Neem oil has become a popular insecticide and fungicide that is used by many gardeners. It is natural, organic, and relatively safe. It can be used to control or kill a wide range of pests, and it’s available on the NZ market.
There are a few different types of Neem Oil and - surprise surprise! – they have different modes of activity and control different organisms in different ways.
Neem comes from the neem tree (Azadirachta indica), (India, Sri Lanka, Burma, etc). The tree produces nuts which are ground and cold-pressed to extract their oil. The active ingredient in neem that harms insects is called azadirachtin and the cold press process ensures that it does not degrade. Further processing removes the azadirachtin from the rest of the oil. The remaining oil is called “clarified hydrophobic neem oil”.
How does Neem Oil work when sprayed on a plant? Well it may disappoint you, but when the leaves are coated, insects are not killed by the deposit, but it makes the leaves taste bad, so the damage is very much reduced. But when juvenile insects eat the azadirachtin-coated leaves, their metamorphosis can be severely disrupted – they simply can’t shed their skin! That means that neem oil sprays are not a quick fix.
Azadirachtin is rapidly degraded in direct sunlight – it last less than a day, although the oily substance itself might linger a lot longer (until it gets washed off by rain). When it comes to systemic activity: azadirachtin does not penetrate the plant very well, but as a soil-drench it appears to be a lot more effective, as long as the soil pH is below 7. It is absorbed by roots.
Azadirachtin is not an effective compound on adult insects so if they eat the sprayed plant parts then it’ll be business as usual. There about few hundred insect species that are affected by neem oil sprays – many “don’t give a toss”! Some of the species that are negatively affected are beneficials, such as lacewings, which are inhibited from laying eggs.
There are varying efficacy data for all sorts of insects when sprayed with Neem Oil; Whitefly might be OK (but target the underside of the newer leaves!) and aphids tend to suffer as well; hard-bodied and adult insects are largely immune, as are spiders and ants.
The Clarified hydrophobic Neem Oil (the stuff without the azadirachtin) is just like any other spraying oil: it suffocates insects when applied in dense-enough quantities to coat most insects. 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.
As with all spraying oils: repeated applications are crucial to cover future generations as well. (and remember to thoroughly coat the plants!
Neem oil and disease control
It’s the oil that does the work by sticking the spores on the leaves and drying them out (preventing spore-germination). What I read in literature is that ordinary conqueror oil does a much better job in sticking the spores to the plants, and a bicarbonate solution works even better!
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