I started out to write one article about organic gardening, but it became apparent that I would not be able to cover all that I wanted to at one time. Three recent articles have addressed some basics of what organic gardening is, soil fertility, and general weed, insect, and disease management. This final article in the series addresses some pesticides (including insecticides, fungicides, herbicides and other products) allowed, under certain conditions, in organic production.
Previous articles have discussed some of the things that must be done before resorting to use of a pesticide in certified organic production. Of course, home gardeners are free to pick and choose among what organic practices they follow, but a focus on good cultural practices and prevention for weed, insect, and disease management should be shared by all gardeners.
Not every product with a given active ingredient will be compatible with U.S. organic standards. All ingredients in a product, not just the active ingredient, must be compliant. The Organic Materials Review Institute (OMRI) is a nonprofit organization that evaluates products to determine if they comply with National Organic Program regulations. You’ll see the term OMRI on some product labels. OMRI evaluates products at the request of companies, so not all allowed products will be OMRI-listed.
One of the easier types of insect pests to find an organic insecticide for is caterpillars. Proteins from the bacterial species Bacillus thuringiensis subspecies kurstaki (Bt or Btk) are toxic to caterpillars when they eat them. (There are products made from other subspecies of Bt that kill certain other insects.) As is true for insecticides in general, Btk products work better when caterpillars are small. A number of Btk insecticides for commercial growers and home gardeners exist.
Like Bt, spinosad is derived from bacteria. In addition to being effective against many types of caterpillars, spinosad products are also used for thrips and ants. Many of these insecticides can be used on a variety of vegetables, fruits and ornamentals.
Horticultural oils (both petroleum-based or mineral oil and plant-derived oils, such as neem oil) and insecticidal soaps have some efficacy against a wide range of soft-bodied insects such as caterpillars, aphids and whiteflies, if they have direct contact with them. This means that good coverage is very important. Because these lack residual activity, they often need to be applied on a regular basis.
There is a group of insecticides used in conventional production called the pyrethroids (active ingredients ending in “-thrin,” plus esfenvalerate). These contain one or more active ingredients that are synthetically produced but chemically similar to natural compounds from pyrethrum daisies.
The synthetic pyrethroids are not allowed in organic production, but some products with one or more of the original, plant-derived compounds (pyrethrins) are. Pyrethrins work on a broader range of insects than some of the other organic insecticides, having some efficacy against insects like beetles and stink bugs. They break down quickly though and are generally less effective than their synthetic counterparts.
Like synthetic pyrethroids, pyrethrin products are toxic to bees and, as with most insecticides, should be applied in the evening when honeybees are no longer active.
For slugs, there are bait products with the active ingredient iron phosphate that are OMRI-listed. These are comparable in effectiveness to some of the best non-organic alternatives.
Sulfur is used as a fungicide and, in some cases, as an insecticide or miticide. It has limited effectiveness against many diseases but works quite well for powdery mildew on a variety of plants. Some plants are sensitive to sulfur, especially under certain conditions, so this should be considered before using it.
Copper sulfate and a variety of fixed copper products (active ingredients such as copper hydroxide, copper oxide and copper oxychloride) provide limited protection against a broad range of diseases. Non-organic growers sometimes use them for bacterial leaf spots, since copper products are some of the only options for diseases caused by bacteria. (Most plant diseases are caused by fungi.)
Few types of herbicides are allowed around organic food crops, but there are some products with caprylic acid and capric acid as active ingredients that are OMRI-listed and labeled for use around many fruits and vegetables.
Soap-based herbicides are allowed in ornamental areas and “farmstead maintenance” but not around fruits and vegetables. Examples of ingredients found in soap-based herbicides include ammonium nonanoate and “ammoniated soap of fatty acids.”
Keep in mind that just because a product can be used in organic production doesn’t mean that it’s necessarily less toxic than comparable non-organic products. When using any type of herbicide, fungicide, insecticide or other pesticide, be sure it’s labeled for the site on which you plan to use it, and read and follow label directions.
Let me know if you have questions.
Dr. Mary Helen Ferguson is an Extension Agent with the LSU AgCenter, with horticulture responsibilities in Tangipahoa and Washington parishes. She handles fruit-, nut- and vegetable-related calls for Tangipahoa Parish. Contact Mary Helen at mhferguson@agcenter.lsu.edu or 985-277-1850.
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