Insecticide Resistance Dynamics and Spinosyn Cross-Resistance in Megalurothrips usitatus in Hainan, China

Megalurothrips usitatus is a major pest of cowpea in tropical regions, where frequent insecticide use has promoted the development of resistance. In this study, field populations of M. usitatus were collected from five major cowpea-producing areas in Hainan from 2023 to 2025, and their susceptibility to five commonly used insecticides was evaluated using a modified leaf-tube residual film method. Resistance levels varied among regions and insecticides, with southern populations generally showing higher LC₅₀ values than central and northern populations. The tested neonicotinoid insecticide, acetamiprid, showed the highest resistance levels. By 2025, the resistance ratios of the Ledong, Sanya, and Lingshui populations to acetamiprid reached 298.48-, 139.60-, and 130.25-fold, respectively. Spinosyn resistance also increased rapidly, particularly in southern Hainan. The resistance ratio to spinetoram reached 110.78-fold in the Ledong population, while resistance to spinosad reached 37.06-fold in the Lingshui population. However, the absolute LC₅₀ values of spinosad and spinetoram remained relatively low compared with those of acetamiprid and chlorfenapyr, indicating that these two spinosyn insecticides retained relatively high bioassay activity in laboratory bioassays. A significant positive correlation was detected between the LC₅₀ values of spinosad and spinetoram across field populations (r = 0.8972, p < 0.0001), suggesting cross-resistance within the spinosyn class. These results indicate that resistance management for M. usitatus in Hainan should prioritize reducing the use of high-resistance-risk insecticides, avoiding consecutive applications of spinosyns, and designing region-specific rotation programs based on local resistance monitoring.