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

For the past 10 years or so, I have been inspecting a large grain-processing plant located along the Mississippi River in the South. This facility has a history of chronic roof rat (Rattus rattus) infestations despite a fairly thorough rat management program. New rats dispersing from the river banks and surrounding grain mills regularly move onto the property and replace some of the rats killed by baits and traps installed at the plant.

Several years ago, a pest manager at this site pointed out something interesting to me. Roof rats were repeatedly nesting within several of the overhead electrical junction boxes that were missing the cover plates through which the wires enter the box. Presumably, the junction boxes are attractive to the roof rats because they are warm from the electrical wires, are compact, are easily accessible via overhead conduit lines, and are located up and off the floor in protected ceiling areas. As any experienced pest management professional will be quick to point out, finding live, healthy rodents resting, sleeping and sometimes nesting inside bait boxes (which are about the same size as these junction boxes) is nothing new. So at first, the roof rats in the junction boxes didn’t really catch my attention as being anything unique.

However, over the past several years since that day, I have repeatedly encountered roof rats — sometimes entire families — nesting not only in electrical junction boxes at other industrial sites I audit, but also within empty and/or overlooked bait boxes. This behavior really began to capture my attention when other pest specialists repeatedly shared similar observations about roof rats establishing nests in bait boxes in urban and industrial facilities, as well as within a couple zoological parks in the United States.

OK, but what significance is this? Well, perhaps for some situations, we can use the rat’s opportunistic nesting behavior to our advantage by installing empty bait stations in specific areas and using the boxes as nest-box live traps. Before discussing why, how and where we can do this, let’s first examine the nesting behavior of Rattus rattus and why in some circumstances this rat uses our bait boxes as substitute nest sites.

ROOF RAT NESTING BEHAVIOR. For rodents and other small mammals, a good nest is essential for survival. The nest provides the rodent with 1) protection from the elements; 2) a place to rest and groom; 3) a chamber to raise and protect young; 4) a safe haven from predators; and 5) a place to store collected food items (sometimes).

The roof rat typically utilizes both natural nests and man-made structures as nests in above-ground locations. In developed urban areas, vegetation plays an important role in roof rat nest sites. Nests are often constructed within dense clinging vines on the sides of buildings and fences, as well within the crowns of palm trees — especially in trees in which the old fronds have not been removed for some time. In some states, the roof rat also builds globular leafy nests much in the same way tree squirrels do. Inside buildings, roof rats locate their nests in attic areas, ceiling voids, and in structural voids or nooks located in roofline areas.

BAIT BOXES AS SUBSTITUTE NESTS. Based on the above discussion, it would appear an empty cavity, such as a junction or bait box is, in many ways, structurally and/or spatially, similar as the nest sites the roof rat uses in both the natural and urban environments. In some ways, an undisturbed bait box may even offer roof rats a superior nest site than what might be available in their natural settings.

For example, rats and mice are often opportunistic when searching for good nest sites and seek a quick "hiding hole" to escape predators. A box containing holes that only allow a rodent-sized animal entry, but deny entry to large birds of prey, cats, dogs and other predators, provides an attractive harborage for the rodent. Too, the openings of the bait box can be defended easily from the inside. One of the two holes also can be used as an escape hole should it be needed. The size and shape of a standard rat size exterior bait box also offers a confined space that provides good tactile feedback to a rodent or rodent family. And finally, commercial bait boxes provide excellent protection from the cold, wind and rain and are compact enough to allow for good heat containment — all critical nesting criteria for small rodents that are subject to rapid loss of body heat.

A disturbance factor is also likely to be involved in how rodents select and establish nest sites. As most experienced field professionals have witnessed, rodents do not establish nests in items or locations that are frequently disturbed. When roof rats are sometimes inadvertently flushed out of bushes and vines around buildings, it is common for them to disperse and not return. This is why mouse and rat nests commonly discovered inside warehouses are in those supplies and boxes that are "rejects" or are in a "hold status" and left for several months.

In residences, rodents commonly move into stationary boxes and items that have not been disturbed for some time. Similarly, it is those bait boxes that are installed in quiet zones and then overlooked or forgotten that become subject to rodent nests, and rarely those that are serviced frequently, or located in areas of high human activity.

CONCLUSION. Finally, it is worth noting that all three species of commensal rodents will, depending on the situation, use bait stations for nests. But in general, exterior bait stations are more subject to occupancy by roof rats than the Norway rat. This is because the roof rat occurs in the more tropical or semi-tropical areas of the world, and thus, exterior stations are not subject to the dangerous cold temperatures as is the case for many locations where the Norway rat occurs. Furthermore, the Norway rat is primarily a ground-nesting species and utilizes the insulation properties of earthen burrows instead of making use of the aerial vegetative and structural "cavities" as used by the roof rat. The house mouse, on the other hand, will readily make use of empty bait boxes or any other undisturbed box indoors (as evidenced by their entry into multiple catch curiosity traps).

In part II of this topic, next month, we will examine how bait boxes can be employed as "specialty nest box traps" for roof rats to supplement conventional roof rat management programs for some sensitive clients such as food plants, warehouses, zoological gardens or areas where there may be elusive roof rats present.

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