[Vertebrate Pests] Exterior Norway rat infestations, part II

Last month we discussed the profiles of exterior rat burrows, how to find them quickly and how to analyze the different levels of infestations. The correct treatment of exterior rat infestations is obviously important. First, safety to people and all non-target wildlife is paramount. Rat burrows occur in urban areas where inquisitive children play in parks, yards and lots; dogs are walked; and urban wildlife (such as tree squirrels and birds) usually occur. Second, the goal is to efficiently eliminate the pest infestation and monitor the program for long-term success.

In the majority of cases, burrow infestations are treated either via the use of bait stations nearby the burrows, or the burrows are baited directly via baits installed down into the burrow. Sometimes, pest professionals also use tracking powders applied directly into burrows. In fact, one of the most frequently asked questions is, "Which is the more effective way to treat rat burrows: baits or tracking powders?

Here in Part II of this column, we review these three treatments, considering both effectiveness and safety.

EXTERIOR BAIT STATIONS. The use of tamper-resistant bait stations containing bait blocks secured on rods inside the stations positioned close to the burrows is the most environmentally preferred method of applying rodenticide baits in exterior areas. The stations protect most (but not all) non-target animals from accessing the baits (dogs, raccoons, skunks, birds, etc.), as well as most people.

However, often exterior rat infestations occur because food has been readily available to the rats. And, if stations are installed without an emphasis on cleanup, bait stations may be ignored or poorly visited, regardless of the quality of the bait installed into them. Still, if the rat’s food sources can be eliminated (and a heavy emphasis should be placed on this), acceptance and activity into the stations are generally good.

It’s best if exterior bait stations are installed nearby active burrow holes or along the rat’s runways that lead to the rat’s food sources. Note: Most rodenticide bait labels state that applications can be made "in or beside burrows." For exterior areas, "besides burrows" implies that bait stations will be used. (Otherwise, such bait would be available to dogs, birds, squirrels, and other non targets.)

To increase the chances of stations being visited, the stations should be installed in discreet locations (i.e., not visible to the public) by using the cover of low-lying bushes, sides of walls, junk piles, etc. In some locales, out-of-sight placements reduce bait station theft.

To sometimes hasten rat visitations to brand new stations, a couple of rat feces collected from the local site can be added inside the entryways of the stations. Once the rats begin leaving their own fecal pellets in the stations, keep the baits fresh, but leave the fecal pellets in the stations (although this cannot be done at more sensitive sites, such as food-handling establishments, food warehouses, etc.).

DIRECT BURROW BAITING. In general, inserting baits directly down into burrows can be done relatively quickly and inexpensively. In fact, because burrow baiting is seemingly so simple and quick, applicators can be misled into hazardous applications if attention is not paid to details. In other words, there is more to professionally baiting rat burrows than stuffing a bait packet down a rat hole and then caving in the burrow.

Technically, each of the rodenticide formulations (pellets, packets, blocks, meals, seeds) can provide good control of exterior infestations. But certain bait formulations are better suited for burrow baiting efforts and safety to non-target animals than others. In general, rodents have more of a tendency to translocate a bait (i.e., carry away and move the bait) back out of their burrow if the bait is large in size and easily movable.

For example, packet-style baits and "wax" blocks offer the best resistance to weather and moisture. Unfortunately, these same two formulations are easily moved, carried or rejected out of burrows. Thus, these formulations do not maximize environmental safety.

The bulk loose pellets (i.e., not in packets) offer both small size and good weatherability. Some professionals crumble the bait blocks into smaller sizes, and then pour these into burrows, which is apt to achieve the same goal as using the pellets. And some professionals simply open up the packets and pour the pellets down into the burrow. If you’re going to do that then why pay for the expense of each package and the handling time associated with all the packets?

The meal formulation offers low translocation potential, but it doesn’t provide the resistance to moisture as well as pellets. Besides, loose pellets ,when inserted correctly into a burrow, are likely to be consumed by rats long before they would be subject to moisture deterioration.

APPLYING PELLET BAITS. The keys to successful burrow baiting are to deliver the baits in a manner which mimics Mother Nature as well as minimizing disturbance to the rat’s burrow environment. This is because rats are sensitive to changes in their environments. In their burrow systems, rats may react towards trauma to their environment by avoiding or expelling (if it is easily moved) a new item that appear suddenly, or appear as part of burrow collapses.

In fields and open areas, the wind blows seeds, berries, nuts and other edible items across the ground. Obviously any holes and burrows in the ground act as a natural pit-trap of such items (as well as for insects and other animals rats can prey upon). Thus, bait pellets carefully inserted directly down into a rat’s burrow system are rarely ejected, or translocated out of burrows compared to packets or whole bait blocks.

Burrows can be baited with bulk-style pellets using manufacturers’ application equipment, or a long handled ladle, or via the use of an ordinary garden hose-and-plastic funnel. To do this, a plastic funnel can be attached to a 2- to 3-foot section of garden hose (¾ inch or more diameter to allow for good pellet flow). This technique facilitates a fast pour and also ensures good penetration of the bait down into the burrow. The hose and funnel technique also helps to save on the physical exertion of constantly bending over when many burrows need to be treated.

