By Paul Oliver, Founder, TreeHelp.com

As someone who's spent decades working with trees and helping homeowners protect their landscapes, I'm often asked: “What’s the best way to protect my trees from insect damage without having to spray constantly or climb a ladder?” One of my top recommendations is Monterey Once-A-Year Insecticidal Drench — and with good reason.

This systemic insecticide has become a go-to solution for many of us in tree care. It’s simple to apply, provides long-lasting protection, and is particularly effective against some of the most common and destructive insect pests. Let me walk you through why I like it, how and when to use it, and some extra tips to get the most out of this powerful tool.

Why I Like It: Long-Lasting, Low-Effort Protection

The Monterey Once-A-Year Insecticidal Drench contains imidacloprid, a systemic insecticide that’s absorbed through the tree’s roots and distributed throughout the vascular system. That means once applied, it protects the entire tree from the inside out — leaves, bark, branches, and roots — all without needing to spray.

What really makes it stand out is the convenience: you only need to apply it once per year. That’s a huge time-saver for property owners, and it eliminates the risks of drift and surface runoff associated with foliar sprays. It’s rainfast, odorless, and can be applied easily with nothing more than a bucket or watering can.

What It Works Against

This drench is 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 infestation
• Elm trees prone to Elm Leaf Beetle
• Maples, oaks, and other ornamentals dealing with aphids, scales, or mealybugs

The systemic nature of the product ensures that insects feeding on your tree will ingest the toxin and die off — often before they can do any serious damage.

How to Use It

Application is straightforward, and you don’t need specialized equipment. Here’s how:

1. Determine the amount you need. The product is typically dosed based on trunk diameter at chest height (DBH). For example, a tree with a trunk 10 inches in diameter would require 10 oz of product mixed with water.
2. Mix with water. Dilute the recommended amount of insecticide in a bucket or watering can with 1–2 gallons of water.
3. Apply at the base. Pour the solution slowly and evenly around the root zone at the base of the tree, making sure the soil is moist so the product can penetrate effectively. Don’t apply to dry soil.
4. Water in if needed. If your soil is dry, follow up with additional water to help move the insecticide into the root zone.

When to Apply

It Timing is key for getting the most out of this drench. The best time to apply is:

• Early spring, just before or as the tree is starting to leaf out. This allows the product to be absorbed and distributed throughout the tree as it begins actively growing.

For some pests like borers, it’s crucial that the tree is protected before the adult insects emerge and begin laying eggs. A spring application ensures the systemic protection is in place when it’s most needed.

In warmer climates, a fall application can also be effective, particularly if trees are still actively taking up nutrients and moisture.

A Few Pro Tips

1. Water first: If your soil is dry, give your tree a deep watering a day before applying the drench. Moist soil helps with absorption.
2. Avoid applying to drought-stressed trees: A tree that’s under stress from lack of water may not absorb the product properly.
3. Mulch smartly: Keep mulch a few inches away from the base of the trunk. This helps prevent fungal issues and allows the drench to penetrate directly into the root zone.
4. Use annually, not excessively: Once a year is sufficient. Applying more frequently won't improve results and could be harmful.

Environmental Considerations

While imidacloprid is effective, it should be used responsibly. Avoid applying it near water bodies, and never apply to flowering plants where pollinators are actively feeding. Since this is a systemic product, it won’t harm bees unless they’re foraging on treated blooms — so timing your application before flowering begins is key.

Final Thoughts

The Monterey Once-A-Year Insecticidal Drench is one of the simplest, most effective tools for managing insect pests in trees. It allows homeowners to take a proactive approach to tree health without constant spraying or specialized equipment. When used correctly and at the right time of year, it provides season-long peace of mind — and healthier, longer-living trees.

If you’re unsure whether this product is right for your tree, or need help calculating how much to use, feel free to reach out through TreeHelp.com. We're always happy to help you care for your trees the right way.