Neem Oil Insecticide and Fungicide For Plants

Neem oil has become a popular insecticide and fungicide that is used by many gardeners. The attraction is that it is natural, organic, and relatively safe. It can be used to control or kill a wide range of pests, and it’s relatively available to the consumer.

Discussions on social media suggest that neem oil controls just about any insect and disease found in the garden. Such an all inclusive product would certainly be good to add as a gardening tool, but before you do that, I suggest you read this post and get some important information about it. It is critical that you buy the right product because a lot of the commercial neem products have a limited ability to control insects.

What is Neem Oil?

When I started researching this topic, I thought that this would be a simple answer, but it’s not.

Neem comes from the neem tree which is called Azadirachta indica, a broadleaf evergreen that grows in India, Sri Lanka and Burma. The tree produces nuts which are ground and pressed to extract their oil, not unlike the way olive oil is produced. For the best neem this process is done at low temperatures resulting in something called “cold pressed neem”. The active ingredient in neem that harms insects is called azadirachtin and the cold press process ensures that it does not degrade.

This neem can be further processed to remove the azadirachtin from the rest of the oil. The remaining oil is called “clarified hydrophobic extract of neem oil”, or “clarified hydrophobic neem oil”.

Why am I telling you all this? Because the general public thinks that neem, neem oil and clarified hydrophobic neem oil are all the same product, but for pest control is is critical you understand the differences.

What is neem oil? Unfortunately, this term is largely misused and can be just about any product obtained from the neem tree fruit. It can be the original neem as I have described above, which still contains azadirachtin or it could be the clarified hydrophobic neem oil. Commercial products also add adjectives like “pure”, “100% natural”, “extracted” and “concentrated”, just to confuse the public even more.

In this post I will use the term ‘neem oil’ to refer to the original extracted product which contains azadirachtin, and I’ll use ‘clarified hydrophobic neem oil’ to refer to the oil without azadirachtin. If I want to refer to both, I’ll use the word “neem”.

Does Neem Oil Control Insects?

Maybe. The azadirachtin in neem oil works in two different ways. It coats leaves and makes them unpalatable to some insects. In this way it does not harm the insect, but it does prevent it from feeding on your plant.

When azadirachtin is ingested by immature insects, like larvae, it prevents them from developing properly and they eventually die. But this takes time, as much as a week in some cases. This is not a quick knockdown insecticide.

It is also important to understand that azadirachtin is not very effective on adult insects because they are no longer going through growth stages. If an adult insect doesn’t mind eating sprayed leaves, then neem oil won’t control them.

Insects ingest azadirachtin when they chew on leaves coated with neem oil. Azadirachtin is poorly absorbed by leaves as a foliar spray and more strongly absorbed by roots through a soil drench. The effect of a soil drench is diminished at a pH above 7. Both of these provide some systemic effects.

Contrary to popular online information, neem oil is not effective on every insect, but It does control somewhere around 200 different insects. Read the instructions on the bottle and use it only for the insects listed. It will affect both beneficial and pest insects. For example, exposure to azadirachtin inhibited egg-laying of green lacewing (C. carnea) females, an important predator of aphids.

Azadirachtin is quickly degraded by sunlight and will last less than a day when sprayed on leaves. The oil remains longer but is washed off with rain.

How Effective is Neem Oil on Insects?

When neem oil was applied to roses to control the Japanese beetle, there was a 60% reduction in defoliation. Less leaves were damaged, but the problem was not eliminated.

Neem oil reduced egg laying of the sweet potato whitefly on tomatoes by 80% (in the lab).

Neem oil does not control spiders, earwigs or ants.

Online information and product promotions would lead you to believe that it works very well, but most studies show that it reduces damage from insects but usually does not eliminate the problem entirely. New gardeners may struggle with that reality – they like zero control.

Does Clarified Hydrophobic Neem Oil Control Insects?

This version of neem is missing the active insecticide that affects insects, so it does not affect most insects.

Clarified hydrophobic neem oil, is an oil, and as such it does coat insects and can suffocate certain types of insects, including adults. It works against things like scale, mealybugs, leafhoppers, mites, whiteflies and aphids, but it does not control many of the other insects that are controlled by neem oil. It can kill insect eggs.

