Rodent Control: Handbook Of Pest Control Book Excerpt

Today, more than ever, pest management professionals should use rodenticides effectively so as to reduce costs, prevent unnecessary exposure by nontarget animals, and minimize pesticide load on the environment. Each group of rodenticides has a recommended general use pattern. Professionals should consider how each rodenticide works and then select the most appropriate compound based upon the control situation.

Modern rodenticides can be divided into two broad groups: anticoagulants and non-anticoagulants. Anticoagulant rodenticides comprise about 90% or more of the baits used by pest management professionals. These compounds cause death as a result of internal bleeding which occurs as the animals’ blood loses its clotting ability and its capillaries are destroyed. The active ingredients are used at low levels of between 25 ppm to 50 ppm.

All anticoagulant rodenticides are relatively slow-acting; death occurs anywhere from three days to 10 days following the ingestion of a lethal amount of bait. Bait shyness does not occur with anticoagulant rodenticides.

ANTICOAGULANT RODENTICIDES. Because of the addition of more advanced anticoagulants within the last 15 years, anticoagulants are now classified as first- and second-generation anticoagulants. First-generation anticoagulants are represented by the active ingredients warfarin, pival, chlorophacinone, diphacinone, and others. These are referred to as "multiple-dose" anticoagulants because they usually cause death only after they are consumed for several days in succession by the rodent. Thus, first-generation baits must be available continuously until the rodent ingests enough doses for death to occur.

The second-generation anticoagulants were invented as a result of resistance developing to warfarin and other first-generation compounds. It was also desirable to produce a more efficient anticoagulant. The active ingredients brodifacoum, bromadiolone, and difethialone are the second-generation compounds used in the United States. Other effective anticoagulants are used elsewhere. Second-generation anticoagulants are also referred to as "single-dose" anticoagulants because they can cause death following only one feeding.

Bait acceptance of all anticoagulants is exceptionally good. Despite the single dose effect of the second-generation compounds, rodents continue to feed on them for three to seven (or more) days following a lethal ingestion. Nevertheless, the second-generation anticoagulants are especially useful in environments where rodents have plenty of food alternatives. Mice, for example, may not visit a toxic bait placement more than once or twice before returning to feed on a regularly available food. In these situations, it is important to poison them as quickly as possible.

NON-ANTICOAGULANT RODENTICIDES. The three most common non-anticoagulant baits used in the structural pest management industry are zinc phosphide, bromethalin, and cholecalciferol.

Zinc phosphide, a blackish powder with a garlic-like odor, has been used for many years. It is said to be attractive to rats and mice, but generally unattractive to people and pets. Against the commensal rodents, zinc phosphide is used to a much lesser extent, due to the increased effectiveness of the anticoagulant and new non-anticoagulant baits. However, it is registered as a restricted-use pesticide for control of rats, mice and other rodents outdoors in crop and noncrop areas, and for vole control in various outdoor situations.

Bromethalin is the only "stop-feed" rodenticide. Once animals obtain a lethal dose, they do not feed again. Thus, compared to the first-generation anticoagulants, only relatively small amounts of this bait need to be available. It is effective against anticoagulant-resistant rodents. This rodenticide chemical causes a decrease in cellular energy production. Death usually occurs within three days.

Cholecalciferol, a form of vitamin D, works by disrupting the use of calcium in the body. The chemical balance in body fluids is changed, the calcium withdrawn from bones is dumped into the circulatory system, and kidney function is disrupted. Usually the animal feeds for several days before appetite disruption occurs. Cholecalciferol is effective against anticoagulant-resistant rodents.

BAITING STRATEGIES. There are three basic keys to effective baiting of rodent populations: baiting with acute rodenticides where baiting is limited to a short period of intensive baiting; baiting with first-generation anticoagulants where continuous maintenance will be provided for several weeks; and pulsed baiting or small dose placements with second-generation anticoagulants or other single-dose rodenticides where bait placements need not be continuous and can be made at intervals.

