Have you ever had a relative (or someone else you just didn’t care that much for) whom you’d rather didn’t stay at your house? Maybe you try to make things so that person doesn’t want to come over — you’re nice, but the person feels uninvited. Don’t you think that’s the way people might think about pests in their house?
Ants have by far have become the most unwanted of all household pests. Imported species, such as white-footed and Caribbean crazy ants, have become well established in Florida and are starting to show up in other states. Other species, including the odorous house ant and some species of rover ants (Brachymyrmex spp.), always have been around but in the past half dozen years have become significant structural pests.
Why ants have elevated themselves into the position of “queen” of the pests is up for debate. The path taken by the odorous house ant in the Midwest and Northeast — from a native but nuisance occasional invader to “the most difficult” pest faced by many pest professionals — is especially intriguing. I have a theory as to why ants have become greater pests, but it can’t be tested or confirmed. Addressing this theory, however, is the central theme to this article.
When I first became a homeowner, I learned that I enjoy yard work and landscaping. As I became more immersed in the past 20 years in types of plants, shrubs and trees, I began to notice people’s landscaping both in my own neighborhood and the homes I’ve visited as a pest professional. As I thought about these things, I noticed that well-established neighborhoods tend to have greater pest problems. Established homes have more lush landscaping — new homes might have a couple of tiny trees and a few small shrubs. Over time, new homeowners spruce up their yards with more and more plants and landscaping.
During the past 20 years, I’ve noticed more and more nurseries and landscaping companies coming about. Big box stores, like Lowe’s, Home Depot and Wal-Mart, have large gardening departments. HGTV is now a staple on cable TV. Landscaping one’s property consumes more time, money and interest than it ever did 20 to 25 years ago. So, when you ask, “Why have ants become such serious pests?” I contend that we invited them into our yards and then into our homes and businesses. As cities and towns grew outward into suburbs, we created ant suburbia around our homes.
Here’s a perfect example: About six or seven years ago in Memphis, I spent a day with a service professional. We visited a home for a regular quarterly service and the customer reported seeing ants in the kitchen. Being Memphis, these were odorous house ants — the Mid-South’s No. 1 pest. To be brief, we went outside to look for the source and found more than 10 sub-nests in the mulch next to the home. Following the service, I pointed out to the customer that the ants were nesting in the mulch. She told me something surprising — the shrubs and plants had only been installed four days before. Prior to that, the ground was bare. It took but a day or two for ant colonies in the surrounding yard to exploit this new “development.” Not surprisingly, it was only a couple days after the landscaping was improved that the ants were seen inside.
By planting shrubs and flowers, installing the mulch and watering the new plants daily, the woman unwittingly invited ants next to her foundation. From there, foraging scout ants gained their way inside and discovered the crumbs under the stove and refrigerator. It is clear that landscaping provides more than everything ants need to thrive: moisture, shelter and food (in that order).
Any pest professional can tell you that Argentine ants in Southern California are found in great numbers around buildings. Ask an entomologist who has looked for Argentine ants out in the dryer ecosystems away from buildings and citrus groves, and he’ll tell you they can’t be found. It is the moisture that allows ants to thrive around structures. All they need is a place to nest (mulch, thick ground covers, flagstones, tree holes, etc.) and a source of food, and they are “good to go.” It is only natural that, while foraging, they discover the inside of a home or office can be just as inviting.
So what do we do? Tell customers the landscaping has to go? Not exactly, but we should take every opportunity to educate homeowners, landscapers, nursery owners and builders how their choices and actions in beautifying a structure contributes to long-term issues with pests, especially ants.
In order to make an area completely uninviting to ants, it would have to be pretty barren. Still, even with a concrete parking lot, ants (such as the Argentine), can be found foraging more than 100 feet at night across the paved expanse and entering a business. The persistent nature of ants means it is impossible to create a situation where ants cannot be present and invade a building. The goal, therefore, is to limit the possible resources for ant survival, thereby vastly reducing the potential for ant infestations.
The most important zone to consider is that closest to a structure. Allowing ant colonies to locate and thrive next to the foundation is the best bet to lead to inside sightings. Thus, the landscaping decisions closest to the structure have the most importance.
GROUND COVERS. Thick mats of greenery are highly desired by most people — just look the efforts we put into our lawns. This desire continues into landscape beds. English ivy, pachysandra, liriope (monkey) grass, mondo grass, archangel, ajuga and creeping junipers are just a few types of plants that grow thick and low to the ground, filling large expanses of landscape beds. Ground covers are desired because they choke out weeds, require little maintenance beyond watering and fertilizing and eliminate the need to mulch those areas.
