Infestation of dried cassava ( Manihot esculenta Crantz) by Prostephanus truncatus (Horn( (Coleoptera: Bostrichidae)

Abstract

Stored dried cassava is known to become heavily infested by Prostephanus truncatus (Horn). A field study was undertaken in Tanzania to determine the extent of losses that this pest could cause in fermented and unfermented dried cassava roots stored over a period of about 4 months. In fermented roots, the mean weight loss (±SD) rose to 73.6 ± 25.9% over this period compared with 52.3 ± 12.0% in unfermented roots. At each time interval that roots were examined, the weight loss in fermented roots was significantly higher. Subsequent laboratory studies, undertaken to identify some of the factors responsible for this difference between the two types of cassava comprised (1) a determination of the length of larval and pupal development at 27°C and 50 or 70% r.h., (2) adult preference for the two types, and (3) their susceptibility to adult boring. P. truncatus developed at a similar rate in both fermented and unfermented roots, but the adults appeared to prefer the fermented cassava possibly because it was easier to bore into. This was certainly one factor making fermented roots more susceptible to damage by adults. It is concluded that although P. truncatus caused lower weight loss in the unfermented compared with the fermented roots, both were so heavily damaged that it is not worth recommending the storage of one rather than the other. The role of cassava as an intermediate host for P. truncatus is discussed and consideration is given to the need to control the beetle in cassava in order to reduce cross-infestation to maize.