Obviously, the amount of bait applied into the burrow is via label directions. For severe infestations, use the maximum amount of bait recommended, and the lower dosages for minor infestations. In common cases, 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 amounts to the correct amount of bait per bait point.

To provide adequate protection against non-target animals gaining access to bait, all baits should be placed deep enough into the burrow so the bait is not visible from the entrance. Depending on the construction of the burrow, this means the bait will be inserted down into the burrow about 1 to 3 feet.

After the bait is inserted into the burrow, the burrows should NOT be closed with dirt or paper. This is arguably the most important procedure for safely baiting rat burrows. Correctly baited burrows should remain open and undisturbed for seven to 14 days. There are several reasons for this. First, it usually takes about five to seven days (but may take even a few days longer) for anticoagulant baits to kill or immobilize a rat. Thus, what is the use 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. Obviously, the rats will re-open any caved-in burrows until they succumb to the effects of the bait.

Second, and most importantly from a hazard concern, is that with a rat’s re-opening of their "disturbed" burrow, any object that is new (i.e., your bait) and unfamiliar to the rat that arrived with the cave-in is often pushed back out on top of the ground along with the dirt needing to be excavated. Those experienced in rat control, and whom have inspected burrows the morning after "baiting and closing," learned this lesson early on.

Treated burrow areas should be flagged with standard pesticide treatment flags. One flag can be used for a general area (i.e., a flag is not needed for each burrow system).

About one week after the initial baiting, previously baited burrows can be caved in with dirt, or if dirt is not easily available, with paper wads. Then re-inspect the closed up burrows a day or two later. Any re-opened (i.e., still active) burrows can be re-baited. Assuming there are fewer rats per burrow, about half as much bait as was used on the initial baiting can be used per burrow for the follow-up baiting.

The process can be repeated until control of the rat colony is complete. Once control has been achieved, all flags can be removed from the area. Just as it is important to leave the burrows open during the baiting program, it is equally important to stress that all burrows following successful control should now be filled in as best as possible with dirt. There are three good reasons for this: (1) community-level rat infestations can be monitored for any new or surviving infestations; (2) old rat burrows are attractive harborage sites for any new arriving rats from periphery areas; and (3) vacated and available rodent burrows are attractive nesting sites for yellow jackets.

TRACKING POWDERS. One of the most common questions about exterior rat burrows treatments is, "Isn’t it faster and more effective to simply dust the burrows with rodenticide tracking powders? After all, even the finickiest rodents that may reject burrow-injected baits can’t avoid the powder."

While there are many, many case histories of tracking powders being used to control tough burrow rat infestations, there are some important considerations associated with their use. To begin with, tracking powder labels are highly restricted on their use in exterior burrows. Any tracking powder label approved for exterior burrows states only the following types of burrows can be treated: "only rat burrows that are located along the periphery of buildings and that are likely to serve as routes of entry into these structures."

To many this label direction is puzzling. Only those burrows that are serving as routes of entry INTO structures? Doesn’t this mean the rodent can carry the powder on their bodies into areas of buildings where humans (especially children), dogs and cats could inadvertently contact the powder?

Similarly, but in the opposite direction when the rats travel outside the structural area, the powder will go wherever the rodent forages. People can step onto powder residues and transporting the powder on the soles of their shoes, dogs being walked can pick up the powder, the feet of squirrels and birds, and any other animals using these areas.

In other words, the applicator has lost control of the pesticide’s fate (on a Restricted Use Pesticide, no less). It will be the rats who ultimately control the toxic powder’s destiny. In this era of litigation, an ultra conservative approach by a pest control company regarding the use of tracking powders seems prudent. Especially considering with some of the more common anticoagulant tracking powders (e.g., diphacinone), the active ingredient is mixed at a 0.2% concentration. This concentration is 40 times stronger than the diphacinone mixed into the edible bait formulation (0.005%).

One possible situation where tracking powders might be used in exterior areas according to label directions would be an abandoned lot next to an unoccupied building where perhaps food trash is a constant battle and the rats are not responding to baits, while also not traveling to any type of sensitive areas.

Besides the safety issue, there are other disadvantages associated with the use of tracking powders in exterior burrows. First, frequent applications must be made to the burrows if there are more than just a couple of rats because powders can be quickly picked up by only a few rats. Also, powder formulations do not do well in moist environments. So, if the ground is moist, or if there is rain, snow, drainage, etc., good adhesion to the target animal is not achieved.

The professional treatments of exterior rat infestations, similar to all rodent pest management, lies in the sound approach of IPM (eliminating the food and cover that are attracting the rats to the area in the first place) and then matching the situation to the selection of any pesticidal tools. But the professional use of these tools requires attention to all the safety and environmental factors beyond being able to just kill the rats. Just killing rats is exterminating. Eliminating rats while also minimizing threats to all non targets and the environment is professional rodent management.

The author is president of RMC Consulting, Richmond, Ind. and can be reached via e-mail at rcorrigan@giemedia.com.