By contrast, ground covers serve as the Ritz Carlton of pest havens. Thick, ground-hugging vegetation traps moisture, which is the key component of a pest microenvironment. Underneath all the leaves, vines and stems of ground covers, organic matter accumulates. Earthworms, sow bugs, millipedes and other creatures fond of such debris thrive. Ants find the conditions in such soil easy to excavate and to regulate temperature. Polydomic (multi-nest) species also will locate subcolonies within the leaves of the vegetation, especially where ground covers sit against a surface, such as a tree trunk or foundation. Additionally, all of the insects, spiders and other creatures within this ecological niche provide a ready food supply for ants to exploit.
An additional concern with ivy as ground cover is that it tends to grow onto and up the side of homes. Although aesthetically pleasing, walls covered in ivy hold moisture against the brick or siding, a condition that ultimately leads to loosening bricks and the rot of wood siding. A myriad of insects and spiders can be found within the protective confines of ivy on walls. Ants readily forage within the ivy to find homopterous insects to tend for honeydew and to prey upon insects. Certain ant species (odorous house, crazy, Argentine) also will nest within the ivy. Once ivy is in place on walls, convincing customers to remove it is usually darn near impossible despite the fact that the ivy is actually causing damage to the walls. Such homes, however, always will likely have some pest problems despite the best pest control efforts.
Ground covers simply should not be used in landscape beds near foundations — they can be critical contributors to infestations of many types of pests, not only ants. Where possible, customers should be advised to remove ground covers and change to plants that are more sparsely located in the beds. This may be especially necessary for homes chronically plagued by ants. If customers just can’t seem to part with ground covers, perhaps they can be convinced to use them in beds at the perimeter of the property.
MULCH. Mulch is a necessary “evil” in landscaping. The reason for its necessity is the reason mulch is a benefit to ants — it holds moisture. The recommended thickness for mulch is 2 inches around the base of plants. Some people (and landscapers) take mulch to extremes, piling it up 4, 5, 6 inches or more around trees, shrubs and along foundations. There is no shortage of moisture here, so it is no wonder one might find numerous ant colonies within the mulch of a single landscape bed.
One also should consider the type of mulch. Loose mulches, such as whole or chunked bark mulch, is the most desired by ants. The innumerable spaces created between the pieces serve as ready nest galleries. It is this type of mulch that was used in the example involving odorous house ants described earlier in this article.
Customers should be urged to remove mulch if thick layers exist and be advised to use little or no mulch within 12 inches of foundations. Anything thicker than 2 inches is undesirable.
GRAVEL BORDERS. Commercial GMPs call for facilities to maintain an 18- to 24-inch border free of vegetation next the building foundation. Often, you also might see a home or business with landscape beds filled with gravel instead of mulch. Gravel borders are better than ground covers and mulch, but they are not immune from use by ants. The type of gravel and how it is installed can play a role in its effectiveness as a deterrent to pests.
In my experience, I’ve found more ants nesting within thick layers of large stone gravel. Thin layers of small stones, like pea gravel, have far fewer pest issues. In one case involving an office building plagued by pavement ants, the border consisted of larger granite (8 to 10 inches) stones buried in a sand base. Needless to say, the pavement ants had little trouble excavating beneath the stones.
Having the gravel placed upon a plastic barrier is helpful but does not, in every case, prevent ants from nesting underneath, especially in shaded areas next to the foundation or at the base of shrubs in the bed. With or without a barrier, a thinner layer of gravel, about 2 inches, is recommended.
SHRUBS. Hedges, azaleas, holly shrubs, yews and other shrubs make up the bulk of landscape biomass near buildings. Based on my experience, most landscapers and homeowners do not take mature size into account when selecting shrubs and planning where to plant them. Shrubs are often planted too close together and too close to the walls of the building where they can grow into dense walls of branches and leaves. The branches grow into the walls and subcolonies of ants have been found on walls beneath thick mats of leaves creating shelter against the surface.
Additionally, the canopy created by thick masses of shrubs creates a microenvironment favorable to ants. Moisture is contained underneath and temperature is more stable during the day due to the shade provided. Thick shrubbery also allows fallen leaves to accumulate underneath, behind and within the branches themselves where they are difficult to reach and remove. Leaf litter is a key nest site for most of the pest ant species that cause the greatest problems. Regular leaf litter removal is also critical for long-term relief from ants so any situation that interferes with easy removal inhibits a successful overall ant management plan.
Varieties of shrubs and other plants vary in their susceptibility to homopterous insects. Aphids, mealy bugs and scales are all used by ants for honeydew and shrubs harboring large numbers of these insects often contribute to large populations of pest ants on a property.
