UF/IFAS Okeechobee County Extension Service

458 Highway 98 North

Okeechobee, FL 34972-2578

Phone: (863) 763-6469

E- mail: okeechobee@ufl.edu 

January 26, 2005

Feature Article - for release the week of February 6, 2005

Dan Culbert - Extension Horticulture Agent

Quick Links:  Sucking insects    Soaps for Sprays    Precautions    References

 

CLEAN UP PESTS WITH SOAP     

Do you remember grandma throwing the dishwater on the rose bush?  There was a lot of wisdom involved with this practice - she somehow knew that it would keep her flowers looking good.  With spring on the way, insects will soon be attacking new growth.  Sudsy soaps may be one of our best defenses against these pesky suckers.

Home owners are often concerned about using chemical insecticides – they can increase the stress on our natural resources.  Many potent bug killers used in the landscape by licensed Pest Control operators are too costly, or the size of the container is too much for the average homeowners.  Today’s column will explore the use of soap as a pest management tool for sucking insects. Information is based on research from retired UF Entomologist Dr. Don Short.  

Insecticidal soaps can help to fill the need for safe but effective ways to manage some of our most troublesome landscape pests.  While soaps have been used to control insects for more than 200 years, they are not the "silver bullet" for landscape insect control.

 

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 aphids, thrips, scales, spider mites, immature white flies and leafhoppers.  If you see a black, papery scum that appears on leaves and stems, suspect these sap suckers are present. This sooty mold grows on the waste materials secreted by these pests.

Insecticidal soaps are used against soft bodied insects.  They can kill by washing away the protective waxy coating of the insect, entering the pest’s respiratory system and causing internal damage.  

Soaps are not as effective in controlling hard bodied insects such as beetles, wasps, bees and flies.  This is why soaps are often considered environmentally friendly, as they selectively kill many of the soft-bodied pests, but rarely bother hard bodied beneficials such as ladybug beetles and predaceous wasps.

Soaps are made from natural substances - they are salts of fatty acids - part of oils and fats.  Oleic acid, found in olive oil, is the most common insecticidal fatty acid. Potassium oleate is the potassium salt of oleic acid. Some soaps contain up to 30% ethanol.  

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, insects will not be harmed by walking over the residue. Coverage is extremely important - spraying the upper leaf surface will miss many of these insects since they are often found under or within curled leaves.

PURCHASE OR MAKE YOUR OWN

There are several commercially available insecticidal soaps that can be found in garden centers. Among the more common brand names are Safer Insecticidal Soap™, M-pede™ and  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.

The UF Insect Pest Management Guide suggests home made products can be used "if a commercial insecticidal soap is not available".  Specific concentrations of dishwashing liquid in water may be substituted.  The down side of using home-made recipes is there are no label instructions to refer to in order to apply soaps safely.

The type and amount of dishwashing liquid is very important.  Avoid the use of products containing a degreaser, and do not use automatic dishwasher detergents.  If you will be combining the bacterial based Bacillus thuringiensis "B.t." caterpillar killer in your spray, avoid soaps with anti-bacterial agents.

UF Entomologist Dr. Short suggests that 2 Tablespoons (not teaspoons: 3 teaspoons = a Tablespoon) 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. Some references suggest a 2% solution of soap - Five Tablespoons of soap in one gallon of water will result in a solution of slightly lower concentration than 2%.

Hard water reduces the effectiveness of insecticidal soaps. Use the purest water possible for insecticidal soaps. Conduct a "jar test" to determine if your water is compatible with the soap: Mix the soap 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 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 burning (phytotoxicity) or injury from the soap treatment.  Leaves with yellow or brown spots, "burned" tips, or yellow brown scorching on the leaf edges may result. Soaps may also cause marking on some fruit varieties Decrease your chances of plant injury by:

• Don't spray wilted plants.
• Avoid treating sensitive plants. Ornamentals sensitive to soaps include 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 the plant before treating an entire plant.
• Tender, young foliage may be sensitive, and plants in bloom should not be sprayed. If 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. The best time to apply insecticidal soap is in the early morning.
• Use a weaker concentration on sensitive plants. Higher concentrations 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, sensitive plants may be washed 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.

I’ve placed more information on our Okeechobee web page,  http://okeechobee.ifas.ufl.edu.  If you need additional information on insecticidal soaps, please email us at okeechobee@ufl.edu or call us at 863-763-6469.  Local residents can stop by our office at 458 Hwy 98 North in Okeechobee, and visit our Okeechobee County Master Gardeners.

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References 

Buss, Eileen A. & Short, Don E.   Insect Management on Landscape Plants.  ENY-338. Gainesville, Florida Cooperative Extension Service, 7/2003.

Buss, Eileen A. & Park-Brown, Sydney G. Natural Products for Insect Pest Management [ENY-350]. Gainesville, Florida Cooperative Extension Service, June, 2009. 

Culbert, Dan.   Clean up Pests with Soap. Vero Beach: Press Journal, 2/25/2001. 

Harrison, Marie.  Consider Insecticidal Soap for Insect Control. Crestview: Oskaloosa County Extension Service, 9/7/03.

Wichman, Tom.  Horticultural Soaps. (Gardening in a Minuet Series)  Gainesville: UF/IFAS Environmental Horticulture Dept. 6/2010. 

 

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.  Millie Ferrer, Interim Dean Last update: 07/11/2011 .  This page is maintained by Dan Culbert