Management of ambrosia beetles in ornamental nurseries relies, in part, on insecticide treatments to prevent beetles from boring into trees. However, data on residual efficacy of commonly used pyrethroid insecticides is needed to gauge the duration that trees are protected during spring when peak beetle pressure occurs. Residual efficacy of bifenthrin and permethrin trunk sprays was examined in field trials which used trees injected with 10% ethanol to ensure host attack pressure. Permethrin consistently reduced attacks by Xylosandrus germanus (Blandford; Coleoptera: Curculionidae) and other ambrosia beetles for at least 4 wk, while efficacy of bifenthrin was inconsistent and lasted only about 10 d. Since previous studies demonstrated attacks are positively correlated with host ethanol emissions, we injected trees with 2.5, 5, and 10% ethanol to determine if residual efficacy was affected by attack pressure. Preventive treatments with bifenthrin reduced ambrosia beetle attacks at all concentrations of injected ethanol compared to non-sprayed controls. There was no interaction between attack pressure and insecticide treatment with respect to total attacks or attacks by X. germanus. However, increasing attack pressure did increase the probability of attacks on insecticide treated trees by X. germanus and other Scolytinae. Results from our current study will improve the ability of growers to make decisions on frequency of protective sprays, but residual efficacy of insecticide treatments may decline as attack pressure increases. Cultural practices should therefore maximize host vigor and minimize attack pressure associated with stress-induced ethanol emissions.