Question of the Week
Answer: In summary, there are two materials that are allowed for rodent control in organic production. However, there are many acceptable methods of control. I have tried to answer your question by commenting on a few key sections of the National Organic Program Standards about pest control that I have pasted in below, and also by providing some general considerations for rodent control. In order to provide you with more specific control options, it is important to identify the problem as specifically as possible. An important first step in any pest management strategy is to seek understanding the life cycle, habits, and damage caused by the rodents that are problems for you.
Please read the Organic Standards carefully. Your certifier should provide you with a copy of the standards, and they can also be found online at www.ams.usda.gov/nop/NOP/standards/FullRegTextOnly.html.
You should also check with your certifier with any remaining questions. Of course, any material you plan to use must be included in your Organic System Plan and approved by your certifier. The standards provide several strategies to help you develop a systems-based approach to managing pests—including rodents—on your farm. Rodent management will be a continuing effort, and it is probably unrealistic to expect to fully eliminate the problem.
Predators: Some people have a great deal of success putting up nest boxes to attract owls to eat rodents. Depending on the wildlife that lives in your region, many types of predators may be able to help you, including snakes, owls, hawks, great blue herons, weasels, bobcats, coyotes, and domestic dogs and cats. More detail on design and placement is provided below.
Trapping, Habitat Reduction, and Physical Barriers: Many if not most organic farmers rely on trapping for some degree of control. Trapping can be very effective when used with persistence, skill, and appropriate type of traps (there are many kinds on the market). To be most effective, trapping must be done daily, especially at critical times in the cropping season and key seasons in the live cycle of the rodent. Many types of rodent problems may be minimized by making the farm and areas around farm buildings less hospitable to them, by removing shelter and potential food sources. Physical barriers to invasion or access to food, such as fences, wire baskets, or even trenches and irrigation.
Rodenticide Materials: National Organic Program standards for pest control (see below) list many methods of control, but only two allowed materials. Allowed pesticide materials should be used only when you have tried other methods and they are not sufficient. Any material, whether natural or synthetic, must be included in your Organic System Plan to be approved by your organic certifier. Natural materials, in general are allowed, but a few, such as arsenic and strychnine, are prohibited. Synthetic materials are generally prohibited, with a few specific exceptions that are allowed and included on the National List of allowed synthetics. For rodent control those are sulfur dioxide (smoke bombs), and Vitamin D3.
Vitamin D3 (Cholecalciferol) is listed as an allowed synthetic material for rodent pest control. Cholecalciferol-containing rodenticides produce hypercalcemia (6), making it an effective poison. Rodents generally die within two days following ingestion and do not appear to exhibit bait shyness. However, care should be used when placing this bait, particularly where dogs and young male cats are present, both of which are somewhat indiscriminate in their eating habits.
In the Generic Materials List published June 2004 by the Organic Materials Review Institute (OMRI), OMRI notes:
Vitamin D-3 cannot be the sole means of rodent control. Alternative methods for rodent control must be documented in the Organic System Plan. Growers must take precautions to prevent killing non-target animals.
There are several trade names of commercial rodenticides containing vitamin D-3 or cholecalciferol, such as: Quintox, True Grit Rampage, and Ortho Rat-B-Gone (6). Some of these rodenticides should be available from a local garden supply store or garden section of a hardware store. You will need to obtain a label from the manufacturer with a complete disclosure of the inert ingredients in the product, and ask your certifier whether these products are acceptable. Many products, although they contain an active ingredient that is allowed, also include “inert” ingredients that may or may not be acceptable for use in organic agriculture (to be allowed they must be on EPA list 4). Other sources of cholecalciferol include multivitamins containing Vitamin D, such as Viactiv (6), feed grade or dietary sources of vitamin D, and plants with calcinogenic properties (1). Cestrum diurnum (Day Jessamine) and Solanum malacoxylon plants are a source of cholecalciferol (6).
National Organic Program standards:
§ 205.206 Crop pest, weed, and disease management practice standard.
(a) The producer must use management practices to prevent crop pests, weeds, and diseases including but not limited to:
(1) Crop rotation and soil and crop nutrient management practices, as provided for in §§ 205.203 and 205.205;
(2) Sanitation measures to remove disease vectors, weed seeds, and habitat for pest organisms; and
(3) Cultural practices that enhance crop health, including selection of plant species and varieties with regard to suitability to site-specific conditions and resistance to prevalent pests, weeds, and diseases.
