Influence of temperature and relative humidity on the combined efficacy of insecticides and diatomaceous earth for the control of Trogoderma granarium on rough rice

Binary combinations of insecticides with diatomaceous earth are gaining importance for the control of stored insect pests; however, efficacy of these combinations and subsequent control of insect pests are greatly affected by the abiotic factors under storage conditions. Here, we studied the effects of temperature and relative humidity on the effectiveness of binary combinations of pirimiphos-methyl, alpha-cypermethrin, thiamethoxam or spinetoram with diatomaceous earth against larvae of Trogoderma granarium on rough rice. Binary combinations of insecticides (at 0.1, 1 or 10 ppm) with diatomaceous earth (500 ppm) were evaluated at 25, 30 or 35 °C and 55 or 75 % relative humidity after 7, 14 and 21 d interval. Overall, the efficacy of binary combinations of pirimiphos-methyl, thiamethoxam or spinetoram with diatomaceous earth generally increased with the rise of temperature, while the opposite was noted in the cases of alpha-cypermethrin + diatomaceous earth combinations. With few exceptions, mortality of T. granarium larvae was lower at 75 % relative humidity than that observed at 55 % relative humidity level. Complete mortality (100 %) was observed at 35 °C in the combinations of pirimiphos-methyl or thiamethoxam (10 ppm) + diatomaceous earth (500 ppm) in both humidity levels; spinetoram (10 ppm) + diatomaceous earth (500 ppm) treatment at 55 % relative humidity, after 21 d of exposure. The results of progeny produced revealed that at each dose rate of the binary combinations, with few exceptions, the number of adult produced increased with rise in temperature in both relative humidity levels. Overall, more numbers of adults were recorded at 75 % relative humidity level than those observed at 55 %. In conclusion, variations in dose rates, temperature and humidity should be taken into consideration when binary combinations of insecticides with diatomaceous earth are included in the management plan for T. granarium in stored rice.