Ash Tree Borer Treatment in Idaho: Can This Tree Be Saved?

An Idaho homeowner has discovered borers in their ash tree, evidenced by multiple bore holes. The tree hasn't shown dieback symptoms yet, and the owner wants to know if fall treatment can save it.

Understanding the Borer Threat

Most wood-boring insects target trees already stressed by drought, injury, or disease. The primary defense is promoting vigorous growth. A two-pronged approach is recommended:

1. Cultural Care: Fertilization

Apply 10-10-10 fertilizer at 1 pound per inch of trunk diameter, scattered evenly under the canopy and watered in thoroughly. This boosts the tree's natural defenses and ability to compartmentalize damage.

2. Chemical Control: Systemic Insecticide

Spray the trunk with Merit (active ingredient: imidacloprid) now and again in April. This treatment kills larvae inside the trunk and, in spring, eliminates adult borers laying eggs. Merit is a systemic neonicotinoid that translocates through the vascular system.

Critical Timing Considerations

Fall application targets overwintering larvae. The spring application is crucial for intercepting the next generation's egg-laying adults. However, if the tree is already heavily infested or shows crown thinning, recovery chances diminish. Emerald ash borer (EAB) is a particular concern in many regions—verify the borer species with local extension services.

Note: Imidacloprid use is restricted in some areas due to pollinator concerns. Check local regulations and consider trunk injection by a licensed applicator as an alternative to foliar/spray application.