Effecacy of selected Entomopathogenic fungi for the management of Sweetpotato weevils (Coleoptera: Brentidae) in Uganda.

Abstract

Sweetpotato (Ipomoea batatas L.) is a versatile and highly nutritious crop that plays a significant role in global food security. The crop is well adapted to all agroecological conditions and serves as an important staple food for many Ugandan households, especially in rural areas contributing to the alleviation of malnutrition by providing vitamins, minerals, and dietary fiber. Despite the vast importance of sweetpotato, sweetpotato weevil (Cylas spp. Coleoptera: Brentidae) has continued to negatively impact the crop in Uganda by significantly reducing the quality and quantity of sweetpotato root tubers through root damage in the garden. These damages are a threat to food security, nutrition and income generation. Microbial control using fungi has the potential of reducing damage caused by sweetpotato weevils. This study aimed at investigating laboratory and field efficacy of entomopathogenic fungal isolates for the management of sweetpotato weevil in Uganda. Laboratory bioassays were conducted on the two African sweetpotato weevil species Cylas brunneus and Cylas puncticollis using five strains including four Metarhizium isolates, and one Beauveria isolate over a period of seven days. Mortality rates of sweetpotato weevils ranged from 20 to 80% among the isolates. Isolates K157, K191, K020 all showed significant differences relative to the control group (p < 0.05) while Isolates K040, K061 were not significantly different relative to the control group (p < 0.05) for Cylas brunneus. For Cylas puncticollis, K157, K191, K040 showed significant differences while K020 and K061 were not significantly different relative to the control group. Four of the isolates tested under laboratory conditions, Isaria, and Cypermethrin were tested under field conditions at two sites of National Crops Resources Research Institute (NaCRRI) in Wakiso district and National Semi Arid Resources Research Institute (NaSARRI) in Serere district. Treatments were applied after every two weeks for three months. Relative to the control, there was no significant difference (p > 0.05) on the sweetpotato vine yield, damaged root incidence and severity, and vine damage among all the isolates of fungi K191, K040, K020, K157 and Isaria. Cypermethrin however had a significant effect (p < 0.05) in reducing the sweetpotato vine damage in NaCRRI but not in NaSARRI. While laboratory evaluations demonstrated promising results for the potential management of sweetpotato weevils, the lack of significant differences in the field suggests the need for refinement. Environmental factors such as UV, temperature, rainfall was identified as potential limitations to the field efficacy. Focus should therefore be to address these constrains to enhance its field applicability.