Evaluation Of Some Sweet Potato Varieties For Susceptibility To The Weevil, Cylas Spp. (Coleoptera: Brentidae)

Abstract

Sweet potato (Ipomoea batatas Lam.) is a staple food crop for a large proportion of the population in sub-Saharan Africa. One important factor limiting its production and utilisation is infestation by weevils belonging to the genus Cylas, commonly known as sweetpotato weevils. The use of host plant resistance is an essential component for the integrated pest management of sweetpotato weevils. In this study, the susceptibility of some Ghanaian sweet potato varieties to infestations by Cylas spp. was assessed under both field and laboratory conditions. Natural levels of infestation and yield loss due to sweetpotato weevil attack of 7 sweet potato varieties (Apumoden, Ligri, Faara, Bohye, Dadanyui, Okumkom and Sauti) were determined and plant root characteristics were measured under field conditions during two rainy seasons in 2015. In the laboratory, feeding and oviposition preferences of Cylas puncticollis (Coleoptera: Brentidae) for eight (8) varieties (the first six mentioned above plus Red skinned and white skinned varieties) through no choice and choice tests were determined at 30.09 ± 1.50o C and 76.12 ± 3.75 % RH. The effect of sweet potato variety on some bionomic parameters of C. puncticollis was also determined. During the major and minor rainy seasons of field experiments, Apumoden and Ligri scored the highest levels of infestation (1.80 and 1.77, 1.84 and 1.96, respectively) and yield loss (43.41% and 41.85%, 23.84% and 23.87%, respectively). While Faara and Okumkom scored the lowest infestation levels (1.20 and 1.24, respectively) and yield loss (9.60% and 8.09%, respectively) in the major rainy season. However in the minor rainy season, there was no significant difference between Faraa, Okumkom, Bohye and Dadanyui with regards to the infestation level and yield loss. The storage root infestation appeared to have a strong negative correlation (r = -0.82 and -0.75, in the major and minor rainy seasons, respectively) with the shortest weevil distance, a strong positive correlation (r = 0.75; and 0.71 in the major and minor rainy seasons, respectively) with the root size index and a weak negative correlation (r = -0.45 and -0.36 in the major and minor rainy seasons, respectively) with root neck length. The highest yield was recorded on Ligri (22.25 and 22.49 t/ha in the major and minor rainy season, respectively). In the laboratory studies, C. puncticollis preferred Apumoden and Ligri for feeding and oviposition more than other varieties in both no choice and choice tests, while Okumkom was the less preferred. Faara was preferred for feeding but not for oviposition. The effect of sweet potato variety on C. puncticollis bionomic parameters appeared to be significant on female fecundity (P < 0.0001), oviposition (P < 0.0001) and post-oviposition (P = 0.0160) periods, female longevity (P < 0.0001), developmental period (P = 0.0190), larval survival (P = 0.0050) and adult emergence rate (P = 0.0190) but not significant on female pre-oviposition period, egg incubation period (P = 0.7260) and survival (P = 0.8880), larval development period (P = 0.0560), pupal survival (P = 0.2510) and development time (P = 0.190) and sex ratio (P = 0.8970).