(b) Pest problems may be controlled through mechanical or physical methods including but not limited to:
(1) Augmentation or introduction of predators or parasites of the pest species;
(2) Development of habitat for natural enemies of pests;
(3) Nonsynthetic controls such as lures, traps, and repellents…
(e) When the practices provided for in paragraphs (a) through (d) of this section are insufficient to prevent or control crop pests, weeds, and diseases, a biological or botanical substance or a substance included on the National List of synthetic substances allowed for use in organic crop production may be applied to prevent, suppress, or control pests, weeds, or diseases: Provided, That, the conditions for using the substance are documented in the organic system plan.
§ 205.601 Synthetic substances allowed for use in organic crop production.
In accordance with restrictions specified in this section, the following synthetic substances may be used in organic crop production: Provided, that, use of such substances do not contribute to contamination of crops, soil, or water. Substances allowed by this section, except disinfectants and sanitizers in paragraph (a) and those substances in paragraphs (c), (j), (k), and (l) of this section, may only be used when the provisions set forth in Sec. 205.206(a) through (d) prove insufficient to prevent or control the target pest….(g) As rodenticides.
(1) Sulfur dioxide—underground rodent control only (smoke bombs).
(2) Vitamin D3.
§205.602 Nonsynthetic substances prohibited for use in organic crop production.
The following nonsynthetic substances may not be used in organic crop production…to treat a physiological disorder associated with calcium uptake.
(i) Tobacco dust (nicotine sulfate).
Control Options, Predators
Since chemical controls such as anti-coagulants are not preferred options, what’s needed is an increased population of predators including snakes such as gopher snakes, corn snakes, rat snakes, owls, hawks, great blue herons, weasels, bobcats, coyotes, and domestic dogs and cats.
Two of the species of rat snakes on the mainland US feed on rodents, such as mice, rats, and squirrels, are the corn snake, Elaphe guttata, and the rat snake, Elaphe obsoleta. However, please note that both species also feed on small birds, so that chicks and eggs might be at risk as well as rodents. Domestic cats are another option and will provide long-term control, although they will also prey on the songbirds on your farm.
More than 95 percent of the diet of barn owls usually consists of small mammals (mostly rodents). However, in some studies substantial bird remains have been found. According to Colvin (3) each adult barn owl may consume about one or two rodents per night; a nesting pair and their young can eat more than 1,000 rodents per year. Dietary studies from California and other states show that a barn owl consumes on average 50 to 60 grams of prey per day (0.11-0.13 pounds per day, 40-48 pounds per year). The actual species consumed depends on the species abundance and availability in the area. Barn owls will readily use man-made nest boxes. It’s clear that there is more than one way to build a barn owl box.
One model for a barn owl nesting box that is made from PVC is online at http://kaweahoaks.com/html/barn_owl_house.html.
Additional guidelines for owl housing (4):
Interior floor size (inches)
Interior height (inches)
Entrance Hole Diameter (inches)
Height to Mount Box (feet)
7 x 7
Forest clearings & edges
8 x 8
Farmland, orchards and woods
8 x 8
Boreal Forests and Bogs
14 x 14
Mature Bottomland Forest
12 x 36
6 x 7
Open Farmland, Marshes
1) National Organic Program, USDA, Organic Standards, Regulatory Text www.ams.usda.gov/nop/NOP/standards/FullRegTextOnly.html
2) An Overview of Cholecalciferol Toxicosis, The American Board of Veterinary Toxicology (ABVT), at: www.abvt.org/tow.htm
3) Colvin, B.A. Barn owls: Their secrets and habits. Illinois Audubon, No. 216 Spring. 1986.
5) Birds and Nature. Bird House Tables. Web site at: http://web.mountain.net/~shalaway/nests.html
6) Rodenticides. Source: Journal of Veterinary Medicine, archives, vol. 27, May, 1998. posted on IPM of Alaska, Solving Pest Problems Sensibly. Rocco Moschetti, P.O.Box 875006, Wasilla, Alaska 99687-5006 (907)745-SAFE (745-7233) email: email@example.com website: www.ipmofalaska.com/files/rodenticides.html
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