Question:
I was told that mixing Dawn Dish Soap in water will kill aphids on your plants. Is this true?
Hardiness Zone: 6a
Edie from Spokane, WA
Answer
Edie,
It's true. When sprayed on aphids, a solution consisting of liquid dish soap and water will kill aphids and many other soft-bodied insects such as caterpillars, thrips, mealybugs, spidermites, leafhopper, lace bugs, and whiteflys. Most of these soaps are made from the potassium salts of fatty acids. When sprayed on soft-bodied insects, these fatty acids disrupt the structure and permeability of the insects' cells, causing the contents to leak out, which quickly kills them.
Most any type of liquid dish soap will work, but DAWN is biodegradable and contains no phosphates, which is better for the environment.
Insecticidal soaps only work on contact and not as a preventative measure. In other words, the soap needs to completely cover the body of the pest you're trying to control. Dried soap residue on the plants will not harm the insects. The disadvantage to using insecticidal soaps is that it causes phytotoxicity (damage from chemicals) in some plants. Here are some tips for using insecticidal soaps effectively:
- Apply insecticidal soaps in conditions that favor slow drying, such as early morning or early evening. Avoid applying during the warmest parts of the day and in temperatures over 90ºF.
- Spot test plants 48 hours before full application to test chemical sensitivity. Signs of chemical damage include yellow or brown spotting on the leaves, burned tips or leaf scorch. Plants with waxy leaves are less likely to suffer damage than those with dull or hairy leaves.
- Plants exhibiting chemical damage within a few hours after treatment should be rinsed thoroughly with water.
- Always wait for new growth to harden off before treating.
- Never spray fruit trees while in bloom.
- Do not apply insecticidal soaps to young transplants or to plants under stress from heat or water.
- Trees and herbaceous plants with known sensitivities to insecticidal soaps include horse chestnut, mountain ash, Japanese maple, sweet gum, jade plant, lantana, gardenia, bleeding heart, crops of thorns, and certain cultivars of azalea, poinsettia, begonia, impatiens, fuchsia, geranium, ferns, Easter lilies, palms and some succulents.
- Soap mixed with hard water may be less effective and may leave a precipitate behind from the metal ions in the hard water.
