Control of insects infesting stored sorghum in Northern Nigeria

Abstract

Store hygiene alone is not sufficient to prevent heavy infestation in unthreshed sorghum stored in sealed mud granaries in Northern Nigeria. Insects that infest before storage, mainly in the field, are liable to cause heavy damage in grain stored longer than six months. Threshing reduced populations of one of the major primary pests, Sitotroga cerealella (Oliv.) in stored grain, but other pests, particularly Sitophilus oryzae (L.) developed more quickly and “heating” was liable to occur if the grain were stored up to the end of the rainy season or longer. Two varieties of sorghum were found to differ in their susceptibility to stored product insect attack but neither was completely resistant. Although air-tight storage of threshed grain gave complete insect control in a large “Argentine” tank, little practical success was achieved in mud granaries treated with bitumen or in sealed clay pots. Some control was achieved in unthreshed sorghum stored in mud granaries treated several times with aluminium phosphide tablets at a rate of 16 tablets per ton. Dusting the surface of a bulk of heads with 0·5 per cent lindane was reasonably effective but a “sandwich” treatment was used in subsequent trials. A 0·5 per cent lindane dust application at a rate of 10 ppm threshed grain was the most effective and economic rate in grain stored in mud granaries for six months or longer. The treatment persists for at least 18 months. Control of insect infestation in grain stored in granaries made from plant materials is less effective because store hygiene is difficult, cross-infestation occurs more easily and a higher ventilation rate causes a faster loss of lindane. In early trials it was found that formulations of 0·5 per cent malathion on kaolin were unstable and thus in a later trial a stable two per cent flour-based malathion was used. Rates up to 15 ppm malathion were less effective than 10 ppm lindane in reducing insect damage. Treatment with two per cent flour-based carbaryl at rates up to 15 ppm were not effective. This series of investigations culminated in a recommendation that farmers could economically protect sorghum which is to be stored for more than six months in mud granaries if 0·5 per cent lindane dust is applied to unthreshed sorghum as a “sandwich” treatment, at a rate equivalent to 10 ppm threshed grain.