Pest Control

Soap, Oil and Other Homemade Insecticides and Fungicides

Because they are often primary components of many home remedies, it helps to have a good understanding of the use of soaps and oils for treating plant pest problems before looking into various other homemade potions.

Soap Sprays

There are three types of soap that gardeners through the ages have used to treat a number of plant insect problems. All soaps are fatty acid-based products, and to varying degrees, are one of the safest sprays to use in the garden or on your indoor plants. They primarily kill soft-bodied insects, such as aphids, mealybugs, spider mites, thrips, whiteflies and immature leafhoppers. Repeated applications at one week intervals are generally necessary to control high pest populations.

I remember my grandmother washing dishes in a large pail in the sink and then dumping the soapy water over various plants in the garden. The belief was that the soapy water acted both as an insecticide and soil nutrient. Many gardeners have been adding small quantities of dish detergent with water and spraying plants with good results for years.

There are other gardeners who prefer to use either natural liquid soap, available at health food stores, instead of dish detergent, and there are some valid reasons. Most dishwashing detergents have anti -bacterial properties that may have some harmful effects on plants, particularly if the sprays are mixed too strong.

The third option is to spend the extra money and purchase an insecticidal soap specifically formulated for use on plants. Commercially formulated insecticidal soaps are better than home remedies because they have been tested to be safe on a variety of plants.

Perhaps the biggest mistake gardeners make in using soap sprays is not in the soap they select, but in mixing the spray too strong. Commercial insecticidal soaps will come with mixing instructions, and it is best to start half strength until you can determine how various plants react to spraying. A conservative mixture for soap spray is 1 to 2 teaspoons liquid soap added to 1 quart of water. A few drops of cooking oil can be added to help the mixture cling to leaves better.

Guidelines for Insecticidal Soap

Insecticidal soap is a contact material, meaning that insects must come into direct contact with spray for it to be effective. There is no residual benefit of the spray, so it is desirable to rinse off soap sprays 12-24 hours after application. This can be done by following sprays with a good water rinse or foliar feeding.

Water Quality is very important to the effectiveness of soap sprays. Hard water reduces the effectiveness of soap sprays and may even render them useless. Using the purest water available is best, so it is recommended to use distilled water with soap sprays. It is best to spray a fine mist over plants rather than drenching them. Spray bottles that can produce a fine mist are available at dollar stores, but commercial sprayers that have wands make it easer to spray the underside of leaves, which is important to achieve complete coverage. The same sprayer may be used for soap and/or oil sprays as well as foliar feeds, but a separate sprayer must be used for herbicidal sprays. As with anything that is sprayed on plants, soap sprays are best applied when it is cool, either in the morning or evening.  

Oil Sprays

Oil sprays have been used for centuries, either alone or in combination with soap or other ingredients to combat many of the same pests controlled by insecticidal soaps. Oil provides an added benefit because it has a different impact than soap on pest’s ability to survive. It has the added benefit of helping sprays to stick to leaves and may provide some control of funguses such as powdery mildew, as well. Oils pose few risks to people or to most desirable species, including beneficial natural enemies of insect pests. This allows oils to integrate well with biological controls.

Many gardeners use common cooking oils in their sprays, but the type of oil used can greatly impact the effectiveness of the spray. Cottonseed oil is generally considered the most insecticidal of the vegetable oils. Soybean oil runs a close second. The advantage of purchasing horticultural oils specifically designed for use on plants is that horticultural oils are normally combined with an emulsifying agent that allows the oil to mix well with water. Regardless of the oil used, the mixture usually consists of a 2 percent dilution, which is like that used for insecticidal soaps. One to two teaspoons liquid soap added to 1 quart of water is a good general guideline.

Another oil that deserves consideration for oil sprays and has received considerable attention as an effective insecticide with some additional fungicidal properties, consists of extracts from seeds of the neem tree (Azadirachta indica).

Now that we have a good understanding of options and uses of soaps and oils as natural insecticidal controls, and hopefully an appreciation for the added benefit of combining the two, we can consider some additional natural insecticide and fungicide tools.

Baking Soda and Cornmeal

Although horticultural oils may provide some fungicidal properties, if various forms of fungus are a big problem, baking soda and/or cornmeal may be just what the doctor ordered. The good news is that baking soda or cornmeal juice can be added to your soap and oil spray formula. Add baking soda at a rate of 1 teaspoon per quart of water for a triple threat against insect pests and fungus.

Cornmeal has been found to have fungicidal properties that may surpass toxic chemical fungicides. Dry cornmeal can be incorporated into soil at a rate of two pounds for every 100 square feet. This method of application has the added benefit of adding nutrients to the soil. A cornmeal spray can also be made by soaking one cup of cornmeal in a gallon of water overnight and then straining it. Cornmeal spray can be mixed with compost tea, or any other natural foliar feeding spray like seaweed extract and/or fish emulsion. Food grade cornmeal can be bought at any grocery, but it is much more economical to buy horticultural cornmeal in bulk from local livestock feed stores.

Manure Compost Tea

Manure compost tea is quite popular as an organic foliar feed but is also effective on many pests because of certain microorganisms that exist in it naturally. To make a simple manure compost tea, fill a bucket half full of manure. Then stuff the manure into an old sock or two. Add a few rocks, tie the sock, and fill the bucket with water. Let the mix sit for 10-14 days and then dilute and spray on the foliage. A rule of thumb is to dilute the tea down to one part compost liquid to four to ten parts water. It should look like weak tea rather than black coffee. Add two tablespoons of molasses to each gallon of spray for more power. Add horticultural oil for even greater pest killing power.

The sprays listed above should be the mainstay of any gardener’s natural pest and fungus control arsenal, but many other natural sprays for a variety of pest problems can be made at home and complete lists of various recipes for different problems can be found online.

Nick Fraser is a free-lance writer and greenhouse enthusiast.

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