Direct Burrow Baiting For Rats

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December 1, 1999

Directly baiting the ground burrows of Norway rats can occasionally provide advantages over installing tamper-resistant bait stations nearby active rat burrows. For example, when food is plentiful nearby their burrows, Norway rats may avoid entering bait stations for weeks or months. In fact, some rats may never enter well-placed bait stations. Too, bait stations often cannot be installed in urban areas due to their accessibility to the general public.

Finally, direct burrow baiting can be done relatively easily, quickly and not entail any bait station costs. Paradoxically however, it is this relative ease that also lends direct burrow baiting programs to cause some problems. Consider that Norway rat burrows in city and rural areas are often directly baited by someone stuffing a packet of bait — or literally throwing bait blocks down a rat hole, caving in the burrow — and then assuming the rats have been poisoned. Unfortunately, such casual baiting methods can easily contribute towards hazardous rodenticide applications. Let’s briefly examine how to bait Norway rat burrows to achieve quick bait acceptance, while at the same time minimizing secondary hazard threats to non-targets animals or people.

BAIT FORMULATION. Technically, each of the rodenticide formulations (pellets, packets, blocks, meals, seeds) can provide good rat control. But certain bait formulations are better suited for burrow baiting efforts and non-target safety than others are. In general, rodents have more of a tendency to translocate (i.e., move a rodenticide) a bait out of their burrow, if the bait is large in size and is easily movable. Obviously, packet style baits and whole bait blocks can be easily moved, carried or pushed about by rats. Bulk loose pellets on the other hand (i.e., not in packets) offer both small size and good weatherability. Some professionals crumble their bait blocks into smaller sizes and then pour these into burrows, which is apt to achieve the same goal as using the pellets. Considering the dampness of the ground associated with burrows, some believe that the packet-style baits and "waxy" blocks offer the best resistance to weather and moisture. But loose pellets when inserted correctly into a burrow are likely to be consumed by rats long before they would be subject to deterioration from the ground moisture. (Although, generally, burrows should not be baited during rainy days, or when significant rain is expected.)

APPLYING THE BAIT. The key to successful burrow baiting is to deliver the bait with as little disturbance to the rat’s home as possible. This is because Norway rats are sensitive to changes in their environments. Often, they react towards changes by avoiding anything that is new or items that are associated with the disturbance in their daily routine. As such, perhaps the most effective way to bait a Norway rat’s burrow is to mimic Mother Nature. It is quite natural in fields and open areas for the wind to blow seeds, small fruits or grass leaves across the ground. As you might imagine, such items are apt to become entrapped within the tunnels of burrow animals. This offers a partial explanation as to why carefully placed pellets inserted directly into the rat’s burrow system are rarely translocated out of the burrow compared to packets or complete block formulations.

Baiting the burrow with pellets can be done using a long-handled large spoons or via the hose-and-funnel technique. Robert Sechriest, Commercial Pest Control, Millersburg, Ohio, once showed me that he uses a plastic funnel attached to a 2- to 3-foot section of garden hose (3/4 inch or more in diameter to allow for good pellet flow) and thus can quickly pour and ensure good penetration of the bait into the burrow. This technique also helps to save on the physical exertion of constantly bending over when many burrows need to be treated. To provide adequate protection against non-target animals gaining access to bait, the baits should be placed about 2 feet deep into the burrow.

Regardless of the above discussion, it should be noted that bait expulsions don’t always occur even with packet or whole block baits. In some cases, bait rejection
and/or expulsion depends upon how much of the rats’ previously available food remains abundant. Should the rats’ normal food supply disappear (e.g., via sanitation programs), the suddenly hungry rats have no choice but to consume whatever baits have been delivered within the burrow (or upon finding new baits near the burrow entrance in a new bait station). However, this is an assumption professionals cannot afford to make and it makes sense to always apply pesticides, erring on the side of safety.

How much bait to install per burrow depends on the bait and label directions and the severity of the infestations. But with most of the second-generation anticoagulants, each active burrow hole should receive about 4 to 5 ounces of bait. Assuming there may be two to three burrow entrances per rat family, this will amount to the correct amount of bait per bait point. Additionally, separate bait installations among the same burrow system can increase the chances of each rat existing in a family unit to have an opportunity to feed on the bait.

BURROW CLOSURE. Baited burrows should remain open and undisturbed for at least one, and preferably, two weeks. There are several reasons for this. First, it usually takes about five to seven days for a second-generation anticoagulant to kill or immobilize a poisoned rat. Thus, what is the value of caving in the burrow immediately after baiting? Active rats will continue to come and go from the burrow system for several nights conducting their daily forays for water, food or other behavioral routines, consequently re-opening any caved-in burrows until they succumb to the more serious effects of the bait. Second, and perhaps most important from a hazard concern, is that with the re-opening of the "disturbed" burrow, any object that is new (i.e., your bait) and unfamiliar to the rat that arrived with the cave-in may be pushed back out on top of the ground along with the dirt they are excavating.

A week or two after the initial baiting, all burrow openings can be closed using paper wads or caving in the burrows with dirt. Then, one or two days after the initial burrow closures, re-inspect all burrows and re-bait any active burrows. Assuming there are fewer rats per burrow, about one half as much bait as was used on the initial can be used per burrow for the follow-up baiting. Seven to 14 days later, repeat the process until control is complete.

As it is important to leave the burrows open during the baiting program, it is equally important that all burrows, following successful control, should be filled in with dirt. In this way, the rat infestation can be monitored for any new or surviving infestations. Additionally, vacant rat burrows are attractive harborage sites for yellow jackets. So, we can play the "pizza-man," and deliver fresh, attractive bait directly to the rats’ front door, while at the same time provide good protection of non-targets animals. But burrow baiting must be done correctly. With pesticide applications, we have a responsibility towards the environment and non-target animals. With direct burrow baiting, the professionalism lies in the details.

Corrigan is president of RMC Pest Management Consulting, 765/939-2829.