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Ask a Master Gardener: Organic Insecticides for the Home Gardener

Updated: Jun 20, 2023

By Carol Dorsey, Mobile County Master Gardener www.mobilecountymastergardeners.org


With year-round growing, area gardeners will notice insects, pests and beneficials, around edible plants. Identifying the damage insect pests cause means decisions must be made to limit this damage. The first step in controlling pests in our gardens is using the Integrated Pest Management process. These tactics to limit harmful insects in the garden may be found at https://www.aces.edu/wp-content/uploads/2021/01/IPM-1305_HomeGardenVeg_012821L-G.pdf


After discovering harmful pests or assessing the damage they cause, it's crucial to determine the most appropriate treatment method. There are basically two approaches: conventional and organic. In the field of chemistry, the term organic refers to chemicals that contain carbon. However, in the world of pesticides, the term organic refers to chemicals derived from natural sources that are minimally processed. These sources can include bacteria, fungal by-products, plant extracts, mechanical deterrents like kaolin clay or diatomaceous earth, or chemically treated plant oils. On the other hand, conventional pesticides are usually synthetic chemicals created in an industrial process.


It's worth noting that both conventional and organic pesticides are given toxicity ratings by the EPA, ranging from highly toxic (Class I) to virtually nontoxic (Class V). Consider it essential to learn the toxicity ratings of any pesticide before using it as a treatment. To reduce exposure to chemicals, the home gardener should note on any pesticide label the ‘days until harvest after application’ and the recommended personal protective equipment to wear while working with products. The label of the product will supply all this needed information. More legible label contents are available online searching the product’s commercial name.


Another aspect of pesticide use is application in the right concentration at the right time. A glug from the jug is not one ounce per gallon. Use a measuring cup appropriate for the volume to mix pesticides that require dilution. Application at the right time is also critical since these pesticides target insects. Beneficial bees and butterflies are insects and can be killed just like the pests. Please educate yourself as to the mode of action, whether contact or ingestion, and time of application so that beneficials are minimally affected. Ambient temperatures less than 90 degrees reduce plant injury with some organic products.


The rising interest in organic gardening has resulted in greater availability of these products in the marketplace. The following is a partial list of easily acquired organic pesticides, their origin, and some applications in the home garden. More information may be found at https://www.aces.edu/blog/topics/lawn-garden/partial-list-of-organic-insecticides-for-the-vegetable-garden/


Bt or Bacillus thuringiensis subspecies kurstaki - A strain of bacteria that targets caterpillars which ingest the product. Then the bacteria quickly work from within making the caterpillar stop feeding. Application is by spraying diluted product on leaves, particularly the undersides where caterpillars are likely to be. Bt is rated as low toxicity to humans and other mammals, and can be used up to day of harvest.


Insecticidal soaps are chemically treated animal or plant-based oils that affect the respiratory system and outer covering of soft-bodied insects like scale, aphids, and mites. The chemical treatment creates potassium salts of fatty acids which are easily broken down in the environment. These products are not dish soap and water. Most of those “soaps” are detergents containing sodium salts which strip the plant’s delicate outer layer. While a low toxicity contact pesticide, concentration is critical because strong solutions can burn the plant. Insecticidal soaps are safe to use up to day of harvest.


Neem Oil is a plant extracted oil with a compound that interferes with insect hormonal systems, reduces feeding, and acts as a repellent. This contact pesticide is useful against adult and juvenile insects. Spray at correct concentration in early morning and again in the evening to reduce effects on beneficial insects. Neem oil may be used up to 5-7 days from harvest.


As we make gardening decisions about pesticide choices and applications, consider effective organic options with lower environmental impact.




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