Bean leaf beetles (Ootheca spp.) (Coleoptera: Chrysomelidae) are one of Africa’s most important pests of the common bean (Phaseolus vulgaris L.). Roots, leaves, floral parts, and young pods are all attacked, leading to a considerable loss in grain yield. In Uganda, there are no comprehensive prescribed management strategies for bean leaf beetles, but farmers typically try to control the pest by delaying bean crop sowing, and to a lesser extent, using insecticides. Although farmers have consistently implemented the two approaches, there is no information on the effects of the approaches in Uganda. To assess the impact of planting timing and insecticide spray regimes on bean leaf beetle populations, concomitant foliar damage, and grain yield, we set up trials in three agro-ecological zones with known presence of the beetles during the second rainy season of 2016 (2016) and the first rainy season of 2017 (2017). The first planting, coinciding with early planting, was conducted within one week after the onset of rains. The second planting, coinciding with mid planting, followed two weeks later, while the third planting, considered late planting in this study, was conducted one month after the second planting. A foliar application of cypermethrin commencing at 7 days after emergence (DAE), 14 DAE, 21 DAE, 28 DAE, and 35 DAE; a soil drench of imidacloprid at planting combined with a foliar spray starting at 7 DAE; and an untreated control were among the insecticide spray regimes evaluated. Higher bean leaf beetle abundance was recorded from mid-planting, while higher foliar damage was recorded from late planting in two of the three agro-ecological zones. However, higher marketable grain yield was recorded from early planting in all agro-ecological zones, suggesting that delayed planting may not be beneficial. Insecticide application reduced foliar damage and increased marketable grain yield, with a combination of soil drench and foliar spra planting time is one of the most useful tactics for reducing the impacts of C. trifurcata [17–19] and has been applied successfully to many pest problems and crops [18,20–24]. In the United States, planting of soybean (G. max) near the end of the recommended planting period reduced early colonization and subsequent pod injury by C. trifurcata [17,18]. In Nigeria, varying the sowing date of cowpea (V. unguiculata) from 2 July to 18 August reduced populations of Maruca vitrata [22]. On the contrary, early and late planting of pigeon pea (C. cajan) in April and October, respectively, resulted in higher infestation of Helicoperva armigera compared to mid-planting in July in Nigeria [22]. Furthermore, while delayed planting reduced infestation by M. vitrata and pod-feeding bugs (Clavigralla tomentosicollis and Anoplocnemis curvipes) in north-eastern Nigeria, this was not consistent with thrips (Megalurothrips sjostedti) [23]. It has been suggested that there is usually a build-up of pests as the season progresses, which results in increased damage to late-planted crops [23]. Furthermore, early planting enables the crop to escape high temperatures during the flowering stages when the crop is sensitive to heat [23]. Farmers in northern Uganda delay bean sowing in order to avoid damage by bean leaf beetles, especially during the long rainy season [10], but it is unknown whether farmers’ practice of delaying common bean planting results in less foliar damage due to bean leaf beetles and, as a result, higher yields. The use of insecticides is the principal recourse for the control of insect pests in cultivated food legumes [22]. In cowpeas (V. unguiculata), application of insecticides lowered pest densities and improved yields [25–28]. In French beans (P. vulgaris), application of insecticides reduced thrips infestation and increased yield [29–31]. In soybeans (G. max), application of insecticides reduced the abundance of C. trifurcata and subsequent leaf loss but did not result in yield increase [20]. There are no officially recommended measures by pest management specialists against bean leaf beetles in Uganda. Nonetheless, some farmers try to avoid the pest impacts by applying insecticides. Application of insecticides is unnecessary if there is no effect on grain yield by insect pests [20], but there is scarcity of information on the effect of insecticides on bean leaf beetle populations, associated foliar damage, and grain yield in Uganda. The purpose of this study, therefore, was to determine the effect of planting time and insecticide spray regimes on bean leaf beetle abundance, associated foliar damage, and grain yield in Uganda. Specifically, the objectives of this study were: (i) to determine the appropriate planting date that would reduce bean leaf beetle attacks and increase grain yield of common bean in Uganda and (ii) to determine the most economical insecticide spray regime for use in the management of bean leaf beetles in Uganda.

show more