Efficacy of microwave irradiation for phytosanitation of wood packing materials

• We tested the efficacy of microwaves at very short treatment times (0.5–3 min).
• We obtained 100% mortality in Cerambycidae larvae at 56 °C for 2 min.
• Fungi species survived up to higher temperatures than larvae (75–90 °C).
• Wood moisture content had no significant effect on treatment efficacy.

Wood packing materials (WPM) are important vectors of invasive xylophagous insects and pathogenic and decomposer wood fungi. The International Plant Protection Convention introduced the International Standards for Phytosanitary Measures No. 15 (ISPM No. 15) to regulate the development of treatments to sanitize WPM and prevent the introduction and movement of forest pests. Dielectric heating (e.g., microwave irradiation) has recently been included as an accepted treatment. In this study, the efficacy of microwave irradiation was tested on Monochamus scutellatus larvae and on four different pathogenic fungi, Gremmeniella abietina, Heterobasidion annosum, Chondrostereum purpureum, and Mycosphaerella populorum, five species of economic significance in Québec, in both jack pine and trembling aspen. We explored different temperature/time combinations on each species in order to accumulate data on the treatment. We irradiated M. scutellatus larvae at 56, 61, and 66 °C for 1–3 min and the four fungal species at 50, 55, 60, 65, 70, 75, and 90 °C for 0.5, 1, or 2 min. Fungi were tested at a wider range of temperatures to account for possible higher variation of resistance between species. We obtained 100% mortality in larvae treated at 56 °C for 2 min and at 61 °C for 1 min. The fungi species were much more resistant to the treatment. G. abietina was eliminated at 75 °C/0.5 min, H. annosum at 90 °C/1 min, M. populorum at 90 °C/2 min, and C. purpureum was still present at the highest temperature/time combination used. We demonstrated the capacity of microwave irradiation to kill the larvae with similar parameters as IPPC guidelines (60 °C for 1 min), though lethal temperatures for the fungi were very high. As the current ISPM No. 15 standard for microwave irradiation was insufficient to kill all tested fungal species, more work should be done on determining optimal combinations for the greatest number of species. Future studies should test a wider range of treatment times and expand trials to include more insect and fungal species to determine which temperature/time combination will allow us to keep both values as low as possible while assuring complete prevention of adult insect emergence and fungal re-growth.