Abstract:
The effective disinfestation of wood products for trade is critical to halting the spread of global invasive insect species. Invasive insects are being transported inadvertently with wood products, despite increasing awareness of this issue. With the arguably most useful and effective disinfestation tool for wood products, methyl bromide (MB), due to be phased out globally, the issue of finding equally effective replacements is pressing.
Phosphine (PH₃) has been in use for several decades to disinfest unprocessed radiata pine (Pinus radiata D.Don) logs during shipment as an alternative to MB in New Zealand. The PH₃ treatment used for pine logs is based on a fumigation schedule for grains. However, little is known about the physiology of the target pests, bark beetles, and how their physiology may affect disinfestation in an enclosed space during log transit. Further, the unique abiotic and biotic variables associated with the fumigation of whole pine logs within relatively airtight ship holds is largely unknown.
In this thesis I focus on three fundamental knowledge gaps related to the use of PH₃ for the disinfestation of pine logs in transit. Firstly, I quantified the respiration of recently harvested radiata pine logs during storage in an enclosed space. I showed how respiring pine logs dramatically changed the surrounding atmosphere to severely hypoxic (low O₂) and hypercapnic (high CO₂) within days of containment. I also demonstrated that there were strong seasonal differences in the amount and rate of CO₂/O₂ from the log sections.
Secondly, I determined the respiratory tolerances and gas exchange patterns of the bark beetle Hylurgus ligniperda (F.), a target pest species in radiata pine disinfestation. Building on the atmospheric data I obtained from the respiration of radiata pine logs, I tested the respiratory response of H. ligniperda to varying levels of O₂. H. ligniperda exhibited a continuous ventilatory pattern and was highly tolerant of hypoxic atmospheres across all three life stages tested: larvae, pupae, and adults. Pupae were the most tolerant to hypoxic atmospheres.
Thirdly, I analysed the efficacy of PH₃ against all life stages of H. ligniperda. Overall, 100% efficacy was not achieved for all life stages; pupae were the most tolerant of exposure to PH₃. Contrary to other studies, I found the egg life stage was the most sensitive to PH₃. PH₃ efficacy was most improved by a longer exposure, rather than an increase dose. iv
Measuring the respiration of both pine logs and bark beetles has highlighted the challenge of controlling endophagous insect pests in an enclosed space. Overall, my research suggests that control treatments could be less effective against bark beetle pests due to their ability to regulate their respiration under the severely hypoxic atmospheres that develop during the transportation of whole pine logs. This adaptation may reduce the insect’s uptake of PH₃ through the respiratory system and, hence, may explain why there has been varied operational success with the current treatment schedule.