Potential of Microbial Insecticides and Diatomaceous Earth Against Three Coleopterous Insect Pests of Stored Grains Under Different Abiotic Conditions

Abstract

Sitophilus granarius (L.), Rhyzopertha dominica (F.) and Oryzaephilus surinamensis (L.) are the damaging insect pests of cereals and their commodities and are able to cause significant losses. Current research was performed to evaluate the lethal and progeny inhibition effect of microbial-based insecticides i.e entomopathogenic fungi (EPF) Metarhizium anisopliae and Beauveria bassiana as well as bacterial based insecticides (spinetoram and abamectin) solitary and with combination of two formulations of diatomaceous-earth (DE) against S. granarius, O. surinamensis and R. dominica. Three concentrations (1x108, 1.5x108 and 2x108 spores/kg grain) of each EPF were used while for bacterial based insecticides concentrations were 0.5, 1.0 and 1.5ppm. Similarly, for diatomaceous earth, three different doses (200, 400 and 800 ppm) of every formulation were applied. Completely Randomized Design (CRD) was used with three repetitions of all treatments. Mortality of test insects was recorded after 7, 14 and 21 days while after 60 days for post-treatment progeny build up. Each bioassay was performed at three levels of temperature (25°, 30° and 35°C) and relative humidity (r.h.; 45, 60 and 75%). Collected statistics were examined with applicable statistical methods using R-Software. The data analyses had shown diversified results regarding test insect susceptibility at different abiotic conditions. In case of Bacterial based insecticides, the response of treatment remained best at high temperature and low r.h. for all test insects. Complete control was achieved after 14 days for S. granarius and R. dominica while for O. surinamensis 100% mortality was not attained even after 21-d of exposure period. Progeny production was remained totally suppressed for S. granarius and R. dominica while in case of O. surinamensis there was significant reproduction at some abiotic conditions with maximum production at temperature of 30°C with 75% r.h. The response of-D.E against test insects was also effective and the most vulnerable species was O. surinamensis followed by S. granarius and R. dominica. It was observed that high temperature, low r.h. levels, higher dosages and long exposure periods increased the efficacy of DEs. Regarding progeny production, low temperature and high humidity remained most favourable for progeny emergence in DE-treated grains. The effects of EPF remained somewhat similar to the response of DE but overall mortality was low. Among tested species of insects R. dominica remained most susceptible to the application of EPF followed by O. surinamensis and S. granarius. The best abiotic condition for B. bassiana was low temperature (25°C) and moderate (60%) humidity while for M. anisopliae moderate temperature (30°C) and moderate (60%) humidity remained suitable at which maximum response was attained. Long exposure intervals and higher dose rates increased the mortality in each case. In the case of progeny development, the emergence of offspring was suppressed at moderate and low temperature for M. anisopliae and B. bassiana respectively with moderate r.h. When DE was applied in combination with the microbial insecticides, a synergistic effect was noticed in all combinations and R. dominica and S. granarius remained more susceptible as compared to O. surinamensis. The response of abiotic condition was significant for combination of DE and EPF while for DE and bacterial based insecticide it was non-significant. The results of the study divulge that all applied microbial based insecticides and DE are very effective for the control of these test insects and different abiotic conditions are responsible for affecting their efficacy. Furthermore, the combinations of these microbial insecticides with DE have a synergistic response against test insects. This study also recommends that attention should be paid to the interaction of abiotic factors with the efficacy of DE and microbial insecticides before planning IPM strategy for stored grain insect pests.