Vulnerability of three stored grain insect pests to bio pesticides and their impact on grain quality parameters.

Abstract

Trogoderma granarium, Sitophilus zeamais and Rhyzopertha dominica are very notorious stored cereals insect pests. This research was accomplished to determine efficacy of some bio-pesticides including entomopathogenic fungi (EPF), bacterial based insecticides and plant extract based commercial formulations, alone and in combinations, against T. granarium, S. zeamais and R. dominica at optimum growth conditions. Also, feeding preferences of test insects for maize was checked and their impact on grains quality parameters was determined by performing biochemical analysis. Regarding mortality assays of bio-pesticides insect diet (maize kernels) was treated with three concentrations of each bio-pesticide i.e for EPF (0.5 x 108, 1 x 108 and 1.5 x 108 conidia/kg grain) for bacterial based insecticides (0.5, 1.0 and 1.5ppm) and for plant extracts (50, 100 and 150ppm). Insects data regarding their percent mortality was noted subsequently by exposure time of 7, 14 and 21 days. Best result producing treatments of each EPF were identified and their combine effect with best treatment of remaining tested bio pesticides was evaluated for percentage mortality. For feeding preferences weighed amount of diet was put into treatment jars and 40 test insects were released into these jars and data regarding weight gain of insect and weight loss of grains was measured. Nutritional indices of R. dominica, S. zeamais and T. granarium was done by calculating the CI (Consumption Index), RGR (Relative Growth Rate), AD (Approximate Digestibility) and ECI (Efficiency of Conversion of Ingested Food into Biomass). Biochemical analysis was done by help of AOAC and ICC procedures for finding of crude amounts of fat, protein, fiber, carbohydrates and ash contents within the grains. Completely Randomized Design (CRD) was used with three repeats of each treatment. Collected data was analyzed with a latest statistical software, R-studio. Among all tested bio pesticides Spinetoram performed best against R. dominica, S. zeamais and T. granarium as complete control against adults of R. dominica and S. zeamais was achieved after 21 days at all dose rates while resulted 64% mean larval mortality against T. granarium with highest dose rate. Performance of Spinosad remained second best after Spinetoram, which has also resulted 100 percent mean adult mortality against adults of R. dominica (at all dose rates) and S. zeamais (at maximum dose rate) while mean mortality was 56.67% against larvae of T. granarium at highest dose rate after 21 days. R. dominica remained most susceptible against EPF (Metarhizium anisopliae and Isaria fumsorosea) followed by S. zeamais and T. granarium while M. anisopliae remained better that I. fumosorosea in causing insects mortality. The toxic effect of Azadirachta indica caused 80, 74.17 and 39.17 percent mean mortality against R. dominica, S. zeamais and T. granarium respectively by using maximum dose rate of 150ppm after 21days. Single use of Pongamia pinnata against R. dominica, S. zeamais and T. granarium resulted in 84.17, 79.5 and 44.5 % mean mortality respectively after 21 days of exposure using higher dose rates (150ppm). Combination of EPFs with Spinetoram, Spinosad, A. indica and P. pinnata against test insects showed significant differences as compared to their alone results. Results showed more mortality of targeted insect in case of their combinations as their alone. The results of qualitative losses proved that maximum reduction of protein, ash and carbohydrate contents in maize grains were noticed by feeding of S. zeamais followed by R. dominica while minimum reduction was observed in case of T. granarium. Maximum fiber loss was observed in maize grains after feeding of T. granarium followed by in case of R. dominica while minimum carbohydrates reduction was observed due to the feeding of S. zeamais on maize grains. Maximum fat loss was observed in maize grains after feeding of R. dominica followed by in case of T. granarium while minimum carbohydrates reduction was observed due to the feeding of S. zeamais on maize grains. From presented study it is concluded that all the tested bio-insecticides are effective to control R. dominica, S. zeamais and T. granarium up to various extents therefore, these bio-insecticides can efficiently be used in integrated pest management strategies against stored grain pests.