When To Spray
Insecticidal soap only works if it comes in direct contact with the target pest while it is still wet. Because insecticidal soap loses its potency when it dries, it is best to use it when humidity is high, such as on cloudy days and during early morning or late afternoon, if possible, so that it will dry more slowly on the plants. If the infestation is bad, however, don’t hesitate to spray.
Be aware that foliage may be injured after spraying on a sunny day. Symptoms include yellowish spots or rings that appear a day or two after the treatment. These are due to sunlight shining through tiny light-focusing “lenses” of moisture on the leaves. This can be particularly troublesome on plants with waxy, horizontal leaves.
Spray To Runoff
Most pest insects are hidden from view when you look down on a plant. They are usually sitting on the undersides of the leaves. Because insecticidal soap works only on contact, you must find a way to make the spray actually touch the pest insect’s body, otherwise you are wasting your time. The technique then is to pay particular attention to spraying the undersides of as many leaves as possible.
Sometimes Repeat Applications
Sometimes, when an infestation of aphids and other small soft bodied insects is severe, they may need to be sprayed several times to get good results; especially because the insecticidal soap does not kill the eggs that are already in place. Repeat sprays are best every 2 to 3 days until the insects are no longer visible. You may have to follow this regimen for as long as 2 weeks if infestation is very severe, those little critters can be tough to control when they get into one of their population explosions. Remember though, we are not looking for zero aphids. We just want to bring the population under control so the natural predators can maintain the balance. For other insects, a weekly spray repeated for 2 or 3 weeks controls most targeted pests. Insecticidal soap keeps indefinitely if you store it where it won’t freeze or overheat.
Checking For Results
Insecticidal soap normally kills in a few hours once it covers the insect. To determine if significant inroads have been made in the pest insect population, examine the affected plant three days after spraying the soap. Remember, you do not need to kill 100% of the pests. If the population is reduced to 10% to 20% of where it was, the survivors will serve as food for beneficial insects, which will then keep the pests at that acceptable low level.
Some Conditions to Avoid
Do not spray soap on new transplants, newly rooted cuttings or very young seedlings without first making a trial application. Do not spray young foliage when temperatures exceed 90°F, or when the plants are stressed by drought. Do not use insecticidal soap on very tender plants because it can burn or discolor leaves, deform them, or cause them to drop off. Avoid spraying blossoms of various fruit and nut trees and vines. Follow guidelines on the product package.