Field efficacy of the mixture of chlorfenapyr and Bt against Plutella xylostella and the residue dissipation dynamic of chlorfenapyr in Brassica oleracea var. italica

This study aimed to evaluate the field control efficacy of mixture of chlorfenapyr and Bacillus thuringiensis (Bt) against Plutella xylostella and clarify the residue dissipation dynamic of chlorfenapyr in Brassica oleracea var. italica (broccoli), which can provide basis for reducing the usage of chlorfenapyr in pest control of broccoli. The field efficacy trials were conducted by spraying insecticide manually against P. xylostella in broccoli field, and chlorfenapyr residues in broccoli were measured by high performance liquid chromatography-tandem mass spectrometry. The results demonstrated that the control efficacies against P. xylostella of the mixture of chlorfenapyr 10% EC with Bt 32 000 IU/mg WP at 50% of their recommended doses at 1d, 3 d and 7 d after treatment were 58.11%, 65.21% and 58.80%, respectively. Although the control efficacies did not show significant difference from that of chlorfenapyr at the recommended dose, they were significantly higher than that of chlorfenapyr at 50% of the recommended dose and that of Bt at 50% of the recommended dose. Moreover, the efficacy of the mixture was also significantly higher than that of Bt at the recommended dose at 3 d after treatment. In addition, the residue dissipation dynamic of chlorfenapyr in broccoli after being treated with the mixture of chlorfenapyr 10% EC and Bt 32 000 IU/mg WP at 50% of their recommended doses was similar to that in broccoli treated by chlorfenapyr at 50% recommended dose, whereas the chlorfenapyr residues in broccoli were significantly lower than that of the treatment of chlorfenapyr at the recommended dose individually. After spraying the mixture of chlorfenapyr with Bt at 50% of their recommended doses for controlling P. xylostella on broccoli, the dissipation dynamic equation of chlorfenapyr in broccoli was fitted the quasi-first order or first-order reaction dynamic equation, and the half-life of chlorfenapyr was estimated as 4.2 d. At 7 d after treatment, the chlorfenapyr residue in broccoli was 0.072 mg/kg, which was lower than the maximum residue limit (MRL) of chlorfenapyr in broccoli 0.50 mg/kg (referenced the MRL in Australia). At 14 d after treatment, the residue of chlorfenapyr in broccoli was lower than the limit of quantification (0.050 mg/kg). This study showed that the mixture of chlorfenapyr 10% EC and Bt 32 000 IU/mg WP at 50% of their recommended doses could control P. xylostella on broccoli, which reduced the usage of chlorfenapyr for P. xylostella control and therefore reduced the residue of chlorfenapyr in broccoli.