Each pest management scenario is unique and should be addressed on an individual basis. Generally speaking however, the preferred insecticide used against the spruce budworm is a naturally occurring bacterium known as Bacillus thuringiensis (Bt). It is preferred because it is specific to Lepidoptera larvae, like SBW, and naturally photo-degrades over time. Bt must be applied to needles and physically ingested by SBW larvae while they’re feeding to be lethal (not a contact insecticide), so the timing of application is linked to the active feeding period (peak activity mid-May to Mid-June in Maine, with younger larvae being more susceptible).
As with all insecticides, application must adhere strictly to the guidelines provided on the chemical label. Usually SBW is only treated during outbreaks and on a grand scale, but there are also insecticides with the active ingredient BT readily available to homeowners interested in treating ornamental trees in an outbreak area. The timing of application is always most effective during times that minimize drift and ensure good coverage directly to the foliage. This means avoiding excessive breeze, heat, and moisture, usually coinciding with morning or evening hours.
For a wealth of additional information on SBW in Maine, please have a look at the Maine Forest Service SBW page here.