The oil is a contact pesticide only. It must coat the insect or eggs to work, so it is important to fully coat all leaf surfaces and apply it repeatedly.

Buying The Right Neem

If you want to buy neem with insecticidal properties, you have to buy the right kind and this is where it gets tricky. Product manufactures use all kinds of descriptions to fool you.

My advice is to ignore the product name and description. Instead, look at the ingredient list. If it does not list azadirachtin, assume it is clarified hydrophobic neem oil with zero azadirachtin.

Don’t be confused by the term “Azadirachta indica” on the bottle. It sounds like the active ingredient but it’s the name of the neem tree.

In Canada, neem is not registered as a pesticide, so you can’t buy it as such. To solve this problem, many recommend getting neem from an Asian grocery store. This is almost certainly clarified hydrophobic neem oil which is not what you want for the control of most pests. There are also lots on online products promoted for beauty care and most of these are also clarified hydrophobic neem oil.

The amount of azadirachtin in the product is important. I saw several products containing low levels and these will be less effective for pest control. The concentration of azadirachtin should be around 2,000 ppm (2 g/l, or 0.2%).

Which neem should you buy? It really depends on the pest you are trying to control. Neem oil contains both azadirachtin and the oil, so it has the widest range of control, but if you want to control aphids, scale or adults, you only need clarified hydrophobic neem oil which is less expensive.

Correct Dose for Neem Oil

Assuming you use a product with 2,000 ppm azadirachtin, the common dilution would be one ounce per gallon (8 ml/l). This produces a solution of 0.8% neem oil. This is an effective dilution for a foliar spray (i.e. spraying on leaves).

When neem oil was tested on stink bugs, it killed about 20% of nymphs, but less than 10% of adults. Increasing the concentration, increased the mortality rate.

The suggested dilution rate does kill some of the pest, but as you can see it does not kill all of them and it’s less effective on adults. The increase in mortality rate with higher concentrations may not be warranted due to the cost. It may be better to spray at the recommended rate and spray more often.

Does Neem Oil Control Diseases?

Neem oil can control some fungal diseases, but azadirachtin has no known effect of these diseases. If you are trying to control diseases, you might as well use the less expensive clarified hydrophobic neem oil.

Does Clarified Hydrophobic Neem Oil Control Diseases?

Clarified hydrophobic neem oil has been shown to retard the growth of powdery mildew. Spores of this fungus require water to initiate growth and infect leaves. It is thought the oil coats the spores and keeps them dry, thereby preventing spore germination. A study found that “a one percent neem oil treatment was effective in managing powdery mildew on hydrangeas, lilacs and phlox.” However, a horticultural oil spray generally works better for powdery mildew control and a bicarbonate solution works even better – see: Baking Soda, a Home Remedy Fungicide – the Cornell Formula.

There is no effect on an existing infection so spraying plants will not cure the plant or remove existing powdery mildew. It only prevents further infection. Therefore you should spray before you see the disease.

For more on powdery mildew see: Powdery Mildew Treatments.

It is unclear to me how well it controls other types of fungal infections. In some cases some control has been reported, but other products may work just as well.

Does Neem Harm Plants?

Neem is an oil and all oils can damage plants. Follow label instructions, but in general oils should not be sprayed in hot weather and they should not be used on recent transplants or on plants that are wilted or under stress. It is always a good idea to do a test spray on a few leaves and wait a couple of days to see if there is damage.

Flowers are usually damaged by neem.

Some neem products can be used on vegetables right up to the day before harvest – follow label instructions.

How Safe is Neem?

Both forms of neem are considered very safe. There is extreme low toxicity for animals (mammalian LD50 >5,000 mg/ kg) and birds. It is slightly toxic to fish and other aquatic organisms. Microbe populations in soil can be disrupted with some types increasing in population and others decreasing.

The half-life of azadirachtin in soil ranges from 3 – 44 days. In water, the half-life ranges from 48 minutes to 4 days. It also rapidly breaks down on plant leaves with a half-life if 1 – 2.5 days.