How and When to Use Neem Oil for Natural Garden Pest Control
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If you’ve been interested in organic gardening for at least ten minutes, you’ve probably already heard about Neem oil. It’s the gold standard for natural garden pest control, among other issues, and folks will recommend it for everything from bug infestations to blight or fungal wilts. Caterpillars? Neem oil. Leaf spots? Neem oil. Deer, rabbits, or the neighborhood kids messing with your garden? Neem oil.
Jokes aside, Neem can certainly be helpful as part of an all-natural pest control strategy. However, it’s not the magic bullet it’s often made out to be. It works better for some problems than it does for others, and in some circumstances, it’s totally useless.
Since Neem oil is somewhat of a broad-spectrum pesticide that can have ripple effects on beneficial insects, it’s important to know when it’s effective and when it’s better to take another route. This way, we’re not spraying it needlessly (and then sitting back to watch as our plants continue to get demolished).
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What is Neem Oil, and How Does it Work?
Neem oil is a seed oil that’s extracted from the seeds of the Neem tree, a large evergreen originally native to India. For many centuries, it was used in herbal remedies for both humans and livestock animals, as well as a pest repellent for fields and harvested grains.
On top of its strong scent, which may throw off certain pickier pests, Neem oil contains a compound called azadirachtin, which interrupts the life cycle of leaf-eating pests when they ingest it. Azadirachtin disrupts insects’ hormonal functions, which eventually limits their ability to eat, molt, and make babies. Neem also works as a horticultural oil to suffocate soft-bodied pests on contact.
Pros and Cons of Using Neem Oil
The obvious benefit of Neem is that it’s 100% natural and largely non-toxic for people, pets, and any wildlife who isn’t a bug. It’s a popular ingredient in skin and hair products, and it’s used in traditional medicine to treat lice and fungal infections on the skin. Newer studies say that Neem oil shouldn’t be ingested for medicinal purposes, but it’s perfectly safe to use the diluted spray on your vegetable plants up to the day of harvest.
By far, the biggest drawback of Neem oil is its broad-spectrum insecticidal properties. For the most part, Neem oil’s residual effects on plants only target leaf-munching pests who eat the azadirachtin. However, any insect that gets hit directly by the spray (good or bad) can be harmed.
Even if you spray your plants while pollinators aren’t active, other beneficials are likely still hanging out in the garden. For example, lightening bugs and ladybugs spend their nymph stage on your plant foliage. At this time, they’re quite vulnerable to Neem oil. Since these beneficial predators help control pests, widespread and heavy use of Neem can reduce their numbers and make pest problems worse in the long run. Unfortunately, pests often rebound faster than the beneficials we accidentally harm while trying to keep them at bay.
What about bees? For a long time, we’ve believed Neem oil was completely safe for pollinators as long as you spray while they’re not actively foraging. However, there’s now some evidence that bees may be sickened by azadirachtin if they forage from flowers recently sprayed with Neem oil. Exposed bees may stop eating or have trouble mating and laying eggs to complete their life cycle. If foraging honey bees bring sprayed pollen back to the queen, it can affect the health of the entire hive.
Another common complaint with Neem oil is the smell. Neem has a strong, distinct sulfur odor with notes of rancid garlic. Some folks swear they can still smell/taste the Neem on produce even after washing it. If you find that to be the case for you, avoid using Neem oil close to harvest time.
Which Insect Pests are Controlled by Neem Oil?
Neem oil works best for sap-sucking and leaf-chewing insect pests, particularly those that are small and delicate. Since the oil has a suffocating effect on soft-bodied insects, it can control an infestation of aphids, mites, or whiteflies pretty quickly. It’ll also provide quick knockdown for the nymph (baby) stage of many pests, such as squash bugs.
For tougher, fast-moving pests like flea beetles, thrips, cucumber beetles, and tarnished plant bugs, control relies more on the ingestion of azadirachtin. Neem works best to head off a problem before it becomes a major infestation, in these cases. It’ll target nymphs, slow down reproduction, and keep them from establishing a village on the crop you want to protect. If lots of adult pests are already causing serious damage, Neem on its own likely won’t put a stop to it.
