By Paul Oliver, Founder, TreeHelp.com

When it comes to protecting your trees, shrubs, and garden plants from pests, sometimes the simplest tools are the best. One of the most reliable and environmentally responsible insect control options I recommend is Monterey Horticultural Oil Insect Spray. It’s a versatile product that’s been around in some form for generations — and for good reason.

This oil-based spray doesn’t rely on toxic chemicals or nerve agents to kill insects. Instead, it works through physical action, smothering insect pests and their eggs while being gentle on plants and beneficial insects when used properly. For gardeners looking for effective control without harsh side effects, horticultural oil remains a gold standard — and Monterey’s version is one of the best on the market.

What It Controls

Monterey Horticultural Oil is effective against many common garden and tree pests, including: 

• Aphids
• Scale insects (both soft and armored)
• Spider mites
• Whiteflies
• Mealybugs
• Thrips (limited effectiveness)
• Leafhoppers
• Psyllids
• Insect eggs and overwintering stages

It’s especially valuable for controlling overwintering pests on dormant trees and shrubs — a key part of any good integrated pest management (IPM) strategy.

Where It Works Best

This product can be used safely on a variety of plant types, including:

• Deciduous and evergreen trees
• Fruit trees (apple, pear, peach, citrus, etc.)
• Shrubs and hedges
• Ornamentals and roses
• Vegetables and herbs
• Indoor and outdoor container plants

It’s a great solution for gardeners looking to avoid pesticide resistance and protect beneficial insects like bees and ladybugs, which aren’t harmed when the product is applied properly (i.e., when they’re not actively flying or present during application).

How to Use It

Monterey Horticultural Oil comes as a concentrate and is easy to use with a standard tank or backpack sprayer:

For general insect control:

• Mix 2 tablespoons (1 fl oz) per gallon of water
• Apply thoroughly to all plant surfaces, especially the undersides of leaves, where pests tend to hide

For dormant or delayed-dormant sprays:

• Use 3–4 tablespoons per gallon for more stubborn pest issues on trees and shrubs

It’s important to shake well and agitate the sprayer frequently, as oil and water tend to separate.

When to Apply It

• Timing is key to maximizing effectiveness and minimizing plant stress:
• Dormant Season: Apply in late winter or early spring before buds break to target scale insects, overwintering eggs, and mites
• Growing Season: Use every 7–14 days as needed to manage active infestations
• Early morning or evening: Apply during cooler parts of the day (under 85°F) to avoid leaf burn

Avoid spraying during hot midday sun or when plants are under drought stress.

Tips for Best Results

1. Apply when pests are present — this isn’t a preventative like some systemics; it works by contact.
2. Thorough coverage is essential — if you miss the insects, it won’t work.
3. Repeat applications may be needed for ongoing infestations.
4. Don’t mix with sulfur-based products or apply within 30 days of sulfur use — the combination can cause phytotoxicity (leaf burn).
5. Use clean water — hard water can affect the oil’s ability to spread evenly.

Safe and Sustainable Pest Control

Monterey Horticultural Oil is one of the best choices for gardeners who want to maintain healthy, productive plants without harsh chemicals or long-lasting residues. It doesn’t harm wildlife, pets, or pollinators when applied correctly, and it’s safe to use around fruits and vegetables right up to harvest (just rinse produce before eating).

It’s also an excellent tool for managing insect populations without encouraging pesticide resistance, which is increasingly important in today’s gardens and landscapes.

Final Thoughts

If you’re struggling with aphids on your roses, scale on your fruit trees, or mites on your evergreens, Monterey Horticultural Oil Insect Spray is a reliable, proven solution. It’s simple, safe, and highly effective when used correctly — and it’s one of the few products I trust equally in both commercial orchards and backyard gardens.

For trees and shrubs in particular, I always recommend incorporating horticultural oil into your annual care routine, especially during the dormant season. If you have questions about how and when to apply it to your specific plantings, reach out to us at TreeHelp.com — we’re always happy to guide you through the process.