[Vertebrate Pests] Landscaping and Vertebrate Pest Issues

In previous columns, I’ve stressed how certain items and practices outside commercial buildings can create pest problems for those buildings and areas. For example, improperly managed food refuse trash bins (i.e., dumpsters) can serve as "magnets," drawing pests to building peripheries. I stressed how selecting the right model of a dumpster, its position on the grounds, when and how the dumpster is emptied and so forth all are important in not attracting pests.

Similarly, the choices we make about our shrubs and bushes, their positions and maintenance will, like dumpsters, affect whether or not we inadvertently attract and harbor pests around our commercial (and other) structures. Yet, landscaping, as it relates to creating or exacerbating pest problems, is usually overlooked by laypersons, facilities maintenance staff and even landscaping companies themselves. Let’s take a closer look.

COMMERCIAL ESTABLISHMENTS. From a pest-pressure perspective, landscaping choices and practices are more important for certain buildings and areas than others. Consider the importance of minimizing pest pressures around facilities that produce, serve or store food, such as restaurants, grocery stores, food plants and the like.

Or, consider buildings in which the health of its occupants must be protected, such as schools, day-care centers, hospitals, hotels, office buildings, multi-family housing complexes and zoological parks. This is because landscaping choices and practices play an important role in the potential impact of pest pressures on buildings and areas — and the chances of vertebrate pests eventually dispersing into buildings.

Although pest levels of zero on the exterior of these types of commercial buildings and areas are not easily attained (or even possible), whenever possible we should strive to minimize both the pest pressure on a specific building and the occurrences that pests may disperse the short distances from vegetation into a nearby building. This point is at the heart of urban IPM — that is, being proactive and not reactive in managing pests around urban environments.

IT’S ABOUT HARBORAGE. Nearly all animals need harborage from the elements and their enemies, as well as a place to rest and rear offspring in a protected space. Humans can build structures of all types to accomplish this. Wild animals utilize hollow trees, ground burrows, small and large caves, nooks and crannies within rocky areas, and within vegetation that grows dense enough to substitute for a structural harborage.

In many cases, wild animals are opportunistic in locating harborages. Thus, once humans "develop" an area that was previously natural, animals are not going to readily distinguish between hollow trees and natural thickets and a hollow wall, crawlspace, attic or the thickets of shrubs used for landscaping.

The common wildlife species that adapt to urban structures and plantings include Norway rats, roof rats, house mice, deer mice, white-footed mice, voles, raccoons, feral cats, skunks, woodchucks, chipmunks, snakes, pigeons, house sparrows and house finches. Thus, our choices of bushes and shrubs for our buildings and parks can directly affect whether or not a particular animal becomes established around a building or moves on to other areas seeking better or more suitable harborage.

SHRUBS AS PEST CAVES. Some shrubs used in urban landscaping provide better harborages to animal pests than others and thus are more attractive to these animals. For example, evergreen shrubs that typically form dense globular or cavernous shapes close to the ground are good mammal and bird harborages.

Common shrubs that fit this description include yew bushes (Taxus spp.), creeping junipers (Juniperus horizontalis) creeping arborvitae (Thuja spp.) and various types of creeping ground ivy.

During pest inspections, I refer to these shrubs as "pest caves" because they are so accommodating to animals for hiding within or burrowing beneath. Additionally, these plantings form tight pockets at their bases — trapping wind-blown leaves from nearby trees. These leaves, together with fallen leaves ("needles") from the shrub itself, form soft and warm nesting environments at the shrub’s dark hidden base. Perfect!

From both a sanitation and a landscape-maintenance aspect, cavernous-shaped shrubs are more difficult to maintain because it is difficult to get rakes below the lowest branches, as well as quickly prune and trim the thick shrubs and bushes. Consequently, wind-blown human trash, such as food wrappers (often containing food scraps), are repeatedly trapped beneath these plantings, providing pests with food basically delivered to their front door.

Finally, when animals construct burrows below these cavernous shrubs and then eventually evacuate the burrows due to death or dispersal, yet another important opportunist commonly occupies the empty burrows, namely yellowjackets. And because these shrubs are often planted close to buildings and doors, we can again see how we sometimes create our own pest problems by not paying attention to details.

RECOMMENDATIONS. In contrast to cavernous shaped shrubs, shrub varieties that are vase-shaped (or pear-shaped) and open at the base are more IPM-friendly shrubs. Because they are open at the base, vase-shaped plants cannot provide pests with protected harborage. They do not trap trash, they can be easily cleaned around and pest professionals can easily inspect them. A variety of vase-shaped shrubs can be selected and purchased from professional urban landscapers.

Unfortunately, in most cases, pest professionals servicing commercial facilities note that "pest-cave plants" are usually already in place and have been for some time. It’s not realistic for a client to rip out all their bushes and re-landscape based on your recommendations. But if a client is experiencing chronic pest pressure or is a sensitive environment client where keeping pest thresholds as close to zero as possible (schools, food plants, pharmaceutical plants, etc.) is necessary, then this may in fact be necessary.

For upright yews, arborvitaes, spruces and so forth, the under-story areas creating the pest caves can be pruned and "trained" to form the vase and not cave shapes. A good height is about 18 inches/46 cm off of the ground.

For chronic rat infestations around city office buildings and parks containing creeping ground covers, there is little choice but to remove and replace this landscaping with open, upright shrubs. Where ground cover material is necessary, the client can install turf ornamental rock coverings or combinations of rocks, turf and well-spaced shrubs.

For complicated and elaborate projects, urban landscape architects should be consulted (as compared to those that merely sell landscaping plants and maintenance services).

TREE SELECTION AND PLACEMENT. Trees and their maintenance also play an important role relative to vertebrate pests around buildings.

The following are but a few tips to help minimize pest problems when trees are being selected for commercial structures:

• Tree varieties that form dense foliage (e.g., standing cultivars of arborvitae) should be avoided for reasons similar to those provided previously for shrubs. Starlings, sparrows and other bird pests gravitate towards dense tree vegetation for year-round nesting.

• For established trees nearby a building, all branches should be pruned back from buildings, preferably by about 6 feet/2m. This helps to prevent roof rats, mice, tree squirrels, raccoons and other animals from gaining access to the roof zones, from which they can enter the building or establish dens inside attics or enclosed soffits.

• New trees should be planted far enough away from building exteriors to avoid creating future "pest ladders" to soffits, parapets and rooftops. A general rule is for a tree to be planted at a distance slightly greater than the expected radius of the mature tree.

It is surprising to me how many shrubs, bushes and trees, are, in fact, quite conducive to harboring and/or channeling pests to our building peripheries and roof zones. Over the next few weeks, with your trained eye, take a look around. Look at the landscaping on your commercial routes or even in your own neighborhood. See if you agree.

Ideally, commercial clients getting ready to landscape or re-landscape their properties would inquire of pest professionals about the importance of shrub selection and maintenance as it relates to pest issues. Unfortunately, this rarely (ever?) happens. But you can help them. Take the information here or from some other source and make a tip sheet with your company’s letterhead. Or better still, make it the "topic of the month" for your company’s Web site.

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