When Neem Oil Won’t Work
While someone will almost always recommend breaking out the Neem oil if you’re seeing holes in leaves, there are quite a few pests Neem oil won’t touch. These include:
-Slugs. These slimy pests are gastropods, a.k.a. land mollusks, rather than insects. This means azadirachtin has no effect on them. See this post for some ways to prevent slug damage: Controlling Slugs in the Garden: 5 Natural Methods That Actually Work.
-Pill bugs/Sow bugs. They’re called bugs, they look like bugs, but they’re another bug impersonator. Pill bugs are actually land crustaceans, closely related to crabs and shrimp. These decomposers only occasionally nibble at small seedlings. When they do, seedling collars made from cut toilet paper or paper towel rolls work better than any pesticide to prevent pill bug damage.
-Caterpillars. While Neem oil may have a smothering effect on teensy, newly hatched caterpillars, larger caterpillars (i.e., big enough for you to notice them) aren’t bothered by it. Bacillus Thuringiensis (Bt), another natural insecticide, works much better for handling a caterpillar problem. It’s also less risky for non-target insects and pollinators.
-Large Beetles and True Bugs. In mine and other gardeners’ experience, Neem oil doesn’t seem to phase large, tough-shelled bug and beetle pests like Japanese beetles, Mexican bean beetles, potato beetles, stink bugs, and squash bugs. While Neem oil may smother young nymphs, these pests are best controlled by hand-picking the adults and other methods.
Can Neem Oil Help with Blight and Fungal Disease?
Neem oil is frequently mentioned as a fungicide to prevent or treat things like powdery mildew, rust, and tomato blight. Like any horticultural oil, it can coat the leaf surface and prevent fungal spores from “sticking” easily. The general consensus is that it works fairly well to slow down milder, less aggressive plant diseases, like powdery mildew.
Like other natural plant fungicides, Neem won’t cure a plant that already has symptoms of disease. It only works as a preventative for healthy plants. It also won’t do much to stop more aggressive pathogens, such as tomato blights, once the weather becomes favorable for the disease. These blights will quickly take hold as soon as an afternoon shower washes the Neem oil coating away.
On top of the fact that it provides weak protection at best, using Neem oil as a regular disease prevention spray needlessly puts beneficial insects at risk. All-natural growers have more effective, more insect-friendly options for preventing disease, such as biofungicides and liquid copper.
One product I like to use for disease prevention is Monterey Complete Disease Control, which works via a friendly bacteria called Bacillus amyloliquefaciens. These bacteria colonize plant leaves and compete with harmful fungi, making it harder for them to establish. It’s also just as safe as plain water for beneficial insects.
Liquid copper fungicide is a harder-hitting option for controlling plant diseases organically. It’s often used by commercial organic growers to stave off destructive, more aggressive disease problems. Although it poses some risk to soil organisms when used too heavily, it’s still safer for beneficial insects than a broad-spectrum pesticide like Neem oil.
Product Matters: What Kind of Neem Oil to Buy
Remember how azadirachtin is the active compound in Neem oil that controls leaf-eating pests? Turns out, many Neem oil products you’ll find on the shelves don’t even contain azadirachtin.
Why? Neem-based pesticides for professional use are often made from pure, extracted azadirachtin. The azadirachtin is removed from Neem oil using refinement methods like heat and solvents, formulated into a potent organic-approved pesticide, and sold for a pretty penny to commercial growers. The leftover product, called clarified hydrophobic Neem oil, is then bottled, slapped with a generic “Neem oil” label, and sold to home gardeners.
Clarified hydrophobic Neem oil can smother soft-bodied insects and provides some weak anti-fungal effects, like any other plain old horticultural oil. Since there’s no azadirachtin to speak of, it doesn’t do squat for tougher leaf-eating pests. Gardeners often use a product like this for a flea beetle or thrips problem and assume Neem oil just doesn’t work.
To get the full effects of Neem, you’ll need to make sure your product is labeled as cold-pressed Neem oil. This is unrefined, pure Neem seed oil that hasn’t been stripped of azadirachtin. I like Captain Jack’s Neem Max by Bonide.
Note: There is a big difference between cold pressed Neem oil and Neem essential oil. Essential oil is processed in a completely different way and is meant for use in skin creams, etc. It’s much stronger and will burn plant foliage if you use it like regular Neem oil.
Making a Neem Oil Solution
Making your Neem oil spray is pretty straight-forward. You’ll mix it with water at a rate of 1-3 tablespoons of Neem oil per gallon (this can vary by product!) and apply it to plant foliage. I like to use my one gallon pump sprayer for this, since the wand makes it easier to get good coverage.
