Quantifying phosphine penetration through the bark of pine (Pinus radiata D.Don) logs

Phosphine (PH₃) is used as an in-transit phytosanitary treatment (10-d fumigation period) for pine (Pinus radiata D.Don) logs exported from New Zealand to China. The ability of PH₃ to penetrate through the bark of the logs is not well known. We designed equipment to accurately quantify PH₃ penetration into the bark of wooden blocks (100 × 100 × 50 mm; n = 12) cut from the upper and lower trunk of recently harvested pine trees fumigated at 15 or 25 °C. Fumigations simulated commercial conditions consisting of two phases; phase I with 2.0 g m⁻³ of PH₃ for the first 5 days (1–120 h) and 1.5 g m⁻³ of PH₃ in phase II for the next 5 days (121–240 h). During the 10-d schedule, we achieved the required commercial CT (concentration × time) of ≥48,000 ppm h at both temperatures. Bark thickness (i.e., trunk location) did not significantly affect fumigant penetration. Phosphine penetration through the bark of the wooden blocks was greatest after each application, then the penetrated concentrations diminished over time. Greater penetration occurred at 15 °C than at 25 °C. Further studies are required to better understand the dynamics of PH₃ penetration particularly at lower temperatures and through insect-infested blocks.