Controlled atmosphere/temperature treatment system (CATTS) is an environmentally friendly postharvest mitigation treatment that uses high temperature forced-air combined with a low oxygen and high carbon dioxide atmosphere to control quarantine pests. The development of CATTS treatments is expensive and time-consuming. For a more rapid assessment of different species and life stages' tolerances to heated controlled atmospheres, the controlled atmosphere water bath (CAWB) system can be used to help advance the development of CATTS treatments for pests. The CAWB system was used to test the response of eggs and larval stages of Thaumatotibia leucotreta (Meyrick) (Lepidoptera: Tortricidae). Eggs and larvae at different developmental stages were treated under regular air and a modified controlled atmosphere of 1% O₂ and 15% CO₂, at two ramping heat rates: 12 and 24°C/h. Typically the faster heat rate and modified atmosphere reduced treatment times required to control the different life stages. T. leucotreta larvae were more tolerant of the treatments than eggs. The most tolerant life stage was the fourth instar. Effective treatments against the most tolerant life stage determined by the CAWB system can now be used to develop CATTS technology against T. leucotreta. Further research will focus on developing CATTS treatments using infested fruit to determine effective treatments that maintain fruit quality.