This article was originally released on February 21, 2001 as a bi-monthly news column for the Vero Beach Press Journal
Date of release: February 25, 2001
Daniel F. Culbert, County Extension Agent
CLEAN UP PESTS WITH SOAP
Spring has sprung - the grass is getting greener, buds are breaking, and Florida Yard owners are re-exploring the wonders of their landscapes. Along with the renewal of our ornamental plants comes an increase in the number of insects that are taking advantage of this fresh succulent growth. Today’s column will explore the use of soap as an alternative pest management tool for aphids and other sucking insects. Information is based on research from retired UF Entomologist Dr. Don Short and several of his colleagues from across the nation.
Home owners are often concerned about using chemical insecticides to mange landscapers pests. The use of insecticides in the landscape increases the stress on our natural resources, and some individuals are looking for alternatives that can reduce plant pests without sacrificing the environment.
Many of the more effective insecticides are unavailable to homeowners because they are too costly or the size of the available container that has to be purchased is too large. Homeowners who feel they need these kinds of insecticides are advised to contact a Pest Control Operator who has the license and know-how to use these tools.
Insecticidal soaps can help to fill this need for safe but effective management of some of our most troublesome landscape pests. Soaps have been used to control insects for more than 200 years. Recently, there has been increased interest in the use of these products. This change is due to a better understanding of how to use soaps most effectively and a desire to try insecticides that are easier and safer to use than many currently available alternatives. But, they are not the "Silver bullet" for all situations, and as with all insecticides, are not without risks.
LIFE THROUGH A STRAW
New growth on landscape plants is attractive to many kinds of insects that suck sap as their food source. They have straw-like mouth-parts that are inserted into the tender tips of plants, and rob the plant of its "blood" needed to support new growth. In the process of feeding, these pests may also transfer virus disease much like the way that mosquitoes transmit many human diseases. (These diseases are specific to plants - don’t worry about getting malaria from an aphid.)
Soft bodied pests include many different kinds of insects and mites. Inspect your plants for aphids, thrips, scales, spider mites and immature white flies and leafhoppers. Another identifying feature is the presence of a black, papery scum that appears on the surface of plant leaves and stems. This is sooty mold, which grows on the waste materials secreted by these pests.
Insecticidal soaps are used against soft bodied insects and mites such as aphids, thrips, white flies, spider mites and immature leafhoppers. Insecticidal soaps can kill susceptible insects by washing away the protective waxy coating of the insect. Soap enters the pest’s respiratory system and causes internal damage by breaking down cell membranes. It also affects the growth of immature insects. Insecticidal soaps are generally about 40-50% effective against these pests. Repeated applications may be necessary to adequately control high pest populations.
While soaps work on soft bodied insects, they are not as effective in controlling hard bodied, mobile insects such as beetles, wasps, bees and flies. This is part of the reason why soaps are often considered environmentally friendly, as they selectively kill many of the soft-bodied pests, but rarely effect the hard bodied beneficial insects such as ladybug beetles and predaceous wasps.
Soaps are made from natural substances - they are salts of fatty acids. Fatty acids are the principal component of oils and fats found in plants and animals. Some soaps contain up to 30% ethanol As they break down in the environment, these complex substances hold some limited potential as nutrients. Insecticidal soaps themselves are about as toxic as any other soap. Oleic acid, found in olive oil, is the most common insecticidal fatty acid. Potassium oleate is the potassium salt of oleic acid.
Soap has no residual value as an insecticide - it is only effective against insects that come into contact with the wet spray. Once the spray has dried, a moving insect will not be harmed by walking over the residue. Coverage, therefore, is extremely important. For example, spraying the upper leaf surface will leave whiteflies alive and healthy, since they usually feed under the leaves. The insects must come into direct contact with the spray droplets for the material to be effective. Insects that cannot be completely wetted, such as aphids within curled leaves, will not be controlled. The immature stages of some of these insects move little or not at all, and will be killed only if the wet material reaches them. Some insecticidal soap products can be purchased that contain additional botanical insecticides such as citrus oils or pyrethrins.
PURCHASE OR MAKE YOUR OWN
There are several commercially available formulations that can be found in better garden centers. Among the more common brand names are Safer Insecticidal Soap ™, M-pede ™ or Concern Insect Killing Soap™. [Trade names given here are not intended as an endorsement, but are given as examples of typical products.] Store-bought insecticidal soaps come with a pesticide label - look for an EPA registration number. Not only is it a good idea to read and follow all label directions - it’s the law. By policy, the University of Florida only recommends pesticides that have been shown by research to be effective when used according to the label.
Even though soaps have low toxicity to humans, they should always be used with caution. Insecticidal soaps can cause severe eye irritation and can cause vomiting and gastric distress if swallowed. Labels have first aid instructions and a lot of other information about their safe use.
Home gardeners have developed several recipes for home-made soap spray mixtures. UF entomologists recognize this trend, and include in our UF Insect Pest Management Guide is the suggestion "if a commercial insecticidal soap is not available", specific concentrations of dishwashing liquid in water may be substituted. The down side of home-made recipes is there are no label instructions to refer to insure the safety of the application.
