosages are wrong or mistimed. As a result, between 15 to 40 per cent of tubers are infested at harvest, even after the use of insecticides. When no controls are applied, the weevil can destroy half of the crop, or more.

But the potato weevil has an Achilles heel: it is flightless.

“Erecting a simple plastic barrier just 30-50cm high and 10cm into the ground is very effective in stopping weevil migration to potato fields and consequently tuber damage,” explained Derrick Kigwe, a scientist working with the farmers. “What’s more, it’s an idea that has been tested collaboratively with local farmers to assess its impact, ease of use and acceptability.”

More than 60 individual field experiments have been carried out over 4 years with farmers in Kinangop to determine the efficiency of the plastic barriers, farmer receptivity and the potential economic benefits to farmers.

The results have been very positive. Barriers were found to reduce the potato weevil damage by up to 70 per cent more than insecticides.

Moreover, the increased yields and higher prices for the undamaged tubers, compared to the lesser cost of the plastic barriers, saw the barriers come up strongly as an investment. Average net benefits for farmers were Sh15,000 per hectare in one of the test villages.

Local opinion on the alternative technology was also very positive. More than 90 per cent of the farmers participating in the project considered the barriers a very useful and easy-to-install tool, and were interested in promoting them further among other farmers. Farmers are also now being encouraged to experiment with the barriers in communal rotation systems, while the technology is being taken up by national agricultural institutes to further promote its use in smallholder agriculture.

In addition, the method scores highly on the environmental impact quotient (EIQ). The EIQ uses a formula to calculate the impact of a pesticide on wildlife, farmers, consumers and agroecologies. Including fungicide applications for the control of late blight disease, the EIQ of 32.9 for the plastic barriers was less than a fifth of the EIQ of 191.5 recorded for farmers’ traditional spraying practices. Moreover, the plastic sheeting is good for at least two seasons, after which it can be recycled.

“The plastic barriers reduce the weevil population enormously,” said Derrick. “So if an entire village applies this method for several seasons, we can effectively minimise the weevil population and ultimately could get to a point where farmers can produce potatoes without needing any special weevil control.”

Also important is the technology’s potential impact on insect biodiversity. The application of broad-spectrum insecticides affects not only weevils, but also their natural enemies, upsetting species’ natural balance and diversity. Prolonged insecticide use in the country has degraded populations of parasitoid and predator insect species, such as ladybirds and hoverflies, which help control other potato pest populations such as potato tuber moth and aphids.

Written by Bob Koigi for African Laughter

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