Water hardness and acidity are the important properties that can influence the performance of pesticides. The effect of water hardness and pH on the efficacy of insecticides on second instar nymph of Bemisia tabaci was determined by leaf dip method. In the first experiment, well water samples at 1869, 645, and 265 mg L-1 hardness, standard, and deionized water with malathion, acetamiprid, and spiromesifen used for bioassay. In another experiment, the effect of water pH at four levels 4, 6, 7, 9 on the toxicity of the above three insecticides was investigated. Increasing water hardness reduced the performance of the experimental insecticides. LC50 values showed that the toxicity of malathion, acetamiprid, and spiromesifen was 40, 157, and 84 times less in water with 1869 mg L-1 hardness than deionized water. The efficiency of malathion, acetamiprid, and spiromesifen was 13, 65, and 39 times less in water with 645 ppm hardness than deionized water. Pesticides were less effective at pH 9 compared to other pH levels. Malathion efficacy was 11.41 and 7.16 times better when it was diluted in water with pH 6 and 7 than pH 9. Acetamiprid provided 10.72 and 5.89 times greater toxicity when applied at pH 7 and 6 respectively than water with pH 9. Spiromesifen in deionized water with pH 7 and 6 has exhibited 5.51 and 3.82 times better efficacy compared to the water at the pH level of 9. Overall, the performance of insecticides reduced at hardness levels above 265 ppm and in alkaline pH.
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