Crop and Pest Management Guidelines

5.5 Rodent Control in Buildings and Other Structures

5.5.1 Pest Species

For an effective control program, you must know what species of mammals are involved. Correct identification is essential for selecting the proper damage management measures. Roof rats are rarely found in New York State and then only in a few locations (usually port cities). Norway rats, voles, and house mice are common throughout the state. Shrews may occupy structures but usually cause few problems.

The Norway rat and house mouse are abundant throughout the United States. They cause enormous economic losses by consuming and contaminating food as well as damaging property.

5.5.2 Feeding Behavior

Most rats and some mice are omnivorous, eating nearly any food, although each kind and population has its own preferences. The Norway rat is an avid consumer of human garbage, whereas the roof rat prefers more natural plant food, if available. They all feed to varying degrees on cereals, nuts, fruits, vegetables, invertebrates, fish, and even manure, depending on the habitat conditions.

Rats usually begin searching for food shortly after sunset. When hungry, or under crowded conditions, they may also be seen in daylight. Rats and mice may cache considerable amounts of solid food, which they may or may not eat later.

5.5.3 Determining the Presence of Rodents

The habits of rodents provide numerous signs of their presence, including species, relative numbers and areas of activity. Species can best be identified after rats and mice have been trapped and closely examined. Evidence of rodents can be determined by one or a combination of factors:

• Sounds: Gnawing on solid objects; clawing and climbing in walls, above dropped ceilings, and under cabinets; various squeaks; fighting noises.
• Droppings: May be found along runways, near shelters, or other places rodents frequent.
• Urine: Wet and dry rodent urine stains will fluoresce under ultraviolet light.
• Smudge marks: May be found on pipes, beams against walls and outside edges of holes where dirt and oil from their fur is deposited where rodents frequently travel.
• Runs: Smooth or worn areas may be found next to walls, along fences, under bushes and buildings.
• Tracks: Footprints or tail marks may be found in dusty surfaces, sand, soft soil, and snow. The use of nontoxic tracking dust such as chalk powder or unscented talc will help determine the presence of rodents inside buildings.
• Gnawing: May be indicated by wood chips around baseboards, doors, basement windows and frames, stored materials, around pipes in floors and walls, and wherever rats might try to enlarge a crack or enter a structure.
• Burrows: Most commonly made by Norway rats in soft ground, particularly where burrow entrances can be concealed in low, dense vegetation or under concrete slabs, foundation walls, lumber, and piles of rubbish. Norway rats also burrow into soil floors in basements, warehouses, and animal quarters.
• Visual sighting: Frequent observation of rats in daylight generally indicates a high population in the area. Mice may be active during the day or night.
• Nests and food caches: Can sometimes be found when cleaning garages, attics, basements, and other storage places. Rodent species (i.e., squirrels and rats) often store food in attics of buildings.
• Odors: Odors (resulting from deposits of fermenting rodent urine and feces and body oils) in a room may provide a clue to their presence.
• Pet excitement: May occur when cats or dogs hear or smell rodents in some inaccessible spaces (e.g., in a wall); pets may sniff or scratch excitedly at the spot where rodents are detected.

5.5.4 Long-Term Population Suppression

Rodent populations have a remarkable capacity for growth and survival, but it is often people who provide the conditions under which that potential can be realized. Pest problems are a function of the design of industrial/ agricultural production systems, ornamental landscapes and architecture, as well as human behaviors and beliefs that create, motivate, and maintain those designs. One must examine all circumstances that initially gave rise to a particular rodent problem and then make basic changes in the environment and human behaviors that resulted in food, water, and shelter for the pests. These measures include improved sanitation, housekeeping, and storage practices; altered landscape designs; and rodent proofing of buildings. When the habitat is modified to reduce the basic factors needed for rodent survival, the control program has a much greater chance of lasting success.

5.5.5 Rodenticides for Structural Use

The use of toxicants has often been the primary treatment or intervention in many rodent control programs. Rodenticide
applications have not proven ideal in all circumstances. They are contradicted in some situations (e.g., where children and pets are at risk or in food processing areas) and should be considered as only one of numerous potential interventions in an IPM program.

It is generally much safer and more effective to use chronic (multiple-dose) toxicants than acute (single-dose) poisons for rodents. There are times, however, when the quick-acting acute poisons are required, such as when the disease hazard is high or when a very large population must be reduced in a short period of time.

Acute (single-feeding) toxicants (Table 5.5.1) are very useful for achieving a quick knockdown of a rodent population. The same acute toxicant should not be used more than twice a year, preferably only once. When an acute toxicant is used repeatedly in the same locality, bait shyness may develop. Hence it is best to periodically change the kinds of bait and active ingredient used.

