Why I Recommend a Once-a-Year Insecticidal Drench for Trees

By Paul Oliver, Founder, TreeHelp.com. 

After decades working with trees, the question I hear most is how to protect a tree from insect damage without spraying constantly or climbing a ladder. My top recommendation is a once-a-year insecticidal soil drench. It contains imidacloprid, a systemic insecticide the roots absorb and carry through the entire tree, so a single annual application gives long-lasting, rainproof protection from the inside out — simple to apply, and effective against some of the most common and destructive pests.

Why I like it: long-lasting, low-effort protection

The drench contains imidacloprid, absorbed through the roots and distributed throughout the vascular system. Once applied, it protects the whole tree — leaves, bark, branches, and roots — with no spraying. The real advantage is convenience: you only apply it once per year. That saves time and eliminates the drift and surface runoff that come with foliar sprays. It’s rainfast, odorless, and goes on with nothing more than a bucket or watering can. Shop Monterey's once-a-year systemic tree drench for season-long, spray-free protection.

What it works against

It’s highly effective against borers, scale insects, aphids, lace bugs, leafminers, and other sap-feeding and wood-boring pests. It’s especially useful for:

• Ash trees threatened by emerald ash borer
• Birch trees facing bronze birch borer
• Elm trees prone to elm leaf beetle
• Maples, oaks, and other ornamentals dealing with aphids, scales, or mealybugs

Because it works systemically, insects feeding on the tree ingest the toxin and die off — often before they can do serious damage.

How to use it

Application is straightforward and needs no special equipment:

• Determine the amount. Dosing is typically based on trunk diameter at chest height (DBH) — for example, a tree with a 10-inch-diameter trunk needs about 10 oz of product.
• Mix with water. Dilute the measured amount in a bucket or watering can with 1–2 gallons of water.
• Apply at the base. Pour slowly and evenly around the root zone; the soil should be moist so the product penetrates — don’t apply to dry soil.
• Water in if needed. If the soil is dry, follow with more water to move the drench into the root zone.

When to apply

Timing matters. The best window is early spring, just before or as the tree starts to leaf out, so the product is absorbed and distributed as the tree begins actively growing. For borers in particular, it’s important that the protection is in place before the adults emerge and lay eggs. In warmer climates, a fall application can also work if trees are still actively taking up nutrients and moisture.

A few pro tips

• Water first. If the soil is dry, give the tree a deep watering the day before — moist soil improves absorption.
• Avoid drought-stressed trees. A tree short on water may not take the product up properly.
• Mulch smartly. Keep mulch a few inches off the trunk so the drench penetrates the root zone and to prevent fungal issues.
• Use annually, not excessively. Once a year is enough; applying more often won’t improve results and could do harm.

Using it responsibly around pollinators and water

Imidacloprid is effective, but use it responsibly. Avoid applying near water bodies, and never apply to flowering plants where pollinators are actively feeding. Because it’s systemic, it won’t harm bees unless they’re foraging on treated blooms — so timing the application before flowering begins is key. Shop the once-a-year insecticidal drench for season-long, spray-free protection.

Frequently Asked Questions

How much drench do I use?

Dosing is based on trunk diameter at chest height (DBH) — roughly 1 oz of product per inch of trunk diameter (a 10-inch trunk needs about 10 oz), mixed into 1–2 gallons of water and poured around the root zone.

When is the best time to apply it?

Early spring, just before or as the tree leafs out, so it’s protected before borers emerge. In warm climates a fall application can also work while the tree is still taking up moisture.

Will it harm bees?

Not unless they forage on treated flowers. Because it’s systemic, timing the application before blooming protects pollinators — and avoid use near water.

Does it work on emerald ash borer?

Yes — it’s one of the pests this imidacloprid soil drench targets, along with bronze birch borer, elm leaf beetle, scale, aphids, and leafminers.