Synergistic effect of two spray adjuvants on thiamethoxam in the control of Megalurothrips usitatus in cowpea

Organosilicon adjuvant Silwet stik and vegetable oil adjuvant Maisi were selected to conduct field experiments with thiamethoxam to verify the preventive effect on cowpea thrips and residual behavior. Dissipation dynamic and residues of recommended application dose 75, 67.5, 60, and 52.5 g a.i/hm² of 25% thiamethoxam water dispersible granules (WG) with two different adjuvants were compared respectively and the preventive effect of 5 and 7 days after the last spray was investigated. The cowpea samples were extracted by acetonitrile, purified with primary secondary amine (PSA), and detected using high performance liquid chromatography tandem mass spectrometry (HPLC-MS/MS). The results of field tests indicated that the initial deposition of thiamethoxam on cowpea were significantly increased by 8.84% and 6.58% respectively when the adjuvant 0.4% Silwet stik or 0.5% Maisi was added. The control effect against cowpea thrips of 25% thiamethoxam WG added with the two adjuvants above were significantly higher than that without adjuvant 7 days after treatments. Five days after treatments, the addition of adjuvant Silwet stik or Maisi increased preventive effect by 4.04%-8.55% and 10.37%-12.25% respectively, while the recommended application dose of thiamethoxam was reduced by 10%-30%. The results showed that the adjuvant treatments increased the deposition of thiamethoxam on cowpea, which would benefit the synergistic effect of pesticide, and improved control effect on thrips. The study provides evidence for the synergistic control effect of adjuvants and the scientific use of thiamethoxam against cowpea thrips.