The type and amount of dishwashing liquid is very important. Avoid the use of dishwashing liquids containing a degreaser and do not use an automatic dishwashing soap or detergent. If you will be using the soap spray in combination with a bacterial based Bacillus thuringiensis "B.t." caterpillar killer, avoid the use of soaps with an anti-bacterial agent as well.
Many references suggest that a 2% solution of soap in water should be used. Five Tablespoons (not teaspoons: 3 teaspoons = a Tablespoon) of soap in one gallon of water will result in a solution of slightly lower concentration than 2%.
UF entomologist Dr. Short suggests that 2 Tablespoons of dishwashing liquid per gallon of water are appropriate amounts of soap to use against mites, aphids, mealybugs, whiteflies, soft scales and thrips on many ornamentals. This recipe will produce slightly less than a 1% solution. The concentration of the spray is more important than the amount of soapy mixture applied.
Hard water reduces the effectiveness of insecticidal soaps. Calcium, magnesium and iron found in hard water turn into soap scum, making your mix useless against the insects. Use the purest water possible for insecticidal soaps. Conduct a "jar test" to determine if your water is compatible with the soap. Mix the concentration of soap that you intend to use with water in a glass jar. Mix and allow to stand 15 minutes. If the mix remains uniform and milky, then your water quality is adequate. If a scum develops on the surface of the water, then water conditioning will be necessary. Hard water can be "conditioned" by using commercially available products such as "Calgon™".
Insecticidal soaps may foam, especially when using homemade mixtures. A defoaming agent may also be added if there are too many bubbles. These products are readily available where pool and spa supplies are sold. Test these defoamers as above, and look for scum.
SOAP CAN BURN
Watch for phytotoxicity, an adverse plant reaction or injury from the soap treatment. Symptoms on foliage include yellow or brown spotting, "burned" tips, and/or yellow or brown scorching on the leaf edges. Soap spray may also cause marking on some fruit varieties. Phytotoxicity is perhaps the greatest concern most people have when using insecticidal soap. However, by observing a few points you can decrease your chances for plant injury:
- Don't treat plants under drought or other kinds of stress. Stressed plants may be especially intolerant of insecticidal soap solution. Make sure plants to be treated are well-watered and otherwise look healthy.
- Avoid treating sensitive plants. Among the ornamental plants reported to be sensitive to soap sprays are gardenia, maidenhair fern, crown of thorns, lantana, nasturtiums, and Easter lilies (during bud formation). Portulaca and certain tomato varieties also are sometimes damaged by insecticidal soaps. Hairy leaved plants may hold spray droplets. These droplets can act as magnifying lenses and cause some burning. Test insecticidal soap first on a small part of palms, delicate ferns, ornamental ivies, and succulents before treating an entire plant or area. These may also be sensitive.
- Wait for new growth to harden off before treating. Tender, young foliage may be most sensitive. Fruit trees in bloom also should not be sprayed. If ever in doubt, test a small part of a plant first. If the plant is sensitive, phytotoxic symptoms should appear after 48 hours.
- Apply when the temperature is below 90 degrees F and not in full sun. High temperatures and high relative humidity may increase plant stress and therefore sensitivity. The best time to apply insecticidal soap is in the early morning. The material works only while wet, and the slower drying conditions in early morning favor better control.
Use a weaker concentration on plants that you feel may show phytotoxicity. Higher concentrations of soap may not only dissolve the wax-like shell of an insect, but may also do the same to the waxy surface of plant leaves. Keep a written record of your mixture so you can remember if your mixture was damaging to the plant. To reduce leaf injury, wash plants within a couple of hours after the application. Limiting the number of soap applications can also be important, as leaf damage can accumulate with repeated exposure.
If you need additional information on insecticidal soap, visit your county Master Gardeners, or call or stop by your county Extension office. For those with other questions about Florida Yards, contact me - my phone number is 863-763-6469 and you can send e-mail to firstname.lastname@example.org.
Biorational or Alternative Controls http://woodypest.ifas.ufl.edu/biorat.htm
Natural Products for Insect Pest Management http://edis.ifas.ufl.edu/IN197
Other University publications
Cranshaw, W.S. Insect Control: Soaps and Detergents Colorado State University, 3/03 http://www.ext.colostate.edu/pubs/insect/05547.html
Anonymous. Insect Control with Soaps and Detergents. Calaveras County University of California Cooperation Extension http://cecalaveras.ucdavis.edu/soaps.htm
Mayer, C. Insecticidal Soap An alternative to chemicals. Penn Sate University, 1/03 http://paipm.cas.psu.edu/pdf/POP/insectsoap.pdf
Sample Insecticidal Soap Labels
Mycogen Corporation M-Pede
Olympic Horticultural Products INSECTICIDAL SOAP™49.52 CF
Safer Concern® Insect Killing Soap
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Trade names, where used, are given for the purpose of providing specific information. They do not constitute an endorsement or guarantee of products named, nor does it imply criticism of products not named. The Florida Cooperative Extension Service - Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research, educational information, and other services to individuals and institutions that function without regard to race, color, sex, age, handicap, or national origin. Florida Cooperative Extension Service / IFAS /University of Florida. Larry A. Arrington, Dean. Last update: 01/31/2005 . This page is maintained by Dan Culbert