[Vertebrate Pests] Roof Rats Nesting In Bait Stations, Part II

In part one of this column in last month’s issue, we examined the nesting behavior of the roof rat, and the fact that it is not uncommon to find this rat taking advantage of empty and/or forgotten bait boxes to build nests and even rear families.

In some situations, we can put this behavior to use by converting some bait stations into rat live traps. In fact, some exterior bait stations can perform much in the same way as the multiple catch mousetraps that capture mice as they enter during their forays and explorations for harborage sites.

CONVERTING STATIONS INTO TRAPS. Converting an ordinary exterior bait station to a nest box live trap for roof rats is quite easy. Only one item needs to be added to the bait station. After that, the station is used virtually in the same manner as is normally done when the stations are used for baiting or trapping purposes.

1. To make the station into a live trap, all that is needed is a cap or plug to capture and contain any rodents that have moved into the station inside the box. To do this, a simple PVC plumbing drain cap of a similar or slightly larger diameter than that of the station’s entry holes is needed. Or, several circular “plugs” made of heavy-duty cardboard, thick Styrofoam® or masonite board can be made to be placed over the entry holes of the stations. The caps can be held in place with duct tape, or from simple pressure if the cap is just slightly larger than the bait box’s entry holes. For large manufacturing plants, the on-site machine shop can improvise plastic or rubber “plugs” that can be quickly inserted into the bait station’s entry holes (similar to the concept of our plugs used to seal termite treatment holes of slabs).

2. The best chances of having a bait box being used as nest box is via good placement of the box into the high activity zones of the rats — areas that are also quiet, dark and out of the way of daily disturbance, or that haven’t been subject to ongoing trapping programs with either snap traps or glueboards.

3. Attempt to locate “rub marks” and droppings in suspected rat activity areas and install the boxes near the rat signs.

4. During the summer, install some boxes in shady, cool locations out of the heat. During winter months, install the boxes in undisturbed areas that are exposed to the sun or near some source of heat (e.g., on shelves in a utility room).

5. One or two inexpensive new washcloths or odor-free cloth rags, or any natural leafy materials, installed into the box may serve as an attractive nesting material.

6. If rat droppings can be located nearby, putting several of the rat droppings around the exterior of the station nearby the entry holes and a few directly inside the station’s entryway may also help encourage roof rats in the area to move into the station.

7. Certain pieces of food (fruits, nuts, slugs, snails) placed around and in the trap may help entice the rats to use the stations.

8. The boxes should be checked during the day on a weekly or bi-weekly basis by installing the cover plugs into or over each entry hole. Gently lift the box and feel the bottom of the bait box for body heat or body movement generated by any rats within the box. The lid of the box also can be opened just slightly to confirm the presence of rats.

9. Trapped rats can be disposed of via the use of carbon dioxide treatments or by some other acceptable means. Or, for bio-centric clients, the roof rats can be released a few miles away from the site in a wooded area where they will not pose a problem for someone else or for any crop production.

10. Successful boxes should be placed back into the same location.

WHERE TO USE NEST BOXES. Nest box live traps may provide an excellent alternative control tool for those clients where poison baits may not be permitted, where baits are not well accepted, or for infestations where the rats exhibit a neophobic or conditioned reaction to snap traps. Such sites might include zoological parks, food manufacturing plants, warehouses and prisons.

Additionally, nest trap boxes can be effective in eliminating an entire family (e.g., eight to 12 rats) of rats, including the more wary breeding adults. In some cases, it can take weeks of snap trapping and baiting to kill the same number of young and experienced breeding rats.

MORE RESEARCH NEEDED. Similar to the results observed with bat houses, bird houses and multiple catch curiosity mouse traps, the factors that affect whether or not rats will use an empty box for a nest are likely to vary and are unknown at this time. Hits or misses may be entirely unpredictable and subject to mere opportunistic behaviors from a foraging rat or rats.

On the other hand, there may be factors that may encourage rats to use or ignore empty boxes within their home ranges and territories. Perhaps the size, shape and structural composition of the box are important (plastic, metal, wood). Other important aspects may include the specific location of the box in relation to environmental conditions (shaded areas in summer; sun-exposed areas in winter; out-of-wind currents) undisturbed areas; proximity to food sources; population density, harborage competition, and the use and marking of the boxes by other (and previous) members of the colony.

Finally, it is not known if one commercial model of bait box is more likely to be investigated by rats for potential harborages than others. So far, the nests I have found and have been told about have all been within the more popular selling brands of tamper-resistant plastic bait stations that are typically used for exterior perimeter baiting programs.

IT’S WORTH A TRY. Does this technique sound interesting to you? Actually, the concept is not new. Scientists have speculated and written about the use of nest boxes for different animals in the past. Moreover, consider how we employ bat houses for bats and bird houses for birds. Similarly, mice, roof rats, chipmunks, voles and shrews all regularly enter various types of “empty boxes” (e.g. junk piles, abandoned cars, multiple catch curiosity traps, etc.) they encounter during exploratory travels and feeding forays.

We have birdhouses, mouse houses and bat houses. Maybe it’s time we looked closer at the possibility of “rat houses” for those unique situations when other tools are not appropriate or ineffective. It’s just the type of lease granted to this specific tenant is a bit different. Try the roof rat nest-box live trap sometime. I would enjoy hearing of your successes, failures or modifications to the concept. 

The author is president of RMC Pest Management Consulting and can be reached at rcorrigan@pctonline.com or 765/939-2829.