The distribution of insects in stacks of bagged groundnuts in Northern Nigeria

Abstract

Decorticated groundnuts (Arachis) are stored in bags in pyramid-shaped stacks in Northern Nigeria over the period November to October. In Kano the dry season extends from October until March and is followed by a short hot season preceding the rainy season, which usually commences during May.The seasonal distribution within such pyramids of the various insect species infesting groundnuts is considered in some detail, with particular reference to the primary pest Tribolium castaneum (Hbst.). This distribution is discussed in relation to temperature and moisture content of the kernels and to the effect on the environment of covering pyramids with tarpaulins, the normal practice for protection during the rainy season. T. castaneum is the only species present on groundnuts initially and throughout the storage season. It is concluded that the elimination, during storage, of other species also present initially, i.e., Cadra cautella (Wlk.), Plodia interpunctella (Hb.), Corcyra cephalonica (Stnt.) and Oryzaephihis mercator (Fauv.), and the ultimate concentration of infestation by T. castaneum within peripheral bags is the result of changes in physical conditions. It would appear that Cryptolestes ferrugineus (Steph.), which was not found until late in the storage season, is a pest of secondary importance.The significance of the results obtained in relation to control measures is discussed, bearing in mind that all but one of the pests considered were found to be present on groundnuts when received at the Kano central storage area from buying stations. It is concluded that a programme of surface spraying of pyramids with insecticides is of little value and recommendations for the introduction of initial fumigation of pyramids have been put forward.