Screening of optimal formulations of insecticide mixtures and validation of the field efficacy against Spodoptera exigua

In order to delay the development of the insecticide resistance and screen the highly efficient mixtures against Spodoptera exigua (Hübner), the interaction assay and co-toxicity coefficient method were used to evaluate the synergistic effects of different mixtures on S. exigua, and the field efficacy trial was conducted to verify the synergistic mixtures. The results of initial screening by interaction assay and rescreening by co-toxicity coefficient method showed that the proportion of chlorfenapyr and beta-cypermethrin at 7 : 3-12 : 1 (mass ratio of active ingredients at 1.20 : 1.00-6.23 : 1.00) had the synergistic effect on S. exigua, and the proportion of 9 : 1 (mass ratio of active ingredients at 4.63 : 1.00) had the highest co-toxicity coefficient of 314.72, and the synergistic effect was the greatest. The three mixtures with the proportion of 7 : 3, 9 : 1 and 11 : 1 (mass ratio of active ingredients at 1.20 : 1.00, 4.63 : 1.00 and 5.71 : 1.00) were selected for the field efficacy test. Compared with the single insecticides, the control efficacies of these three mixtures were improved at middle dosage (75.0 g a.i./hm²) and high dosage (100.5 g a.i./hm²) on each day after treatment against S. exigua, and the highest control efficacy reached 93.58% on the third day after treatment. According to the above results, the mass ratio of active ingredients of chlorfenapyr and beta-cypermethrin at 1.20 : 1.00-6.23 : 1.00 had the synergistic effect on S. exigua, which could be used for the control of S. exigua in Wuhan, and the recommended dosage was 75.0-100.5 g a.i./hm².