Insecticide efficacy and emergence timing of the Douglas-fir twig weevil, <i>Cylindrocopturus furnissi</i> (Coleoptera: Curculionidae)

The Douglas-fir twig weevil (<i>Cylindrocopturus furnissi </i>Buchanan) has recently emerged as a significant pest of Christmas tree species grown in the Pacific Northwest United States. The larvae girdle and disfigure twigs, which adversely affects tree marketability. Trees produced for export are also routinely destroyed for phytosanitary reasons when <i>C. furnissi </i>is discovered at border crossings. Due to historically being a sporadic and benign pest on planted and natural Douglas-fir, there is a lack of chemical management recommendations. We assessed the knockdown effects of four insecticides commonly used on Christmas trees: one assay tested knockdown after direct contact for 24h and the other assay tested knockdown after being allowed to feed on treated twigs with 2d, 7d, and 14d residuals. Concurrently, we monitored temperature and adult <i>C. furnissi</i> emergence at a noble fir bough farm for two years to estimate the ideal degree-day window for applying insecticides. Bifenthrin and esfenvalerate knocked down all weevils on contact within just 4h, whereas chlorpyrifos and acephate failed to achieve 100% knockdown within 24h. Only acephate failed to knock down more weevils than the control (water) after feeding on treated twigs, regardless of the insecticide residue age. Degree day modelling revealed a variable emergence window between the two years but 50% of adult emergence occurred between approximately 1000-1100 degree days (January 1^st, 50 °F, single sine). Future work should assess the resulting management recommendation: apply bifenthrin or esfenvalerate just after 1000 growing degree days, once the year prior and once the year of harvest.