Extension Communications
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
8/14/00
Contacts:
Mark Shour, Extension Entomology, (515) 294-1101, mshour@iastate.edu
Elaine Edwards, Extension Communications, (515) 294-5168,
eedwards@iastate.edu
Yard & Garden Column for the Week Beginning August 18
Pesticides from Nature
By Mark H. Shour
Extension Program Specialist
Iowa State University Extension
A visit to the pesticide section of a garden center can be an education. In addition to being greeted by colorful labels, different package sizes and formulations and a variety of odors, there is a flood of hard-to-pronounce chemical names. In addition, some products are identified as "natural" or "organic." To satisfy your curiosity, let's spend a few moments examining these pesticides from nature.
Organic pesticides are derived from naturally occurring substances. They need to be extracted, processed or concentrated from the natural source, but they are generally not chemically changed. In contrast, "synthetic" pesticides are usually derived from petroleum sources and the chemistry of the molecules is changed to give a desired product.
Organic pesticides fall into three broad groups: botanicals, microbials and minerals. Botanical pesticides are obtained from the roots, leaves, seeds, stems or flowers of plants. Many plants manufacture chemical compounds that protect them from pest attack. Once identified, people began using these chemicals to protect valued plants against similar pests.
The second group of organic pesticides, called microbials, contains pathogenic bacteria, viruses, fungi or nematodes. These microorganisms serve as natural enemies to some insects and control natural population outbreaks under certain conditions. Manufacturing processes have allowed the application of these microbial pesticides in the form of sprays, dusts, baits or granules to control pest outbreaks in the home garden or orchard.
The third group is called mineral-based pesticides. These are derived from inorganic chemicals of the earth's crust. They are crystalline, stable chemicals that do not evaporate but generally dissolve easily in water.
Why are organic pesticides preferred by some gardeners? They breakdown rapidly in the environment after application, they act to prevent further insect feeding and many have a low toxicity to people and animals. Some disadvantages to using organic pesticides include a higher sticker price, differences in quality from batch to batch, a gross misconception that these products are not dangerous and that they are "broad spectrum," that is, they can kill more insects and other organisms than desired (such as honeybees and natural enemies).
The poisonous nature of a pesticide (called toxicity) can be determined by the consumer in two ways. One of four possible signal words are printed on every pesticide label; these provide product toxicity in a relative way. "Caution" is required for products considered slightly toxic to relatively nontoxic. "Warning" is required on pesticide labels involving moderate toxicity. "Danger" or "Danger/Poison" with a skull and crossbones appears on all product labels that are highly toxic.
The second indication of a product's toxicity follows a standardized, industry-wide criteria called the LD50, or the amount of pesticide that kills 50 percent of a group of test organisms. You can find this value on a pesticide's material safety data sheet (MSDS) but not on the pesticide label. Ask the garden center to provide you with a copy of the MSDS if you are interested in comparing precise product toxicity.
The units for an LD50 value are milligrams of poison per kilogram of body weight (mg/kg); this allows the extrapolation of toxicity from test organisms to people. A key rule to remember is the SMALLER the LD50 value, the MORE toxic the pesticide . Thus, if product A has an LD50 value of 300 mg/kg and product B has a value of 2,000 mg/kg, product A is more toxic and could cause more harm to you. A pesticide with an LD50 value of 5,000 or greater is referred to as "relatively nontoxic."
Pesticides which can be classified as natural or organic include:
Abamectin is derived from fermenting Streptomyces avermitilis, a bacterium. It has an oral LD50 value of 300 mg/kg. Very low amounts are effective. It stops insect feeding days before they die. Unfortunately, this product is not available to homeowners.
Bordeaux Mix is made from copper sulfate and hydrated lime. It acts as a fungicide and an insect repellant. It is relatively nontoxic.
Boric acid is a mineral-based organic pesticide derived from the element boron. It is a relatively nontoxic insecticide used in cockroach and ant control.
Bt is derived from fermentation of Bacillus thuringiensis, a soil bacterium. One widely available strain only controls caterpillars. It is relatively nontoxic.
Clandosan (trademarked) is made from ground crab and shrimp shells. It is relatively nontoxic. Clandosan stimulates the growth of beneficial soil organisms, which in turn control nematodes. This product is difficult to find.
Corn gluten is made from cornmeal. It is a relatively nontoxic, preemergent herbicide that usually requires 2 to 3years for effective weed control.
Limonene is derived from citrus peels. It is used to repel or kill mosquitoes, cockroaches, silverfish and some external pests of pets. It is relatively nontoxic.
Neem is made from seeds of the neem tree. The active ingredient is azadirachtin. It has a garlic-like odor and is relatively nontoxic. Neem stops insect feeding and interferes with shedding of their skin.
Nicotine comes from tobacco leaves and is one of the oldest and most dangerous organic insecticides (oral LD50 = 50 mg/kg). Black Leaf 40 was a popular garden product for many years; it is still sold as NicoSoap (trademark). Chemically, nicotine is related to several noted botanicals (with the plant source), including caffeine (cocoa beans), quinine (cinchona bark), morphine (opium poppy), cocaine (cocoa leaves), ricinine (castor oil beans), strychnine (Strychnos trees), coniine (spotted hemlock; used to poison Socrates) and LSD (ergot fungal contamination of rye).
Pyrethrum is extracted from the flowers of Chrysanthemum (daisy) species grown in Kenya and Ecuador. The oral LD50 value is 1,500 mg/kg. Pyrethrums give a fast knock down of pest insects and are broad spectrum. There are no residues left on crops after spraying with this pesticide.
Quassia is a botanical insecticide extracted from wood and bark of bitterwood, Quassia (Picrasma) amara. It is a relatively nontoxic, broad spectrum product sometimes found in products containing rotenone.
Rotenone is derived from the roots of certain bean plants (Derris and Lonchocarpus species). It has an oral LD50 value of 350 mg/kg. Rotenone stops insect feeding and is broad spectrum in action. It leaves no residue on crops and lasts only 3 days once applied to vegetables. Rotenone is very toxic to fish.
Ryania is made from ground roots of the ryania shrub of Trinidad. It has an oral LD50 value of 750 mg/kg. Ryania stops insect feeding and leaves no residue on crops. It is chemically related to nicotine.
Sabadilla is extracted from seeds of a lily family. When orally ingested, it is a relatively nontoxic product. However, this insecticide is irritating to human eyes and causes violent sneezing in some people. It leaves no residue on crops. Sabadilla is difficult to purchase.
Soap is the alkali salt of fatty acids. Unlike the other organic products, this product has resulted from simple chemical changes. Insecticidal soap is included with these pesticides because it is relatively nontoxic and has a broad spectrum of action.
Spinosad is a fermentation process of Saccharopolyspora spinosa, a soil bacterium. It has low toxicity to people and animals (oral LD50 = 3,700 mg/kg). Very small amounts of this product are needed to control insect pests. Unfortunately, spinosad is not yet available to the homeowner.
Sulfur is a very old, mineral-based organic pesticide derived from the element sulfur. It is relatively nontoxic when orally ingested, but it is a skin irritant to some people. It controls mites, thrips and scale crawlers and is also a fungicide.
Zinc phosphide is a mineral-based pesticide made from the elements zinc and phosphorus. It is very toxic to mammals (oral LD50 = 45 mg/kg) and is used to control rats, mice and other rodents. Like Neem, it has a garlic-like odor.
ml: isugarden
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