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Army worms cause havoc in Rift Valley

Thursday March 29 2001


The African Armyworms (Spodoptera Exempta) which destroyed more than 40,000 hectares of maize and wheat plantations in the Rift Valley Province in 1999 are on the march again.

The migratory worms sighted in Tanzania towards the end of last year later entered Kenya via Trans Mara district and have since invaded farms in Bomet, Kericho, Buret and Nandi district.

In Trans Mara, the worms invaded 1,990 hectares of pasture and cereals, forcing farmers to replant between 200 and 300 hectares of maize.

In Kericho, the army worms infested 268 hectares of maize, and 2,330 hectares of crop and pasture in the neighbouring Buret district.

The worms also invaded 311 hectares of maize in Nandi and 218 hectares of pasture in Koibatek district.


Agricultural officials said that the worms were dealt with in Kericho, Buret, Nandi and Koibatek and that no crop failure was expected.

Crops such as maize regenerate if spraying is done immediately the crop is attacked.

Officials from the Crop Protection Unit of the Ministry of Agriculture and Rural Development released between 6,000 and 7,000 litres of pesticides for distribution to farmers immediately after the worms were sighted.

More than 350 knapsack pumps were also distributed to small scale farmers in the affected areas.

In Rongai (in Nakuru District) the worms invaded 40 hectares of pasture but their migration was halted immediately by spraying.

The Rift Valley Provincial Director of Agriculture, Mr David Nyasani, told The Horizon that prompt action had been taken to combat the infestation and that at least five motorised sprayers had been dispatched to the affected districts.

Mr Nyasani said that farmers were being issued with the appropriate pesticides for spraying the affected areas either using hand pumps or motorised ones in areas where infestation was heavy.

Agriclultural officials in the province are on the alert because another invasion is expected to occur between April and May.

During the 1999 invasion, the number of worms per metre was between 500 and 1,000. The government supplied farmers with more than 7,000 litres of pesticides at the time.

The Rongai infestation, according to Agricultural Extension officials was estimated at 60 worms per square metre.

Three types of pesticides – Brigade, Sherpa and Fenitrothion are being issued to the farmers by the government free of charge. Agricultural Extension Officers are also assisting farmers to combat the menace.

A farmer at Kandutura in Rongai told The Horizon that although the worms had not invaded the area, there was fear that most farmers could not afford the pesticides.

A litre of Brigade costs about Sh2,000 while Sherpa and Fenitrothion cost Sh830 and Sh840 per litre respectively.

A document on the African armyworm compiled by the Regional Armyworm Programme of Desert Locust Control Organisation for Eastern Africa, says that many different insecticides, in a variety of formulations can be used effectively against armyworms.

The choice of material used vary with the extent of outbreak to be treated.

While DDT is still used in the control of armyworms outbreaks in some countries in East Africa, there is the world wide concern expressed in recent years about contamination of the environment.

DDT is banned in Kenya because it is toxic and harmful to the environment.

Persistent compounds such as dieldrin and endrin have been used and are known to be effective against the army worm but are not also recommended on account of their high mammalian toxicity.

Safer and faster acting materials such as cypermethrin are increasingly being used.

An agricultural official when told that some small scale farmers were too poor to buy the pesticide said that it was the onus of the government to control migratory pests. "We are prepared this time around unlike in 1999 when we were caught unawares," one of the officials said.

In Eastern Africa where frequent outbreaks occur, larval densities are often in excess of 1,000 per square metre and may cover tens or even hundreds of square kilometres.

Army worms forecasts are based on information from a number of sources, the most important being networks of light and pheromone traps distributed over Eastern Africa and operated nightly under the supervision of trap operators.

Various other factors are taken into account such as the incidence and extent of recent outbreaks of larvae and the current weather conditions, particularly the wind-fields which influence the direction of moth migration.

The army worm outbreak in East Africa usually begins in Tanzania between November and December.

In Kenya and Uganda, the outbreaks occur between February and June and between May and September in Ethiopia, Somalia and Sudan.

The young larvae at first eat the upper and lower surface tissue of the leaves which result in the skeletonisation of windowing of the leaves.

As the larvae become older and increase in size, they are able to bite through the entire leaf, starting from the edges and usually eating all but the mid rib. Heavy infestation results in total loss of leaves often leading to severe crop loss or necessitating replanting.