[Vertebrate Pests] Exterior Norway Rat Control, Part I

The Norway rat and the common house mouse originated on the grassy, wide-open steppes of central Asia. The primary harborage for small mammals in this environment was to dig out burrows into the earth. Mammals here forage about for wind blown seeds, insects, birds and a variety of other natural foods they encounter during nightly forays.

With the Norway rat being translocated into hamlets, towns and cities, it readily adapted by constructing ground burrows along the exteriors of buildings and other developed areas where food is readily available. Instead of (or in addition to) natural foods, a new rat colony will thrive on the food discards of humans. Soon, a burrow or two can become a dozen or more and this infestation will require control. But because such rat infestations are outside where wildlife, pets, birds and even people might be affected by rodent control efforts, safety must be carefully matched to efficacy.

In most cases, three control approaches are used for exterior burrow rat infestations: 1) establishing exterior bait stations nearby the burrows; 2) treating burrows directly with bait; or 3) treating burrows with tracking powders. In some cases, there are some specialized situations where burrows may be fumigated with approved, labeled materials.

Which of these is most effective? Research hasn’t addressed this question yet. Too, when conducting pest control in exterior areas, safety must take priority over efficacy.

This month, I will discuss bait stations and direct burrow baiting efforts (placing baits directly down the burrows). Next month, we’ll cover the use of tracking powders, answer some questions about the applicability of burrow fumigants and discuss the importance of proper monitoring of exterior rat infestations to ensure long-term control.

ESTABLISHING BAIT STATIONS. Compared to directly baiting the burrows or using tracking powders in burrows, establishing exterior tamper-resistant bait stations nearby active rodent burrows has the following advantages and disadvantages:

• Bait consumption can be monitored within the stations and thus the progress of a control program is readily visible.

• Bait dosages can be adjusted according to the level of the infestation.

• With bait containers, the professional has better control over the applied pesticide.

• Clients can see a tangible item is in place for controlling their rats. For some clients, this visual "sense of security" is an important component of their sense of value.

• The disadvantage of bait stations occurs where there is an abundance of food and/or sanitation is difficult. In these situations, some rats have established feeding patterns and are reluctant to enter bait boxes that suddenly appear in their territory.

• When burrows occur around building peripheries of high public visibility (e.g., restaurants, etc.), stations may be too visible and/or readily accessible to inquisitive persons, pets or wildlife. In most cases, the use of the fake rock tamper-resistant bait stations can be employed for these situations.

DIRECT BURROW TREATMENTS. Installing baits directly into rat burrows is a common approach for treating exterior rat infestations. This can be done easily, quickly and not entail supply costs beyond the bait itself. Direct burrow baiting also has some advantages over installing bait stations. But, there are important safety considerations for burrow baiting efforts that should be considered to avoid harming non-target animals, people or contaminating the environment.

• The particular rodenticide formulation selected for burrow baiting is important. The loose pellet formulation delivered directly down into a rat burrow is perhaps the safest. This is because rodents occasionally push back the larger bait formulations (packet and block baits) out of the burrows. When this occurs, non-target animals will have access to any rejected rodenticide.

• Bulk-style pellets can be inserted into rat burrows using a long-handled soup ladle or salad-style serving spoon or via a hose-and-funnel technique.

• Spoons and ladles can be modified by taping a 3- to 4-foot piece of wood latching to the spoon handle.

• For the hose-and-funnel technique, a plastic funnel can be attached to a 2- to 3-foot section of garden hose (at least 3/4 inch). This technique will allow for a quick pour and ensure good penetration of the bait into the burrow. The hose-and-funnel technique also helps to save on the physical exertion of bending over when many burrows are treated.

• To provide adequate protection against non-target animals gaining access to bait, all baits should be placed down into the burrow so it is not visible from the entrance. Depending on the construction of the burrow, this distance can vary from 12 to 36 inches.

• The amount of bait applied is determined by each product’s label directions. For severe infestations, the maximal amount of bait allowed is recommended. Similarly, for minor infestations, only the minimal dosage is needed. In most 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.

• Immediately after a burrow is baited, it should not be caved in with dirt or paper. Correctly baited burrows should remain open and undisturbed for seven to 10 days. Although this seems contrary to the older techniques of caving in the burrows or stuffing them full of paper immediately, there are several reasons for this. First, it usually takes about three to seven days for anticoagulant poison baits to kill or immobilize a rat. Thus, what is the use of caving in the burrow immediately after baiting? Until the time they become too sick to travel, poisoned rats will continue to exit and return to their burrow system for several nights. 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.