Major pests of African indigenous vegetables in Tanzania and the effects of plant nutrition on spider mite management

Abstract

Pest status of insect pests is dynamic. In East Africa, there is scanty information on pests and natural enemy species of common African Indigenous Vegetables (AIVs). To determine the identity and distribution of pests and natural enemies in amaranth, African nightshade and Ethiopian kale as well as pest damage levels, a survey was carried out in eight regions of Tanzania. Lepidopteran species were the main pests of amaranth causing 12.8% damage in the dry season and 10.8% in the wet season. The most damaging lepidopteran species were S. recurvalis, U. ferrugalis, and S. litorralis. Hemipterans, A. fabae, A. crassivora, and M. persicae caused 9.5% and 8.5% in the dry and wet seasons respectively. Tetranychus evansi and Tetranychus urticae (Acari) were the main pests of African nightshades causing 11%, twice the damage caused by hemipteran mainly aphids (5%) and three times that of coleopteran mainly beetles (3%). In Ethiopian kale, aphids Brevicoryne brassicae and Myzus persicae (Hemipterans) were the most damaging pests causing 30% and 16% leaf damage during the dry and wet season respectively. Hymenopteran species were the most abundant natural enemy species with aphid parasitoid Aphidius colemani in all three crops and Diaeretiella rapae in Ethiopian kale. Coccinellids beetles were present in all crops. Based on the survey findings, further studies were carried out to determine the effects of fertilizer and water regimes on host selection, population and damage of Tetranychus evansi on Solanum scrabrum. The effects of these regimes on leaf alkaloid content were also evaluated. When fertilizer and water regimes experimentally varied, mite population and leaf damage increased with a decrease in soil moisture and nutrient with 35% leaf damage observed in the highest regime and 73% in the lowest. In addition, alkaloid content was highest in the high moisture and low fertilizer regimes and low in all regimes with the lowest moisture level. The alkaloid content of the tested plants negative correlated with mite population at extreme regimes. Knowledge generated from this study will inform farmers and extension service providers on the major pests of these vegetables and help in prioritizing pest management resources. African nightshades farmers ought to ensure crops are well watered and receive adequate fertilizer particularly during the dry season when the crop is mainly grown under irrigation. This could suppress red spider mite populations and increase yields.