Effect of sequential planting on occurrence and population dynamics of sweet potato weevils on selected varieties in Central Tanzania

MSc. Dissertation 2018

The sweetpotato (Ipomoea batatas L.) is one of the most important food crops in Africa and the world that is in Tanzania produced in largest quantities in the central Regions where, however, yields are very low due to abiotic and biotic constraints. One of the most serious biotic constraints is the sweet potato weevil, an insect whose cryptic feeding nature of the very destructive larval stages makes chemical control often not effective. A survey was conducted in two Districts of central Tanzania (Ikungi – Singida and Gairo - Morogoro) to map the sweet potato weevil distribution and damage incidences. Ten villages in each District were randomly selected whereby three farmers in each village were interviewed and their fields intensively assessed for weevil infestation. Results showed varying distribution of weevils with locations (villages) surveyed. Gairo District was having more weevil infestation and damage incidences than Ikungi District. Due to the nature of damage in both vines and roots it was concluded that the species available in the area is Cylas spp. Further study was conducted to establish variety response to the weevil infestation at different times of planting. Four sweetpotato varieties were tested for weevil infestation in three timings of planting (December, January and March). Again, adult weevils were captured in traps throughout the growing season. Significant differences in percentage weevil infestation and yield amongst various planting periods was observed (P≤0.05) with the least infestation registered in December while the highest infestation was realized in March planting. Simama variety was most susceptible to weevil infestation followed by Polista and Mataya while Gudugudu was least susceptible. Despite its susceptibility, however, Simama yielded the highest. The weevil capture experiment suggested existence of only one species in central Tanzania, whose distribution was uneven among the two Districts. This species was Cylas puncticollis. Trends of population build up in the study locations indicated that adult weevils increased progressively along the growing season. The study concluded that Cylas puncticollis will remain a problem in sweetpotatoes in central Tanzania. Concerted efforts in form of IPM and education to farmers are required to minimize the pest’s damaging effects.