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| Photo Credit: ©2006 Buglady Consulting |
| Scale insects, which don’t resemble insects at all, can be controlled with horticultural soaps. |
Insecticidal soaps kill pests in different ways. One way is by penetration through the insect’s outer skin (the cuticle) and causing cells to rupture, resulting in severe dehydration. Another way is by suffocation. (Lack of respiration is a quick recipe for death.) This type of control can be used on all kinds of insects: aphids, scales, mealybugs, lace bugs and whiteflies. It also kills spider mites.
As great as they are, insecticidal soaps are not a cure-all – they do have limitations. One of the biggest is that the soap MUST come in direct contact with the pest in order for the product to kill. You can’t spray insecticidal soap on the top of a leaf expecting to kill the pest hanging out on the bottom of it. And dried soap residue on the leaves has almost no impact on pests. So once the soap dries, there’s no residual activity. Another limitation can be phytotoxicity issues: Soaps can burn foliage if not applied at the correct rate or if a plant is under stress. Symptoms can include yellow or brown spotting on the foliage, burned tips or leaf scorch. And plants are just plain sensitive to soaps. If you’re uncertain about using an insecticidal soap on a particular plant, spot-treat a portion of the plant and wait at least 24 hours to see if any phytotoxic symptoms develop. Soaps can also be affected by water quality. If you buy insecticidal soap concentrate, you’ll have to mix it up. If your water is hard, it can result in a chemical change, producing insoluble soaps. (You know this in the shower as soap scum.) This can reduce the effectiveness of the soap.
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