Hessian fly, Mayetiola destructor (Say), puparial mortality was evaluated in three modern hay compressors that produce compressed standard and large-size bales for export to Asia-Pacific countries. Pressure on bales ranged from 93.4 to 139.4 kg/cm², causing 90.0–99.9% mortality of 10,891–23,164 puparia. Puparial response to a cylinderized hydrogen phosphide (1.8–2%) and carbon dioxide (97.8–98%) gas mixture was evaluated as a potential quarantine treatment using 2–4 d-exposures to low, medium, and high doses of 0.73–0.86, 1.05–1.26, and 1.39–1.56 mg/liter, and temperatures of 5.87 ± 1.14, 9.84 ± 0.05, 16.14 ± 0.14, and 20.35 ± 0.11°C. Accumulative concentration multiplied by time products (mg h/liter) at all fumigation temperatures for low, medium, and high fumigant doses were 34.9–37.7, 52.2–54.3, and 67.9–73.1 for 2d; 52.7–60.6, 77.9–89.2, and 102.1–110.7 for 3d; and 69.9–82.0, 99.4–118.2, and 132.3–146.8 for 4 d, respectively. An increase in mortality was significantly related to an increase in fumigation duration at 5, 10, and 15°C, and an increase in fumigant dose at 10 and 15°C. Puparial mortality ranged from 97.2 to 100% at all doses and durations at 20° C with no survivors at the highest dose for 3 d and the mid- and highest dose for 4 d. Bale compression is currently used in the first phase of a multiple quarantine treatment to control potential Hessian fly contaminants in exported hay. The novel fumigant may have application as a single quarantine treatment for noncompressed, standard exported bales.