The combination of heat and low levels of oxygen increases mortality to insects infesting fruit compared with either heat or low oxygen alone. This combination treatment shows promise to disinfest commodities of quarantine pests. Heated air/modified atmosphere treatments employ the modified atmosphere (e.g., low oxygen) during the entire treatment interval. There is a positive relationship between temperature and efficacy of heat/modified atmosphere treatments. Efficacy of delaying atmospheric modification in a heat/modified atmosphere treatment was studied with the Mexican fruit fly, Anastrepha ludens (Loew) (Diptera: Tephritidae), a quarantine pest of citrus and other fruit in Mexico, Central America, and southern Texas. Larvae were subjected to heat/low oxygen treatments in vitro as well as in grapefruit, Citrus paradisi Macfayden. The relationship between time delay of the modified atmosphere and estimated time required to kill 99% of Mexican fruit fly third instars was not linear, which would indicate an additive relationship, but followed a sigmoid relationship. When infested grapefruit were heated with 47°C air in three atmospheric regimes: 1) air; 2) N₂ at 99 kPa plus O₂ at 1 kPa; or 3) air for 55 min then N₂ at 99 kPa plus O₂ at 1 kPa for the remainder of the treatment, estimated 99% prevention of pupariation was 157, 127, and 141 min, respectively.