Oxygen enhances phosphine toxicity for postharvest pest control.

Phosphine fumigations under superatmospheric oxygen levels (oxygenated phosphine fumigations) were significantly more effective than the fumigations under the normal 20.9% atmospheric oxygen level against western flower thrips [Frankliniella occidentalis (Pergande)] adults and larvae, leafminer Liriomyza langei Frick pupae, grape mealybug [Pseudococcus maritimus (Ehrhorn)] eggs, and Indianmeal moth [Plodia interpunctella (Hübner)] eggs and pupae. In 5-h fumigations with 1,000 ppm phosphine at 5 degrees C, mortalities of western flower thrips increased significantly from 79.5 to 97.7% when oxygen was increased from 20.9 to 40% and reached 99.3% under 80% O2. Survivorships of leafminer pupae decreased significantly from 71.2% under 20.9% O2 to 16.2% under 40% O2 and reached 1.1% under 80% O2 in 24-h fumigations with 500 ppm phosphine at 5 degrees C. Complete control of leafminer pupae was achieved in 24-h fumigations with 1,000 ppm phosphine at 5 degrees C under 60% O2 or higher. Survivorships of grape mealybug eggs also decreased significantly in 48-h fumigations with 1,000 ppm phosphine at 2 degrees C under 60% O2 compared with the fumigations under 20.9% O2. Indian meal moth egg survivorships decreased significantly from 17.4 to 0.5% in responses to an oxygen level increase from 20.9 to 40% in 48-h fumigations with 1,000 ppm phosphine at 10 degrees C and reached 0.2% in fumigations under 80% O2. When the oxygen level was reduced from 20.9 to 15 and 10% in fumigations, survivorships of Indianmeal moth eggs increased significantly from 17.4 to 32.9 and 39.9%, respectively. Increased O2 levels also resulted in significantly lower survival rates of Indianmeal moth pupae in response to 24-h fumigations with 500 and 1,000 ppm phosphine at 10 degrees C and a complete control was achieved in the 1,000 ppm phosphine fumigations under 60% O2. Oxygenated phosphine fumigations have marked potential to improve insecticidal efficacy. Advantages and limitations of oxygenated phosphine fumigation are discussed.