Variety-Specific Residue Dynamics and Optimal Dose of Thiamethoxam for Borer Control and Harvest Safety in Sugarcane

Although thiamethoxam is frequently used in sugarcane production in Guangxi, China as a systemic insecticide and bioactivator, little is known about its field control effectiveness and residue dissipation across commercially significant genotypes grown in the region. In this study, we have addressed this gap by assessing the residue dynamics and borer control effectiveness of thiamethoxam in three significant sugarcane types (GT 40, Haizhe 22, and ROC22). Three application doses (Low: 750 mL/hm2; Medium: 1500 mL/hm2; High: 2250 mL/hm2) were sprayed as a soil drench along with urea for field and greenhouse studies. Over the course of 75 days, residues in juice and leaf tissues were measured using LC-MS/MS, and stem borer damage and control effectiveness were recorded. Thiamethoxam residues in juice peaked at 45 days after treatment (DAT), with ROC22 exhibiting the highest concentration (11.07 mg/kg). All treated plots continued to have thiamethoxam residues above the Chinese maximum residue limit (MRL) of 0.3 mg/kg at 75 DAT, suggesting an inadequate pre-harvest interval. Leaf residues remained a long-term reservoir for more than 56 days and were an order of magnitude greater. While the low dose demonstrated increased efficacy over time, the medium and high doses produced statistically equal borer control (50–75% efficacy), exhibiting a saturation effect. There were notable varietal differences: Haizhe 22 achieved steady control with fewer residues, while ROC22 was most vulnerable and needed greater doses. These results demonstrate that a 75-day pre-harvest period is insufficient for consumer safety, requiring additional experiments for longer intervals for susceptible cultivars. The ideal economic threshold is determined to be the middle dose (1500 mL/hm2), which balances environmental load, chemical input, and efficacy. To enable precision application techniques suited to certain sugarcane genotypes, future research should concentrate on clarifying the genetic basis of varietal uptake discrepancies.