[Rodent Control] Frequently Asked Questions About Rodenticide Baits

Editor’s note: The following are frequently asked questions by clients of pest management professionals regarding on-the-job use of rodent baits. They cover a wide range of subjects, including rodenticide resistance, baiting frequency, secondary poisoning and rodent behavior following consumption of a rodenticide bait.

Q: ow important is it for rodent baits in service vehicles or on the storeroom shelf to be protected from chemical odors (e.g., insecticidal, cleaning agents, etc.)?

A: In some cases, it may be crucial. In infestations where rodents may have readily available food sources, they have the option of rejecting new-appearing foods that have chemical tastes. Therefore, it makes sense to maximize the palatability of grain-based baits. Rats can taste chemicals in their food in parts per billion. Thus, it is smart to prevent rodenticides from becoming contaminated with chemical odors, as well as from nicotine residues from the fingers of technicians who smoke.

Q: What does it mean to see droppings the color of the bait in the station if the bait is supposed to be killing the rodents?

A: Colored droppings indicate a rodent has ingested and digested the bait and its active ingredient. It takes several days for most baits to kill a rodent. Mice produce upwards of 50 or more droppings every day, rats produce 30 to 40. Thus, some droppings will be passed through the body containing the green, blue or red dye of the rodent bait. The rodents that produced the colored droppings are usually dead by the time the colored droppings are noted inside the bait station or they are likely to be near death.

Q: If a cat or dog consumes two or three mice that died as a result of any of the popular rodenticides being currently used (second-generation anticoagulants or non-anticoagulants), what is the likelihood of the pet dying of secondary poisoning?

A: Almost nil. The dead mice already have assimilated the poisoned baits and, from a practical perspective, there are no secondary poisoning characteristics associated with the anticoagulants. Secondary ingestions of undigested baits that still remain within the mouse’s stomach can occur, but there is not nearly enough bait in these few pellets to kill or even render a cat or dog sick.

Q: Will mice or rats dying from the effects of the rodenticide bait leave the building to die?

A: Poisoned rodents may die anywhere, including in some very inconvenient areas. There is no evidence that rodents exit buildings “seeking water” and die outside, although this would be very convenient for all parties. Most times, poisoned rodents succumb in their nests. It is also common to find rodents dead or near death in bait boxes.

Q: Using the pellet formulation as an example, how many pellets of the second-generation anticoagulants does it take to kill a mouse and a rat?

A: Usually only a few pellets will kill a mouse. In some cases, only a pellet or two may be effective. For rats, usually six to 30 pellets, depending on the brand, are required. Both rats and mice will continue to feed for several days until they succumb to the effects of the poisons.

Q: What is the typical length of time required for a second-generation anticoagulant to kill a rodent?

A: Several factors may affect the time to death, but it varies from rats and mice and from the particular active ingredient used by the pest management professional. Rodents subjected to those compounds with the lowest LD50 values tend to die sooner than those with higher values. But it also depends on specific colony factors. In general, anticoagulant baits can produce death as early as three days, but in some cases it may take up to 18 days. For practical purposes, it is best to be conservative and inform clients that it could take seven to 10 days before the first rodents succumb and cease their activity in the account. Severe infestations may require three to four weeks to achieve a high level of control.

Q: If the rodents are eating pet food or some other food high in vitamin K1 can these foods offset the effects of the anticoagulant baits?

A: Not likely, at least not on a practical scale. Regular ingestions of foods high in vitamin K1 may be able to delay the time of death to a rodent that has ingested an anticoagulant, but only to a slight degree. There are not enough dosages of vitamin K1 in processed foods to have a practical impact on the average baiting program.

Q: How often should the rodent baits in bait stations be replaced if they still appear fresh?

A: Most recommendations suggest changing rodent baits on a 4 to 6 week schedule on a need-it-or-not basis. When maintenance baiting or in low-risk situations (i.e., not protecting a food plant or some other sensitive account), if the bait has remained undisturbed by dirt, dust or moisture, change-out periods may be extended to slightly longer periods as determined on a case-by-case basis. Very moldy or decayed baits are unattractive to rodents. In most cases, it is more efficient to overprotect the area with the freshest baits.

Rodenticide resistance is often used as an excuse for obtaining poor results. However, several factors other than true genetic resistance are responsible for baiting failures. The following discussion was adapted and modified from Marsh and Howard (1977). Both anticoagulants and non-anticoagulants are represented below and offer an on-the-job guideline for diagnosing baiting failures.
 
When Baits Are Well Accepted, But Control Is Poor:

• Baiting time period was too short; baits should be available for three weeks.
• Insufficient amounts of bait are administered and none remains from one baiting to the next.
• Too few baiting points are used or some are not spaced closely enough together. In some situations, baiting points may have to be spaced 15 to 25 feet apart for rats and as close together as 8 feet for mice.
• The control program does not cover a large enough area, permitting rodents to move in from untreated, adjacent areas.
• Possible resistance, especially if the same amount of bait is taken daily over several weeks and no appreciable reduction in the population is noticed and above factors do not seem to be responsible.

When Rodenticide Bait is Poorly Accepted:

• The bait or the bait formulation is not attractive to the rodents.
• The bait container is not placed in the rodents’ high-activity areas or is in an area of too much human activity for the rodent.
• Other attractive foods are abundant and easily available to the rodents, and they have no need to switch to a new food. (This is a common occurrence with rats around zoos, livestock operations and granaries, and with mice in grocery stores.)
• The bait has become rancid, moldy, insect-ridden, or contaminated with insecticide or other odors (e.g., cleaning agents, nicotine, etc.), which reduces acceptance. Control is best with fresh baits. Exterior baits should be changed every four to six weeks on a need-it-or-not basis.
• The quality of the anticoagulant is poor and rodents can detect poison in the bait.

The author is president, RMC Consulting, Richmond, Ind.

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