Because each rodenticide product has a recommended use pattern, professionals need to consider in which situations the various compounds are best used. The single-dose rodenticide zinc phosphide, for example, should be used only at intervals (such as six months) to prevent the development of wide-scale bait shyness.

Each effort should be a maximum one. With these types of baits, it is best to use as many bait placements as possible during several days of baiting and then stop. The professional may decide to shift to alternate rodenticides, if necessary, to kill remaining animals.

With first-generation anticoagulants, an adequate bait supply must be maintained during the entire control period because the rodent must be able to feed daily for a week or more. With some of the first-generation compounds, as many as 10 successive feedings are required. Missing a day cancels the effect of previous feeding. Maintaining such amounts of bait may increase the chances that nontarget species will consume it.

The second-generation anticoagulants have much greater activity with much smaller doses. Thus, relatively small amounts (usually acquired with a single feeding or within one day of feeding) of bait are required to be effective. Unfortunately, it still takes several days for death to occur, and in the meantime, rodents continue feeding.

One method called "pulsed baiting" was proposed and studied by researchers in the early 1980s. The researcher A. Dubock believed that only a single feeding is required to produce a lethal result and can be used as a basis for an efficient baiting regimen in those cases where the professional expects to conduct follow-up visits on a weekly basis. (These should be done with an initial cleanout program against rodent infestations.) Bait initially placed to satisfy the estimated number of rodents should be increased if rodent signs or activity persists. However, bait initially placed will result in death to those rodents that ate it. Similarly, this result will be true of the second (and third) baiting. Thus, the effect of each baiting is maximized.

Not all rodents will come in contact with the bait in the initial placement. Especially with mice, populations may exist in ceilings, attics or other relatively inaccessible sites. Not until the dominant individuals are removed can these other animals expand their ranges and find the bait. Thus, several waves of feeding may occur.

Several different types of rodenticide bait stations are used in the industry. Some are more effective and/or appropriate for use in one type of facility than in another or within certain areas of a facility (e.g. indoor baiting vs. outdoor baiting).

Placing baits in containers offers the advantages of keeping the bait fresh and protected from dirt and moisture, and also reduces the chances of accidental poisoning of people, livestock, pets and nontarget wildlife. It also allows the monitoring of bait consumption and activity in particular areas.

When bait stations are used for elimination and maintenance programs, they should be spaced at 8- to 12-foot intervals for mice, and 25- to 50-foot intervals for rats in areas showing rodent activity. Use closer spacing for severe rodent infestations.

SAFETY CONSIDERATIONS. When used according to label directions and following all safety precautions, rodenticides present little hazard to nontarget animals and the environment. Nevertheless, it is important for all users of rodenticides to keep in mind that these pesticides have the potential to seriously harm people, dogs, cats, birds, livestock, wildlife and zoo animals. Therefore, every effort should be made to analyze situations where companion animals or wildlife may be at risk, and to take extra precautions to ensure against accidental poisonings or secondary hazards. Some anticoagulants are more toxic to dogs, cats, birds and livestock than others, and the professional needs to be aware of the differences.

GUIDELINES FOR SAFE BAITING PROGRAMS

In addition to the safety precautions appearing on rodenticide labels, the following points must be emphasized.

1) Minimize bait translocation by using bait blocks secured on rods within secured bait stations.

2) Always keep baits out of the reach of people, livestock, dogs, cats and wildlife.

3) Have any dogs or cats which have access to the affected area or facility confined during baiting operations.

4) Handle rodent carcasses with rubber gloves, long tongs, or newspaper, and bury or incinerate all dead rodents.

5) Label all outdoor bait containers clearly with appropriate warnings and keep unused baits in their original containers.

6) Store baits in a locked cabinet and post appropriate warnings on the outside of cabinet doors.

7) Remove and properly dispose of all uneaten bait at the end of the baiting program.

Editor’s Note: This is the sixth in a series of excerpts from the recently published 8th Edition of The Mallis Handbook of Pest Control.