Evaluation of Inert and Permethrin Powders against Cylas puncticollis Boheman (Coleoptera: Brentidae) on Stored Sweet Potato

Abstract

The sweet potato weevil Cylas puncticollis is one of the insect pests of stored sweet
potato in Africa. Due to its perishable nature, infested tubers cannot be stored for
long period. Laboratory experiments were conducted to evaluate the efficacy of
two inert powders namely Insecto® Diatomaceous Earth (DE) and Oil Palm
Inflorescence Ash (OPIA), and a permethrin powder against C. puncticollis on
white-fleshed sweet potato tubers. Each treatment was applied at 0, 0.3, 0.9 and
1.5 g per 30 g sweet potato, and then infested with ten unsexed adults of C.
puncticollis in glass jars. The set up was arranged in a completely randomised
design with each treatment replicated three times and maintained at 70±5%
relative humidity and 28±2°C. Data was collected on adult mortality daily (up to
7 days) while progeny emergence and weevil damage (perforated tubers) were
recorded after 28 days. Adult mortality was dose-dependent of insecticide and
exposure time, and there were significant differences among all the tested
insecticides. Permethrin induced 100% mortality and total progeny suppression
against C. puncticollis. At the highest dose, DE and OPIA were less effective
causing 67.2% and 54.7% mortality, and below 78% progeny suppression,
respectively. All powders significantly caused lower weevil damage on tuber
compared to control which recorded 37.3%. Permethrin proved the most effective
against C. puncticollis, followed by DE and OPIA. However, more research is
required to explore how these inert powders can provide additive effect with other
bio-pesticides against sweet potato weevils.