Biology of ootheca mutabilis on common bean and effect of alternative hosts on its abundance

Abstract

Common bean (Phaseolus vulgaris L.) is an important crop in eastern and southern Africa by providing 25% total dietary calories, 30 to 60% of protein intake, and is a source of income for the smallholder farmers. The production of common beans is threatened by, among others, damage from insect pests such as Bean leaf beetles (BLBs) (Ootheca spp.). In Uganda, the adult and larvae of O. mutabilis feed on mainly leaves and root system of common bean, respectively, hence causing loss in yield reaching 48.9%. The heavy damage during first season rains often forces some farmers to abandon growing common bean. Due to its polyphagous nature, O. mutabilis survives and reproduces in the absence of common bean, which complicated their management. The information on the biology and ecology with respect to life history and host plant species preference of O. mutabilis would aid the designing of effective control tactics, however such information is lucking. This study determined the developmental biology pertaining to oviposition, morphological changes and development durations of O. mutabilis on common bean, and provided vital information on the abundance of pest’s life stages on alternative hosts, viz. common bean, cowpea, and soybean. The biology was studied while monitoring oviposition and development of O. mutabilis on potted common bean plants in the screenhouse. The host plant effects and interaction with seasons on the abundance of life stages were studied under natural infestation in the field. The field studies were conducted during three seasons (2017B, 2018A and 2018B) at two selected hotspot areas in Arua and Lira districts in northern Uganda. The field experiments were set up following a Randomized Complete Block Design with three host plants (common bean, cowpea, soybean) as treatments, and replicated four times. Results indicated that O. mutabilis females laid in hidden spaces such crevices and below plant debris near plant stalk or protruding roots. The beetles completed their life cycle in the soil, which lasted for 120 days (range: 103 to 137). Egg, larva, and pupa development lasted for 22, 75, and 24 days, respectively. This life cycle is longer than the lifespan of most bush bean varieties implying that larvae and pupae of O. mutabilis remain in the soil after crop maturity. Cowpea with longer maturity and exhibited indeterminate growth habits duration had higher larvae and pupae than common bean, which also suggest higher preference. Adult beetles were observed in the first and second cropping seasons albeit in smaller numbers in the latter than former season. Subterranean larvae were also significantly higher during the 2018A season than in 2017B and 2018B. This means that the crops grown during first season rains are much affected by both foraging adults and subterranean larvae suggesting higher impact on yield than during second cropping season. It is, therefore, recommended that control tactics must focus on controlling beetle population on alternative hosts most especially cowpea because beetles use these as alternative sources of food for juvenile stages and adults. The control tactic must also target pupae and teneral adults that live in the soil during the second cropping season. The findings from this study are, therefore, useful in informing farmers and pest managers on selecting a set of management tactics and their timing.