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Simple Summary: Spinetoram is a semi-synthetic bioinsecticide. This study evaluated the efficacy of spinetoram, the main insecticide against cowpea thrips, on cowpea in the main production areas of Hainan. The maximum dose bioassay method was used to assess field populations of cowpea thrips collected from 20 villages in Yazhou District. The results showed that mortality rates of cowpea thrip populations ranged from 3.31% to 100%, with 66.98% of the populations exceeding 80% mortality and 33.96% exceeding 90% mortality. Significant differences in mortality rates were observed between populations from different villages. In conclusion, the maximum dose bioassay method provided valuable insights into the field efficacy of spinetoram against cowpea thrips and highlighted the importance of caution when using it in combination with other methods to reduce the potential for resistance. The bean flower thrip Megalurothrips usitatus (Bagnall) is a severe pest on cowpeas and causes a 20–30% reduction in cowpeas in Hainan, China, with even complete crop failure in severe cases. Spinetoram is currently the most important pesticide against M. usitatus in cowpea production. In the main producing areas of cowpeas in Hainan, however, the efficacy of spinetoram against M. usitatus is not well known. In the present study, we employed the maximum dose bioassay to evaluate the efficacy of the mortality rates of adult thrips at F0 in spinetoram, freshly collected from 212 field populations of M. usitatus collected from 20 villages in the Yazhou District of Hainan. Our results showed that the mortality rates of these thrip populations exposed to spinetoram were from 3.31% to 100%. Among them, the mortality rates of 66.98% (142/212) of the populations exceeded 80%, while that of 33.96% (72/212) of the populations surpassed 90%. Only a small proportion of 0.47% (1/212) the populations exhibited a mortality rate below 10%, and 4.72% (10/212) displayed rates below 50%. Furthermore, significant differences were also observed in the mortality rates of thrips among different villages. Taken together, the maximum dosage bioassay method is a rapid and easily implemented approach providing valuable insights into the field efficacy of insecticides and offers guidance in determining the optimal dosage required in the field. Spinetoram is still effective against M. usitatus in the main producing areas of cowpeas in Hainan, but caution should be exercised in its combined use with other methods to reduce potential resistance. [ABSTRACT FROM AUTHOR]