Distribution of bean leaf beetles and associated yield losses in Uganda

Abstract

Common bean (Phaseolus vulgaris L) is the most important and most widely grown legume crop for food and incomes of smallholder farmers in Uganda. However, its production is constrained by many factors including bean leaf beetles. Bean leaf beetles are reported to have forced farmers to abandon growing beans during first season rains in northern Uganda. Adult beetles skeletonize the leaves while the larvae feed on lateral roots. Heavy bean leaf beetle infestation may result into complete loss of the crop. Nevertheless, information on bean leaf beetle species and their distribution in Uganda is lacking. Additionally, there are no available yield loss estimates due to bean leaf beetles in Uganda. This has limited development of management strategies. This study was carried out to determine the distribution of bean leaf beetles and associated yield losses in Uganda. To determine the species of beetles and their distribution in Uganda, field surveys were conducted in six major bean production agro-ecological zones. This was for a period of three seasons of 2016A, 2016B and 2017A. The results revealed 17 species of bean leaf beetles infesting beans in Uganda. Of these, only three species belong to genus Ootheca (O. mutabilis, O. proteus and O. orientalis). Ootheca mutabilis was the most prevalent and accounted for 70.3% of the total species and existed in all agro-ecological zones while O. proteus accounted for only 6.8% of the total species and was restricted to only four agro-ecological zones. Only two specimens which were identified as Ootheca orientalis were recovered in Central wooden savannah agro-ecological zone. Among the six agro-ecological zones, the Northern moist farmland was the most infested with a bean leaf beetle density of 0.15±0.02 and a foliar damage per plant of 1.17±0.05 while southwestern grasslands was the least infested with a bean leaf beetle density of 0.03±0.02 and a foliar damage per plant of 0.53±0.0. Yield losses due to bean leaf beetles were studied in three most infested agro-ecological zones during 2016B and 2017A seasons. A spray schedule consisting of foliar application of Cypermethrin starting at 7, 14, 21, 28 and 35 days after emergence (DAE) was imposed. Additionally, a soil drench of Imidacloprid at sowing combined with a foliar spray starting at 7 DAE was involved to control larvae of bean leaf beetles hence providing maximum crop protection. Yield losses were assessed by comparing all the other treatments with the treatment that offered maximum protection (foliar spray with soil drench). Results indicate that all the insecticide treatments reduced foliar damage due to bean leaf beetles and increased grain yield depending on the time at which spraying was started. A combination of soil drench with foliar spray had significantly higher yield than all the other treatments. Yield losses from the individual treatments were 48.9% from the control and 28.4%, 32.6%, 40.3%, 43.5%, 45.8% from foliar spray starting at 7, 14, 21, 28 and 35 DAE, respectively. This study provides a documentation of bean leaf beetle species existing in Uganda, their distribution, and extent of damage in different agro-ecological zones. The findings suggest that efforts geared towards management of bean leaf beetles should first prioritize the northern moist farmland and northwest farmland, which are the most infested agro-ecological zones. Management of bean leaf beetles should put into consideration control of bean leaf beetle soil stages together with adults since higher yields have been obtained from combining a soil drench with foliar spray. More so, the study indicates that the timing of pesticide application is important in alleviating losses due to bean leaf beetles with early insecticide application resulting into high yields. Therefore, application of insecticides for the control of Ootheca spp should be conducted within 14 days after crop emergence for greater yields.