Simple SummaryTransgenicBtcrops are important tools for growers to manage insect pests, but their use is threatened by the evolution of insect resistance, and monitoring programs are essential in detecting and responding to resistance. ForBtproducts in which insect control is not complete (“non-high-dose crops”), resistance monitoring is challenging, because insects and insect damage will be present even without resistance. Given these challenges, “sentinel plots” (designated monitoring plots) consisting ofBtand non-Btcontrol plots have been used to monitor for insect resistance to non-high-doseBtcrops by assessing changes in the efficacy of aBtcrop over time relative to a non-Btcontrol. We used this approach for ThryvOn™ cotton, a new non-high-doseBtproduct targeting two sucking pest types—Lygusand thrips—and report here on the thrips monitoring program. Monitoring for insect resistance over time requires knowledge of the baseline susceptibility, which is the initial assessment of the insect population response to a givenBtcrop prior to its widespread adoption. To characterize the baseline susceptibility of thrips to ThryvOn, we tested several approaches and found that the number of immature thrips on ThryvOn relative to the control cotton best characterized the efficacy of the trait.AbstractTransgenicBtcrops are important tools for growers to manage insect pests, but their durability is threatened by the evolution of insect resistance. Implementing a resistance monitoring program is essential to detect and mitigate resistance. For non-high-doseBtcrops, resistance monitoring is challenging, because insect control is not complete, so targeted insects and insect damage will be present even without resistance. Given these challenges, sentinel plots have been used to monitor for insect resistance to non-high-dose crops by assessing changes in the efficacy of aBtcrop over time relative to a non-Btcontrol. We optimized a sentinel plot resistance monitoring approach for MON 88702 ThryvOn™ cotton, a new non-high-doseBtproduct targeting two sucking pest taxa—Lygus(L. lineolarisandL. hesperus) and thrips (Frankliniella fuscaandF. occidentalis)—and report here on the thrips monitoring methods and results. Quantifying thrips immatures was the best metric to characterize the impact of the trait, with at least a 40–60% average reduction of thrips immatures on ThryvOn relative to the control cotton at all sites with higher thrips densities. These data can be used within a ThryvOn resistance monitoring program and represent a case study for establishing a resistance monitoring approach for a non-high-dose trait product.