Bean Leaf Beetle (Ootheca spp.) (Coleoptera: Chrysomelidae) Management via Planting Timing and Insecticides

Abstract

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 spray resulting in much less foliar damage and, as a result, higher grain yield. However, this did not result in economic benefits. Furthermore, marketable grain yield was higher when insecticide spray regimes were combined with early planting in all agro-ecological zones during both seasons. Our findings suggest that the common bean should be planted early and that the control of the bean leaf beetle should target both the adults and the juvenile stages in the soil. Therefore, there is a need for farmers to be able to access less-expensive soil treatments.

Population Dynamics of Selected Field Pests and Their Effect on Grain Yield and Yield Components of Common Bean in Uganda

“…Our observation in WMAFSF, however, indicate otherwise and this requires further investigation to determine whether this can be consistent over many seasons. The differences in pest abundance in different agro-ecologies may be attributed to differences in weather [20].…”

“…Combining imidacloprid soil drench with cypermethrin foliar spray reduced insect pests' populations except for bean leaf beetles and caterpillars. Generally, insecticides have been reported to reduce insect pest populations and increase grain yield [25][26][27][28], including bean leaf beetles [20]. Low insect populations as a result of soil drench and foliar spray combinations may be attributed to imidacloprid unique ability to combat a variety of insect pests, when applied either as a soil drench or a foliar spray since it is easily translocated in the xylem [29].…”

“…For instance, the bean fly is managed by soil fertility enhancement and mulching [9], variety mixtures [8], intercropping [16], early planting [17] and insecticide seed dressing [18]. Recommended management practices against bean leaf beetles include intercropping with non-hosts, deep tillage, crop rotation [18], trap cropping [19], early planting [20], insecticides [20,21] and use of aqueous botanical extracts [22,23]. Aphids and thrips are given little attention by farmers, although resistance to insecticides by bean flower thrips has been reported [15].…”