The development of postharvest quarantine treatments can be both expensive and time consuming. It is necessary to determine the species and stage of the pest most tolerant to the treatment, if more than one species is the target of the treatment. Initial laboratory studies often include infesting the commodity with various egg and larval stages of the pest and performing treatments and evaluations of the fruit. We have previously developed combination high temperature under controlled atmosphere treatments (CATTS) against two quarantine pests in apples, peaches and nectarines. We decided to develop an artificial system that can be used for these initial tests without the need for infesting large quantities of the fruits. We tested the system on the immature stages of the pests under regular air and controlled atmospheres using the controlled atmosphere water bath system (CAWB). We compared these data to previous in-fruit treatments and found that the CAWB gave a good approximation of the most tolerant stage and species to the CATTS treatments. This system can be used for rapid assessment of the most tolerant stage and species of a pest to CATTS without the expense of infesting, treating, and evaluating the commodity.