About Neem Oil
For any that might be interested, I wrote this as an article for our bonsai club newsletter, but it applies to houseplants as well.
Neem extract as an insecticidesize>
In India mainly, but also Asia and Africa, grows a tree all bonsai enthusiasts should be aware of: Azadirachta indica. You are probably already wondering if it makes a good subject for bonsai. Well, yes and no. I don't know enough about the tree's culture to say it makes a good bonsai medium, but it makes a very good bonsai subject. I'll explain: Azadirachta indica is commonly known as the "neem" tree. Extracts from the tree contain azadirachtin, a relatively safe and effective naturally occurring insecticide. Let me preface the following comments by reminding you that the terms "naturally occurring and/ or organic" do not universally mean safe. Pyrethrums, rotenone, and even the very dangerous nicotine are all organics that should be handled with great caution.
Neem extracts, on the other hand are used in a wide variety of cosmetics, as a topical treatment for minor wounds, to treat stomach ailments, as an insecticide in grain storage containers, bins, and bags, and a whole host of other applications. I'll limit this discussion to its use as an insecticide.
Neem works in many ways. It is effective both as a topical and a systemic. It is an antifeedant, an oviposition deterrent (anti-egg laying), a growth inhibitor, a mating disrupter, and a chemosterilizer. Azadirachtin closely mimics the hormone ecdysone which is necessary for reproduction in insects. When present, it takes the place of the real hormone and thus disrupts not only the feeding process, but the metamorphic transition as well. It interferes with the formation of chitin (insect "skin") and stops pupation in larvae, thus short-circuiting the insect life cycle. Tests have shown that azadirachtin is effective in some cases at concentrations as low as 1 ppm.
Neem oil or extract is most often used in an aqueous (water) suspension as a foliar spray or soil drench. Commonly, it is diluted to about a .05% solution, but the suggested ratio for use in bonsai culture is 1 tsp. per quart of warm water. A drop or two of dish soap (not detergent) helps keep the oil emulsified. The mixture is then applied as a mist to all leaf and bark surfaces and as a soil drench to the tree's root system. It should not be applied as a foliar spray on hot days or in bright sun as leaf burn may occur. Remember to agitate the container frequently as you apply and do not mix anymore than you will use in one day. Neem breaks down rapidly in water and/ or sunlight. (Since writing this, I have discovered that a 50/50 mix of water/rubbing alcohol works very well as the vehicle)
Some users of insecticide need to be able to observe the instant results of their efforts in order to be convinced of the effectiveness of their choice.The application of neem derivatives does not provide this immediate gratification. There is virtually no knockdown (instant death) factor associated with its use. Insects ingesting neem usually take about 3 - 14 days to die. Its greatest benefit; however, is in preventing the occurrence of future generations. It is also interesting to note that in studies it was found that when doses were given, purposefully insufficient to cause death or complete disruption of the metamorphic cycle, up to 30 surviving generations showed virtually no resistance/immunity to normal lethal doses.
I have been using neem oil for about 7 years as both a preventative and fixative and have had no insect problems on my bonsai. It is said to be effective for mites, whitefly, aphids, thrips, fungus gnats, caterpillars, beetles, mealy bugs, leaf miners, g-moth, and others. It seems to be fairly specific in attacking insects with piercing or rasping mouth parts. Since these are the pests that feed on plant tissues, they are our main target species. Unless beneficials like spiders, lady beetles, certain wasps, etc. come in direct contact with spray, it does little to diminish their numbers.
Neem oil does have an odor that might be described as similar to that of an old onion, so you may wish to test it first, if you intend to use it indoors. I've found the odor dissipates in a day or two. As always, read and follow label instructions carefully.
Neem oil can be purchased from many of your favorite bonsai suppliers or via the net.