Sustained Control of the Pine Wilt Disease Vector Monochamus alternatus in Pinus thunbergii Forests Depends on Residual Efficacy, Not Initial Knockdown

Pine wilt disease control often depends on reducing adult beetle activity, but how aerial spraying performs under operational forest management conditions remains poorly understood. We evaluated a 2022 operational aerial spray program in Pinus thunbergii stands in Shandong, China, by combining droplet deposition measurements, branch-feeding bioassays to assess acute and residual toxicity with Monochamus alternatus, and seasonal trap monitoring of both M. alternatus and Arhopalus rusticus, a relevant co-occurring cerambycid species. Spray cards showed that insecticide reached the stand in both spray rounds, although vertical differences between upper and lower strata were stronger during the second application. Branches collected immediately after spraying caused rapid mortality of M. alternatus, and both strata reached complete mortality within 72 h of exposure. In contrast, branches collected one month later caused little additional mortality beyond control levels, indicating that biologically effective exposure declined quickly and provided an insufficient window of protection relative to the full period of adult beetle activity. Trap data matched this pattern. After the first spray, M. alternatus captures dropped sharply during the immediate post-spray period but rebounded before the second spray. A. rusticus showed a similar short-term response, but its seasonal activity pattern differed from that of M. alternatus. Overall, the main limitation of the spray program was not weak initial toxicity, but the short duration of effective control relative to beetle activity in the field. This study shows that better aerial control of pine wilt vectors will depend on matching spray timing and residual persistence with local beetle phenology to improve the design and timing of aerial control programs in pine wilt management.