Neem oil is a natural, non-toxic product that is a fungicide as well as an insecticide. It kills common plant pests, such as mites, flies and aphids, and common diseases, such as powdery mildew and rust. Unlike chemical pesticides, neem oil does not kill beneficial insects, such as ladybugs. You can buy neem oil pure, or buy it pre-mixed with soap--soap helps it mix properly with water and helps it stick to plant leaves.
Mix your batch of neem oil with other necessary ingredients the same day you plan on using the spray. Ottawaorchidsociety.com advises you shouldn't make up a batch sooner than four hours ahead of time as the oil, being natural, will start to break down and may lose some effectiveness. Make up only the amount you think you'll need for that application.
Pour the oil, along with water and dish soap, into a garden sprayer, if you are spraying a large garden area, or a plastic spray bottle, if you are treating only a few plants, or plants indoors. The ratio of neem oil to dish soap and water depends on the oil strength (e.g. 70 percent or 100 percent) you are using, so check the label first. An example, however, is provided by neemking.com, which suggests using 4 tsp. neem oil and ¼ tsp of dish soap per gallon of water, adding the water only gradually so the oil emulsifies (mixes well) in the soap first.
Spray your garden plants in the morning, before it gets too hot, or in the evening when temperatures are starting to cool off. Neem oil, as any other product, should not be sprayed on plants in the heat of the day--doing so can damage plants.
Cover all parts of the plant's foliage, including the stems and undersides of the leaves (many insects like to hide there), with the mixture.
Repeat application of the oil (make a new batch each time) in a week, and continue each week until pests are gone. If it rains, you may have to treat more frequently--hort.uconn.edu advises rain will degrade neem oil; so, after a rainfall, when the foliage has dried, you will need to treat plants again.