Insecticide toxicity and degree-day modelling of an emergent Christmas tree pest, the Douglas-fir twig weevil (Cylindrocopturus furnissi)

Previously known only as a minor pest in Oregon and Washington, the Douglas-fir twig weevil (Cylindrocopturus furnissi) has recently caused substantial economic losses for Christmas tree growers of all sizes. The small, native beetle has also become a regulatory concern – from 2014 to 2018 over 50% of the rejected Christmas tree loads shipped from Oregon to Mexico were rejected due to twig weevil. In an effort to develop a pest management strategy on adult weevils, the most susceptible life stage to contact insecticides, we (1) conducted laboratory experiments to determine which commonly used pesticides are most effective and (2) developed a degree-day model for adult emergence. Two pyrethroids, bifenthrin and esfenvalerate, were the superior chemistries, killing weevils faster on contact and more reliably after a long residual period than the organophosphates we tested. The number of growing degree days (base 50 °F, January 1^st biofix, single sine method) for 50% of the adult weevil population to emerge ranged from 1000 to 1200, with the entire emergence window ranging from 500 to 1500. These results have provided a springboard for further research into Douglas-fir twig weevil management in Pacific Northwest Christmas trees. Field testing the efficacy of the optimal insecticides and timing from this research will be a future focus.