Rodents incur costs associated with purchase and placement of rodenticides, damage to structural components including insulation, deterioration of equipment including fiber egg-conveyor belts and electrical insulation which is gnawed. They serve as reservoirs of bacterial infections such as salmonellosis and pasteurellosis and they may function as mechanical vectors of viruses which can be entrained in dust.
House mice are susceptible to Salmonella enteritidis (SE) and survivors remain permanent carriers of the infection. Since a single mouse voids 35,000 droppings in its lifespan of 15 months, with each dropping containing up to one million Salmonella, there is a significant potential for contamination of flocks.
An infestation of 10,000 roof rats in a one-million hen in-line operation will consume feed to the value of $20,000 a year. In the case of organic operations, the cost will be proportionately greater based on a three-fold differential in the price of feed and limitations on the use of baits for control.
Quantifying rodent populations
Since rodents are nocturnal, large populations of house mice and roof rats can exist without management being aware of the extent of infestation. The presence of mice and rats can be determined by physical observation. Bag stores and feed mills will show the presence of droppings and damage due to gnawing bags will result in spillage of grains.
Burrows associated with Norway rats may be found adjacent to houses and within dried manure cones in high-rise houses.
Damage to the bottom of wooden doors from gnawing indicates points of entry and activity. Equipment failure due to damaged insulation and water leaks from gnawed piping confirms the presence of rodents and suggests a relatively high population. Nocturnal inspection of attics, aisles, the pits of high rise houses, especially at the feed and egg collection end, and around dumpsters should indicate the extent of the problem. It is advisable to examine egg belts in the morning before activation to detect the presence of droppings.
Quantitative measures may include the use of video cameras, trapping using multiple capture mouse traps (Tin Cat or Ketch-All) or snap traps arranged with the triggers adjacent to a wall. Tracking powder can be used to detect mice and rats when placed along walls or on beams which reveal movement over a 24-hour period. Tail and foot prints are usually characteristic and can differentiate between mice and rats. Smudge-marks are left by mice and rub- marks by rats on rafters, walls and other structures along frequently traveled routes.
It is generally accepted that control of rodents requires exclusion exemplified by a comment from a producer who claims to successfully "build 'em out." Structural defects such as gaps beneath doors larger than a quarter inch will allow entry of mice. Overhead or underground pipes or feed augers passing through walls, and poorly fitting roll-up doors allow entry of rodents. Roof rats may gain access to a house through defective flashing or electrical cables which pass through roof components, eaves or walls. Norway rats burrow beneath foundations and can enter unprotected earth-floored houses with ease.
It is essential that houses should be surrounded with a concrete or crushed stone perimeter barrier three-feet wide to prevent burrowing. An area free of vegetation other than mowed grass at least 20-feet wide should surround houses. Disused equipment, surplus building material including roof and wall sheeting and stacked timber should be removed from the vicinity of feed mills, packing plants and poultry housing for a distance of a least 300 feet.
Rodenticides must be placed in approved bait stations around the exterior of feed mills, poultry houses and packing plants and along the side walls and aisles of cage houses. Only curiosity traps and sticky traps are allowed within processing plants or units operated according to organic rules.
Rodenticides may be formulated as pellets, small granules in packets, wax blocks or in liquid form. Only approved containers should be used to prevent accidental contamination of the environment and poisoning of non-target species. Rodents will not consume spoiled or fungus-infested baits which require frequent inspection, cleaning of containers and replenishment.
Generally mice are effectively baited using wax and extruded blocks. Pellets, granules and liquid baits are available and multiple types should be used depending on the extent and range of rodent infestation.
Generally bait stations compete with available feed sources in mills and the feeders in houses. Effective control requires placement of baits in areas of high rodent activity and in sufficient numbers to encourage consumption. For severe infestation with mice and Norway rats in high-rise houses, baits must be placed along walkways within the pit area at a distance of approximately 25 feet for rats and 12 feet for mice. It is frequently advisable to concentrate bait containers in areas where rodent activity occurs such as adjacent to doors, along walls and beneath egg conveyors.
Liquid bait may be placed in one-gallon fonts in attics to kill roof rats. Bait containers are required along walls, aisles and in pits and packets of granules can be positioned on beams.
Wax blocks can be placed on nails along walkways beneath the collecting belts on the lowest tier. Rotational baiting can be followed; this involves moving bait stations around the perimeter of houses in 8-foot increments at three day intervals.
Rodent suppression should be intensified at the time of flock depletion and continue during the interflock interval. With no feed in troughs, rodents will avidly access both granular and liquid bait. Unfortunately, in high-rise houses migration occurs after flock depletion which creates the potential for dissemination of SE from infected flocks to susceptible hens in adjacent houses.
Generally, egg-production companies employ a professional exterminator to place and service bait stations. Although this approach may be more expensive than in-company control, bonded professionals maintain records, assume legal responsibility and follow procedures which may be required in terms of qualification as a supplier or to comply with state regulations.
If an in-company program is followed, the procedures must be committed to writing. Clear responsibility for placing and servicing bait stations and monitoring rodent populations must be assigned and the cost of a program should be monitored against predetermined budgetary values and industry benchmarks.
Classifications of rodenticides
Approved rodenticides, see Table 2, are classified according to first-generation anticoagulants, second-generation anticoagulants and non-anticoagulants. It is possible that some rodent populations have developed resistance against first-generation anticoagulants since they have been widely used since the early 1950s. The first-generation anticoagulants require multiple feedings over several days to produce death, requiring constant access to bait.
The slow action of first-generation anticoagulants precludes the development of "bait shyness."
Second-generation anticoagulants may produce death after a single feeding although a cumulative effect occurs with consumption of small quantities over successive days. Brodifacoum is the most potent of the anticoagulants and will produce death within four days of a single feeding.
Each of the three non-anticoagulant rodenticides has a specific action. Bromethalin affects the central nervous system and is a single-dose compound which is not associated with bait shyness. Cholecalciferol (Vitamin D3), mobilizes calcium from the skeleton and results in death from hypercalcemia three to four days after ingestion of a lethal dose. Zinc phosphide releases phosphine in the stomach after ingestion, resulting in death. Bait shyness has been associated with this compound.
Occasionally placement of anticoagulant baits is only partially effective. Although resistance is frequently implicated, it is relatively rare and is usually associated with Warfarin the original first-generation anticoagulant. Most failures result from insufficient bait stations placed too far apart with infrequent replenishment of bait or lack of rotation of rodenticides.
Control and suppression of rodents requires a coordinated approach involving exclusion, structural modifications and housekeeping. Selection and placement of baits according to accepted and approved practices should be carried out by a licensed professional exterminator or a designated trained employee in strict accordance with statutory label instructions. Rodent control requires planning, implementation and control both of procedures and costs.
Effective rodent control is a necessary component through the entire chain of egg production.