Depending on your product, you may also want to add a teaspoon or two of a gentle, unscented soap such as castile soap to your solution as an emulsifier. Since pure Neem seed oil is hydrophobic (like vegetable oil), the soap helps it mix with the water instead of floating at the surface.
Note that some garden-specific Neem oil products, like Captain Jack’s Neem Max, already contain an emulsifier. It’s always a good idea to read through the product instruction booklet for specific mixing rates and whether or not you need to add soap.
Avoiding Phytotoxicity/Leaf Burn
As with many oil-based sprays, using Neem oil carries a risk for phytotoxicity on plant foliage. In simpler terms, oil on the leaves can cause your plants to get a bad sunburn. The best way to prevent this from happening is to mix at the label rate and apply Neem oil at dusk or dawn when the sun isn’t intense.
Sometimes, vulnerable plants might take damage from Neem oil even when you do everything right. It’s particularly common with young seedlings or plants that are already stressed out for some reason. This is why I always do a test spray before applying Neem oil to an entire crop.
To do a test spray, mix up a small amount of Neem solution and apply it to only a few parts of a single plant. Be sure to spray both a few older leaves and a little of the new growth. If you don’t see any browning, bleaching, or curling leaves after 24 to 48 hours and some sunshine, it’s safe to treat the rest of the plants.
Spraying Tips for Best Control
Spray thoroughly. Neem oil will work best if you can get full coverage of the plants to be protected. It’s especially important for dealing with tiny pests like mites, aphids, and thrips, since they like to hide in nooks and crannies. Make sure you hit both the tops and bottoms of the leaves, spraying each plant until you see a few drips from the leaf tips.
Shake the bottle/sprayer often. Even when emulsified, Neem oil is bad to separate a bit from the water it’s mixed in. Shake it up between plants to keep it evenly mixed. Otherwise, some plants may get too much or not enough, leading to either leaf burn or continued pest activity.
Reapply after 7-10 days. Neem oil won’t kill pest eggs, which will later hatch and re-infest your plants. Pests like thrips and tarnished plant bugs have a pretty quick life cycle, so a second spray a week or so later should mop up any newly hatched nymphs. For best control, do a little research on the pest you’re dealing with. This way, you can learn about their life cycle and time your sprays appropriately.
Reapply after heavy rains. Neem oil easily washes off foliage during a heavy afternoon storm. If you’re trying to get a handle on leaf-eating pests, you’ll need to re-spray after a big washout.
How to Protect Beneficial Insects When Using Neem Oil
Since Neem oil can pose a risk to bees and other pollinators, never spray while they’re out doing their foraging. Pollinators usually pack up and go home after sunset and start foraging again right at sunrise. Near dusk is my go-to time to spray things like Neem oil to avoid accidentally spraying a bee.
There’s also the aforementioned issue of bees being harmed by contaminated pollen. For that reason, I prefer not to use Neem oil on plants that are heavily blooming, particularly those the pollinators are extra fond of, like cucumbers. Neem is best for catching and controlling a pest problem early on, when plants are still young.
Secondly, consider whether the pest situation 1) warrants using a pesticide, and 2) can be effectively controlled with Neem oil. Healthy plants can tolerate some feeding from pests like flea beetles, aphids, or thrips without any impact on your harvest. Some little holes on the leaves may not immediately require action on your part.
In other circumstances, Neem oil may not provide enough control to make it worth the trouble. Again, squash bugs and Japanese beetles won’t be too concerned about your Neem oil efforts. Pests like these are better dealt with by scouting and hand-picking to lower their populations in your garden.
Neem oil is best reserved for use on pests that are definitely vulnerable to azadirachtin, and those you know from experience will cause significant damage. For example, I won’t spray for a bit of flea beetle nibbling on lower tomato leaves. On the other hand, seeing tarnished plant bugs on my young pepper plants merits a Neem oil treatment. They destroy flower buds and growing points, stunting plants and reducing production.
Finally, consider integrated pest control strategies to reduce the need for pesticides like Neem oil in your garden. For example, crops that don’t require insect pollination (such as potatoes or beets) can be covered with netting using a simple hoop tunnel structure to keep pests out. This way, you can set it and forget it rather than trying to keep up with spraying all season long.
