August Home-Garden

National Gardening Association Soap Sprays Can Control Pests Safely
by Cheryl Dorschner

Many gardeners searching for safe yet successful insecticides experiment with home remedies of household soaps. Some swear by concoctions like a cup of soap flakes to a gallon of water. Others whip a mild solution of Fels-Naptha and water into a rich suds, then syringe or spray it onto pest infested plants. A few garden books discuss home remedies and offer similar "recipes."

But within the last decade commercial insecticidal soaps arrived on the market to take the guesswork out of home brews. Soap has be- come a tested, regulated, bona fide insecticide for gardeners looking for alternatives to both pests and petrochemicals.

Soaps have been used with success to combat aphids, scale, mites, whiteflies, leafminer sawflies, psyllids, citrus mealybugs, some thrips and leaf-feeding beetles. It's not been clearly determined how soap kills insects. Old theories said it simply drowns them. Among the latest educated guesses is that within about 30 to 60 seconds of contact it removes the waxy coating on insects' bodies and disrupts their osmotic balance. others say it penetrates the insects' membranes, binds up certain "messenger" chemicals (perhaps calcium) and causes cells to rupture.

Soap spray insecticides are not new to gardening, of course. In the 1840s when the Massachusetts Horticultural Society offered a whop- ping $120 reward for the cheapest, most effective cure for rose chafers, the winning entry was none other than a whale-oil soap spray. By the latter half of the 19th century and the early 20th, soap became one of the principal insecticides used commercially and in the home. With the advent of DDT after World War II and gardeners' ensuing enthusiasm for chemical pesticides, many Americans turned to more se- vere methods of pest control for a few decades. The prevalent thinking was that pesticides should kill almost every insect in sight.

Gardeners' attitudes have once again changed. Now, many opt to regulate pest populations with those pests' natural enemies, or with insecticides that don't demolish all the varmints - just enough to allow for abundant harvests. Recent trends toward using less toxic methods and integrated pest management have again brought soap sprays to the fore.

Scientists take another look

Lance Osborne, researcher and assistant professor at the University of Florida Agricultural Research Center at Apopka, found a similar pattern in the scientific community's interest in soap sprays. Before 1940 scientists wrote about using soaps as insecticides. But he found no references dealing with insecticidal soaps from 1940 to the early 1970s. Since then, "there has been a resurgence of interest," he said. Experiments have been conducted throughout the country in the past decade testing the effectiveness and possible harm of several soaps on a host of insects and plants.

"By experimenting with minimum and maximum dosages we try to determine the minimum effective dosage that still gives good control without phytotoxicity (plant damage caused by chemicals)," Osborne said. When testing for phytotoxicity, usually deliberately strong doses are used in order to identify soaps that could potentially harm plants if gardeners used them under adverse conditions or with accidentally higher doses.

One early test (repeated in 1982) conducted by Carlton Koehler, entomologist at the University of California at Berkeley Cooperative Extension, put both household and commercial soaps to the test against aphids, spider mites, psyllids, greenhouse thrips and spittlebugs on ornamental shrubs. Tide laundry detergent, Ivory Liquid dishwashing detergent, Ivory bar soap, FelsNaptha bar soap, Williams' shaving bar soap, green soap tincture, glycerin soap, Shaklee's Basic H, Safer Insecticidal Soap and Acco Highway plant spray soap were tested.

Results indicated that soap sprays reduced the numbers of all pests except spittlebugs up to 97 percent. Dry formulations were not more effective than liquids (and dry soaps are more bother to use), and common liquid dishwashing soaps were as effective as the more expensive commercial insecticidal soaps.

The experiment also tested phytotoxicity of some soaps. Two percent solutions of Acco, Basic H, and Ivory Liquid and comparable concentrations (108 grams per gallon) of Tide and Fels-Naptha were used. Tide caused leaf burning or spotty residues on all plants tested except camellia. Basic H caused burning in five of eight plants. Acco, ivory and Fels-Naptha each burned one or two plants.

The manufacturers of Safer Insecticidal Soap, the only commercial insecticidal soap now marketed for home use, strongly object to comparisons between commercial insecticidal soaps and household soaps. "We spend all the money doing research on a product and getting it registered, and a company makes a pure soap and sells the product (as an insecticide) because other people recommend it, but the (soap manufacturers) are not breaking the law because they don't recommend it. The reason household soaps cost less is because they didn't have to spend a half a million dollars getting them registered," said Safer's vice president of technical development, Michael Atkins.

EPA regulates pesticides indeed, Safer's is the only soap now registered for use as an insecticide by the Environmental Protection Agency (EPA) and thus subject to EPA regulations. "The label is the law," as Osborne says. "If a product has an EPA registration number on it, as does any pesticide, one must use it as directed."

There has been some confusion among gardeners and even Cooper- ative Extension agents and entomologists about the legality of using and recommending household soaps. The EPA has declared it illegal to distribute, sell, even give away, any unregistered pesticide. But it is legal to concoct one's own pesticides for home use. "It's a free country and people can poison themselves if they want to, but we don't recommend it," said Tim Gardener, EPA project manager for pesticides.

EPA entomologist Phil Hutton points out that when it comes to home brews of insecticidal soaps, "as long as you just use it, it's OK. There's hardly an entomologist that hasn't used a soap spray," he said.

