Question: My mother is convinced dishwashing soap is the best thing to use to kill insects on plants. She has been doing it for years. Is this a safe and effective practice?
Answer: Various types of soap sprays have been an effective control method for insects for more than 200 years. They are used as an alternative to stronger insecticides. Home gardeners have used dishwashing soaps as a common ingredient in homemade insect sprays. Dishwashing soap is very different from horticultural insecticidal soaps. Even though it may seem effective in killing the pest, there are some limitations you might want to consider.
Certain brands of household soaps and detergents have been used to kill insects because they are less expensive and readily available. However, since they are not designed for plants, they can be too harsh to be used on the plant. There are also no instructions for dilution amounts on the bottles or which - if any - plants may be harmed. If using them, you are doing so at your own risk.
Researchers investigated the effects of dishwashing soaps as an insecticide for plants and found they were effective, but the more effective they were the more damage there was to the plant. Soaps work by penetrating and dissolving the cells covering the insect's body, resulting in dehydration and death. Plants also have a wax covering that is affected by the soap. The plant may not die instantly. But, disintegration of the leaf's outermost layer may result in the loss of water and dehydration of the plant.
If you prefer to use a less toxic insecticide, then horticultural insecticidal soap is the best option. The ingredients are selected for certain insects, in order to minimize plant damage, and are consistent in their manufacture. Insecticidal soaps are most effective against small, soft-bodied insects such as aphids, young scales, white flies, mealybugs and spider mites. Most beneficial insects - including lady beetles, green lacewings and pollinating bees - are less susceptible to soap and will not be harmed.
When using an insecticidal soap, thorough coverage is essential. It will have no effect on insects that arrive after the soap has dried. If an insect is curled up in a leaf, they will emerge unharmed. Also, certain plants are very sensitive to soap sprays. One of those particularly sensitive plants is tomatoes. When in doubt, test the spray on a small area of the plant and wait a few days to see if there is any effect on it. They should be applied during the colder parts of the day, with repeat applications made every four to seven days.
Mona Bawgus is a certified master gardener and consumer horticulturist with Rutgers Cooperative Extension of Atlantic County. Write to her c/o Rutgers Cooperative Extension, 6260 Old Harding Highway, Mays Landing, NJ 08330. E-mail: