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.
Scale insects, which don’t resemble insects at all, can be controlled with horticultural soaps.
Photo Credit: ©2006 Buglady Consulting
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.
Despite these limitations insecticidal soaps may have, their benefits make them worthwhile. One nice feature is they’re very friendly to beneficial insects and predatory mites. Once the soap solution dries on a plant, these good guys can crawl or land back on the leaves safely. Insecticidal soaps can be a great tool in a pest management program, too. They kill a wide range of pests, are relatively inexpensive and are soft on birds and the environment.
When treating plants with insecticidal soap, follow these tips:
- Make sure the plant has been well-watered and isn’t wilted.
- Don’t treat during the heat of the day. Early morning or evenings are the best times.
- Wear protective gloves or any other gear recommended on the label.
- If you purchase concentrated insecticidal soap, use the proper rates on the label when mixing. (If mixed too strong, the product can damage plants. If you mix below the labeled rates, you can make the spray ineffective.)
- Try to only mix as much soap as you need at a time.
- Don’t spray on a windy day. (This could cause your pesticides to drift and end up where you don’t want them.)
- Apply your spray until the insecticidal soap just starts to drip off the foliage or stems.
- Follow directions on the label for cleanup. If you have leftover mixed product when you’re finished spraying, read the label to learn how to dispose of it properly.