NGA talked to a sampling of Cooperative Extension entomologists across the country and found some wholeheartedly recommending ev- erything from ivory to Murphy's while others, as Koehler from Berkeley said, "would not technically recommend household soaps, though we certainly report our research findings showing that they work."

"For certain pests, I've seen a one percent solution of Ivory Liquid soap be more effective than Safer's. I've also seen ivory kill plants. If you're trying to control pests, you are responsible for using products as directed. I can't recommend the misuse of products when it comes to killing insects," Osborne said. On the other hand, "if you want to wash your plants with a household soap, it's up to you." just as insecticidal soap is probably not something you want to wash your dishes in, what's "new and improved" for your dishes may not be for your plants. The EPAs Hutton cautions that "many household soaps have other ingredients added that could be phytotoxic. There may be some household soaps that are fine. You can't know which. And soap manufacturers can change their formulas quickly"

If your plants are damaged by household soaps, you have no re- course. Hutton noted that if you report to the EPA that your plants are damaged after correct use of a commercial insecticide, the EPA checks into the claim. Valid claims have resulted in pesticide companies changing the claims they make about their product. Hutton said he had received no claims against Safer insecticidal Soap.

"We have to answer to the homeowner. We stand behind our products," said Safer's Atkins. "If someone uses our, product according to directions and it does something we say it doesn't do, we're responsible."

Because of variations in formulas, household soaps may perform inconsistently against pests. This gave household soaps a bad name in the 1930s, Atkins said. "The reason they're unreliable is that nobody had taken the trouble to research, standardize and register them. That's one of the battles we face. it's not that we're concerned about the competition, we're concerned that people go out, buy other soaps, find they don't work or that they burn their plants, then lump us in with all those products."

Safer claims that all fatty acids (the basic ingredient in soap) are not alike; Safer determined which ones kill insects and which harm plants, and came up with a formula that "doesn't harm 99 percent of plants it's used on and does reliably kill insects."

Experiments have also compared soap sprays to chemical sprays. Osborne, testing spider mite control on various houseplants in green- houses, reported in June 1984 that Safer insecticidal Soap was as effective as Pentac (dienchlor, a common chemical control for spider mites). Both provided effective control during the two-week test, although a second application of Safer's was made about 10 days after the first spray

Another experiment with schefflera concluded that of eight treatments for the control of citrus mealybug, Safer's soap and Zoecon Houseplant Mist were very effective. Murphy's Oil Soap showed some control at a three percent solution.

Soap sprays tested outdoors

Most of the experiments have been with indoor plants, but a few more recent trials have brought soap sprays outdoors. A test conducted at the University of Minnesota in Minneapolis in August 1985 tried high concentrations of Safer's soap on bedding plants to test phytotoxicity. it was found that snapdragons and Ringo Dolly geranium were undamaged. Rose periwinkle and petunias showed some damage after two or three sprays of twice or three times the recommended rate, and celosia and impatiens showed damage after spraying at all rates. Even in the worst cases, the damage did not persist. Either the plants recovered after a temporary setback or new growth did not show damage.

Tomatoes and potatoes were in on some tests done at Colorado State University at Fort Collins by entomologist Whitney Cranshaw. He tested Safer's and Ivory Liquid against potato psyllids and aphids on potatoes, and against aphids on tomatoes with good results. Both in- secticides performed equally without damage, he said. "Potatoes and tomatoes seem to be very hardy when it comes to soap sprays. I'm recommending soap for any softbodied insects."

And in Germany tests showed good results with Safer's against various aphids and mites on Golden Delicious apples, Processor bush beans, white cabbage, Chrudiemer plums, Conference pears, kohlrabi, grapes, tomatoes and lettuce.

Why home gardeners try soap Experiment after experiment shows that soap sprays work. But there are more reasons why gardeners are becoming big on bubbles. Gardeners like soap sprays primarily because of their safety. In storage, because they're not so caustic, soaps are less likely to harm children and pets. In use, they're biodegradable and leave little residue so it's safe to spray up to harvest time as long as the vegetables are thoroughly washed before eating. Because they are not severe enough to kill every pest in the vicinity, soap sprays are especially usefull when gardeners are trying to encourage beneficial insect populations. A soap spray may kill enough of the pest to prevent serious damage to plants, yet leave sufficient numbers to provide food for natural predators and build that beneficial population. Because it leaves less residue on plants and vegetables, a dose does not continue to kill insects as long as a similar dose of a chemical control, but most gardeners don't object to repeated applications.

Since soap sprays seem gentle, it's easy to be lax in using them. All soap solutions should be used with caution. Even those specifically designed for use on plants can burn particularly sensitive plants. Here are a few reminders from soap spray researchers across the country:

Use the correct dilution rate: 1 1/2 to two percent solution for most household soaps. Follow the directions on commercial insecticidal soaps. The EPA reminds gardeners that following directions extends beyond the dosage and frequency to wearing the proper clothing and taking all precautions listed. - Use a high spray pressure if possible. Some insects will be knocked off the plant and die before they're able to return.

Cover the plant thoroughly. Soap sprays are contact insecticides so spray the plant top to bottom and under the leaves, a favorite insect hangout.

Don't be afraid to repeat applications when the population increases again.

Don't spray during very hot weather when plant damage is more likely to occur. Some gardeners prevent plant damage by rinsing soap- treated plants a few minutes after application. If you're unsure how your plant will react to the spray, test a small area, then watch for about five days for damage.

Copyright NGA

Reprinted with permission HouseNet, Inc.

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