Pest damage in sweet potato, groundnut and maize in north-eastern Uganda with special reference to damage by millipedes (Diplopoda)

Field experiments were conducted in Soroti District, north-eastern Uganda, an area with two rainy seasons per calendar year, the first one with long, reliable rains and a second one with short, less reliable rain. The trials were with sweet potato (Ipomoea batatas (L.) Lamk), groundnut (Arachis hypogaea L.) and maize (Zea mays L.) and aimed at collecting information on the incidence of millipede damage. Failure of sweet potato cuttings to establish caused by biotic stress varied from 4 to 33%. A significant but variable proportion of that biotic stress was caused by millipedes. Millipedes of the species Omopyge sudanica were responsible for the loss of up to 84% of the sweet potato cuttings if the crop was planted early in the first rainy season. During bulking hardly any damage was inflicted on the storage roots. When the tubers were stored 'in-ground on plants' during the dry season, millipedes in combination with other insect pests affected up to 86% of the tubers at the onset of the rains of the following growing season. Data on groundnut and maize were taken on plots where in the previous season sweet potato had been grown. Early in the first rainy season, O. sudanica also caused damage in germinating groundnut, causing plant losses of 12-29%. Maturing groundnut seeds were affected for 39%. Millipede damage in germinating maize seeds in the first and second rainy seasons amounted to 34% and 29%, respectively. The species O. sudanica, Spirostreptus ibanda and Tibiomus spp. cfr. ambitus were found in the vicinity of the maize seeds but were only found feeding on them during the second rainy season. More research is needed to quantitatively assess economic damage to crop production caused by millipedes.