Probably 95 percent of all rodent-control operations in the United States use multiple-dose, anticoagulant baits (Table 5.5.1). The effects of anticoagulants are cumulative, and most baits must be consumed several times during a two-week period. That is why they are considered relatively safe for humans and other nontarget animals. Rodents rarely develop shyness to anticoagulant baits of good quality. Anticoagulants act by interfering with blood clotting. The onset of symptoms is relatively slow, which allows time for administering vitamin K, an effective antidote, or giving whole-blood transfusions to non-target organisms in the case of accidental poisoning.

The past good safety record of anticoagulants is no reason for complacency. Recent reports indicate that more care should be exercised in their use because some of the products (brodifacoum, diphacinone, and bromadiolone) are acutely toxic to some rodents and nontarget species. Brodifacoum and diphacinone are particularly toxic to dogs and have relatively long biological half-lives. These are important considerations from the standpoint of accidental primary or secondary poisoning of companion animals or wildlife.

For greatest effect, rodents should feed on the treated bait each day. The intervals between feedings should never exceed two days. Depending on the anticoagulant and the amount consumed, the first deaths in rats should occur

about the fourth day post-exposure. Keep fresh bait exposed continuously for at least two weeks or until all feeding ceases. With some anticoagulants, shorter periods of exposure are recommended, so follow label instructions carefully.

Prebaiting. Prebaiting is the placement of nontoxic baits to create a high acceptance before the rodenticide is exposed and can enhance later acceptance of rodenticides. Prebaiting improves the effectiveness of acute rodenticides but is not needed for less toxic anticoagulant rodenticides. The most acceptable bait is frequently a local food the rats and mice have already become accustomed to eating. Evidence of food preferences can be determined by carefully examining the infested area for damaged food items or scraps left behind. These baits should contain all the same ingredients (dyes and other additives) as the toxic bait, aside from the toxicant. After two to three nights of prebaiting, the rodents will overcome natural aversion to the new food items and will have learned to feed at these locations. The prebait should be handled with the same thoroughness and care anticipated for the actual poisoning. In using the more toxic chronic baits, prebaiting should make it possible to reduce exposure time and thereby reduce the probability of accidental intoxication of nontarget animals.

Bait Type and Placement. It is best to place the bait where it will be in the rodents' normal line of travel. Baits should be placed under cover so the animals will feel secure when they feed. Put out more bait than you think necessary, unless prebaiting for three to five days has already indicated how much is needed.

Toss-type packets containing about 4 ounces (100 grams) of anticoagulant baits are commercially available from several sources. Rodents will readily gnaw into these bags to get at an acceptable bait. These bait packets have the advantage of easy placement beneath pallets, etc., where bait boxes are unsuitable because of their size.

Bait Boxes or other enclosed bait stations should be large enough to accommodate several target rodents at one time. Each station should have at least two appropriate-sized openings. Bait boxes usually contain a feed hopper for holding the bait. Such boxes should be placed next to walls or in places where rodents will intercept them. Some bait

boxes may be large enough that both water and dry baits can be placed inside. Bait boxes are available commercially in a variety of sizes and shapes. They may be constructed of water-resistant cardboard, wood, plastic, or metal. Locks, seals, and concealed latches are often used to make the boxes tamper-proof. All bait boxes and other permanent bait stations should be clearly labeled "Poison-Rodent Bait" or other appropriate wording.

When first put out, bait boxes should be checked daily. After a while it will be necessary to check the stations only once every other week to replenish or replace old bait with fresh. They must be visited often enough so that ample fresh anticoagulant bait is available.

Paraffin Bait Blocks for rat and mouse control are available through pest control suppliers. Some firms offer bait blocks in several flavors, or containing different bait ingredients, that are advertised as more appealing to certain populations of rodents. Again, these bait blocks reportedly give good control, particularly where food is limited. Bait blocks have been found particularly valuable in control of roof rats where the blocks can be fastened to telephone poles, fences, or the rafters of buildings. Dogs will occasionally chew on a block as if on a bone, which creates a hazard. Therefore, bait blocks should be placed out of the reach of dogs and other curious domestic animals. Because

loose cereal or pellet baits are invariably accepted more readily than are paraffin blocks by Norway and roof rats, blocks should be used only in situations where other baits are unsuitable.

Traps can be an effective method of controlling rodents but requires somewhat more skill and labor than baiting. Traps alone are sometimes not completely effective for removal purposes, but can be very useful in conjunction with habitat modification. Trapping is recommended where use of poisons seems inadvisable and is the preferred method in homes, garages, and other structures where there are few rodents, and nontarget species risks may be great. Trapping has several advantages: (1) It does not rely on inherently hazardous rodenticides, (2) it permits the user to view his or her success, and (3) it eliminates rodent deaths in inaccessible locations, which frequently creates odor problems when poisoning is done within structures.

5.4 Repellents for Deer and Rabbits

Tips for Laundering Pesticide-Contaminated Clothing