Recent research on treating Hemlock woolly adelgid with Imidacloprid injected into the soil is showing five to seven year control of this insect. This makes treating with Imidacloprid once every five years or more cheaper in the long run than using horticultural oil every year, especially for taller and hard to reach trees.
Research also shows that control is achieved at the lowest application rate and that at this lower rate Imidacloprid is not as hard on the predators of two other Hemlock pests, Scale and Eriophyid mites.
More good news is that the recovery for trees in significant decline due to woolly adelgid is better than previously thought.
Of course biological control of this pest is the preferred long term strategy, but Imidacloprid may be the best way to keep some hemlocks alive until this becomes viable.
Research on the long term control using Imidacloprid.
Recovery of trees in decline from Hemlock woolly adelgid infestations.