Studies on Damage by Prostephanus Truncatus (Horn) (Coleoptera: Bostrichidae) And Araecerus Fasciculatus (Degeer) (Coleoptera: Anthribidae) to Dried Yam Chips

Abstract

In West Africa, especially Nigeria, Benin Republic, Togo and Ghana, dried yam chips constitute an important food material. The recent introduction of the larger grain borer into Africa and its devastating effect on cassava chips necessitated a study of the biology and damage caused by Prostephanus truncatus (Horn) (Coleoptera: Bostrichidea) and Araecerus fasciculatus (Degeer) (Coleoptera: Anthribidea) on dried yam chips in Ghana. Three yam varieties were investigated in the studies; these were: “Asaana” and “Pona” (Dioscorea rotundata Poir) and “Afasie” (Dioscorea alata L.). The dried yam chips were obtained following different treatments as follows: Parboiled oven-dried, Non-parboiled oven-dried, Parboiled sun-dried and Non-parboiled sun-dried. Two hundred grammes of each treatment of dried yam chips were placed in a glass jar and ten insects of P. truncatus and A. fasciculatus were placed in each separately as single cultures. Mixed cultures with a starting population of twenty insects: ten insects from each species of P truncatus and A. fasciculatus and a control were also set up. The studies were carried out for three durations: one month, two months and three months at 30 } 3°c and 70 - 92% R.H. The proximate compositions of dried yam chips were also investigated before and after infestation by P truncatus and A. fasciculatus. P. truncatus survived and established on all the treatments processed from the three varieties of yams. However, A. fasciculatus did not survive on oven-dried yam chips, but survived and established on sun-dried yam chips. Ten adults of P. truncatus (age 0-7days) produced 578 progeny and ten adults of A. fasciculatus (age 0-14 days) produced 328 progeny after three months of storage. The mixed culture with a starting population of twenty insects produced 501 progeny (322 of P. trucatus and 179 of A. fasciculatus) after three months of storage. Based on varietal differences, “Afasie” dried yam chips supported the highest progeny of P. truncatus and A, fasciculatus which was significantly more than that supported by “Asaana” and “Pona”. There was no significant difference between the number of progeny supported by “Asaana” and “Pona”. Therefore, “Afasie” dried yam chips were the most susceptible to infestation by P. truncatus and A. fasciculatus. P truncatus survived and established more significantly on dried yam chips processed from the three varieties than A. fasciculatus. Consequently, the percentage damage and weight loss produced by P. truncatus was high. The highest percentage weight loss produced by P. truncatus was 94.91% while A. fasciculatus produced 91.51% after three months of storage. Parboiled sun-dried yam chips and non-parboiled sun-dried yam chips in all the varieties used recorded the highest weight loss by P. truncatus and A fasciculatus. P truncatus produced significant weight loss on parboiled oven-dried yam chips and non-parboiled oven-dried yam chips for all the varieties of yam used for the studies, while A. fasciculatus did not, as it was unable to survive and breed on these chips. Based on varietal differences, P. truncatus and A. fasciculatus produced the highest weight loss on Afasie dried yam chips which was significantly more than those produced on “Asaana” and “Pona”. There was no significant difference in weight loss produced between “Asaana” and “Pona”. The proximate composition of the varieties of yam used for the studies showed slight differences after infestation by P. truncatus and A. fasciculatus. moisture increased significantly, while fat, ash, protein and fibre show no significant difference after infestation, carbohydrate, decreased significantly. Reducing and non-reducing sugars decreases drastically after infestation. Decreases in proximate compositions may be due to degradation and use by the insects for their biological activities. Based on these studies, P. truncatus was found to multiply rapidly and was more destructive than A. fasciculatus. Parboiled oven-dried yam chips and non-parboiled oven-dried yam chips were less damaged by P. truncatus but were not damaged by A. fasciculatus. P. truncatus and A. fasciculatus significantly damaged the parboiled sun-dried yam chips and non-parboiled sun-dried yam chips. Also the presence of insects on dried yam chips leds to increases or decreases in some proximate compositions. It can be concluded therefore that, “Asaana” and “Pona” are significantly better than “Afasie” for the production of yam chips that can be stored for longer periods.