Shrubs can be expensive, so advising customers to change out aphid-prone plants for others is often not an immediate option. In such cases, having them employ a tree/shrub company to treat the plants to control homopterous insects is highly beneficial for ant management.
Advise customers that if they ever decide to relandscape that they should (1) ask their landscaper or nursery to supply shrubs that are less prone to aphids and similar insects, (2) take into account the mature size of the plants when selecting them, and (3) space plants far enough way from each other and away from the structure to avoid them growing into each other or onto the building itself. Concerning the latter, it is okay to plant shrubs to form “fences” at the borders of property although if such shrubbery barriers are located next to wood fences, removing leaf litter can become problematic and contribute to ant infestations. (Obviously, better at the perimeter of the yard than next to the home.)
SUMMARY. Fewer resources for ants to exploit in terms of moisture, shelter and food close to buildings will lead to fewer ants. Many of the items discussed here are beyond the control of the pest professional responding to a customer’s call to control ants. Changing out landscaping can be expensive and is not a short-term option. However, educating customers on the things on their property that serve as invitations to ants can ultimately lead to changes later, especially for those clients who have such problems with ants they will try almost anything to get a handle on them.
In the short term, convincing a customer to change out ground covers is not too costly and can be helpful. Also, customers should be reminded that leaf litter is a key contributing factor in pest infestations and that getting rid of leaves not just in the yard, but especially in landscape beds and under decks, is beneficial. Addressing aphid, mealy bug and scale issues by hiring a tree/shrub company is also a big positive in those cases where large ant populations are present.
The author is a board certified entomologist and professional sanitarian. He is director, technical services, Terminix International, Memphis, Tenn., and can be reached at shedges@giemedia.com.
*****
My Own Case History
When I first moved into my home in Tennessee, I found that the previous owner had landscaped the front of the house with two large privets, “trained” into tree-shaped shrubs, crepe myrtles and box hedges. The beds also had thick rows of liriope monkey grass. The privets, hedges and crepe myrtles annually had issues with mealy bugs, scales and white flies, and large populations of odorous house ants (OHA) developed during summer around the house. After several years of battling homopterous insects and the resulting OHA invasions, I finally revamped the landscaping in the entire yard.
Today, the beds have low-spreading azaleas, camellias and holly shrubs with a couple of Japanese maples to add visual height to the landscape. The liriope grass is gone and replaced with either nothing or bulb flowers. It is easy to remove what little leaf litter accumulates and to keep the branches off the walls of the home. Beginning the first year after renovation, I noticed a distinct reduction in the numbers of OHA around my house. Nine years later, it is still difficult to find OHA nests on the property except at the perimeter of the yard. Uninviting ants through intelligent landscaping does work in minimizing issues with problem ants. — Stoy Hedges
*****
Act Like a Tree and Leave
Like shrubs, landscapers and homeowners rarely seem to take into account the mature size of a tree when planting it in the yard. Having to cut back branches 6 feet away from the roof and walls of the home or building every other year is a time-consuming pain. Also, trees overhanging homes result in more leaves on roofs and clogging gutters.
Some trees, like shrubs, are more aphid-prone than others. Fruit-bearing trees are commonly used in home landscaping and are often attacked by aphids, mealy bugs and scales. When pest professionals become involved in dealing with ants, tree replacement is generally not an option so a tree/shrub company should be consulted to treat trees infested by aphids or scales. Obviously, fruit-bearing trees must be treated with care using products that are labeled for treatment of homopterous insects on fruit trees intended for human consumption.
If one ever gets the chance to advise a customer, builder or landscaper about trees, impress upon them the need to select varieties or species that remain small if located near a building and to locate large maturing trees away from the structure. Trees should be planted so that the mature span of branches is not likely to come into contact with the building.
*****
Other Ant Attractions
Landscape borders
They look great, but using stacked flagstones, river rock or unmortared bricks are not good choices if one seeks to minimize ants near a building. Stacks of stones mimic natural settings that ants and other pests find attractive, as the spaces between the stones hold moisture and provide protective shelter. Large stones cast a thermal shadow that helps keep the soil underneath warm in cool weather and cooler in hot weather. Railroad ties and untreated lumber rot over time and readily serve as harborage for carpenter ants and other ants.
No border is best, but tight-fitting retaining wall blocks, mortared bricks or treated landscape timbers fastened tightly together delineate landscaping from lawns and don’t allow ants to nest in between their pieces. Of course, ants still can nest underneath and behind such borders, but these are better choices than loose stones or railroad ties.