To Bait, or Not to Bait?

Numerous surveys of the structural pest control industry have ranked ants (Hymenoptera: Formicidae) as one of the most-common-yet-difficult-to-control pests in residential environments. No matter where their business might reside in the United States, pest management professionals (PMPs) engaged in residential pest control typically include ant management as part of their array of residential pest control services.

Depending on the location in the U.S., the diversity of ant species that are troublesome to property owners differs considerably. For small startup companies, especially in the South and Southeast where ant diversity is greatest, ant control services may comprise a great deal of their new pest control business because the active season is longer, the climate is mild and the diversity of ant species is greater.

No matter where a PMP resides, the principles of managing ants are common throughout the industry and include proper identification of the offending species (more on the why of this later) and inspecting the property for insights into why the ants are pestiferous and where they might be originating (nesting). The information gathered from the inspection is in large part the fuel that drives decisions on what course of action the technician might take to solve the problem.

This is the essence of integrated pest management (IPM). The details and information that come from a proper inspection, no matter the pest involved, help the technician decide what to do. IPM is that simple, and inspections are the cornerstone of the process. Dr. Austin Frishman, a long-time teacher, educator, consultant and icon in the structural pest control industry, tells technicians that the most important piece of equipment on their truck is their brain.

Indeed, good information leads to good decision making, while no (and bad) information leads to poor decision making.

Although ants are social insects, differences in their biology, colony size, foraging and food preferences differ greatly among species. These differences, and the selection of various chemical control strategies that might be included in an ant management program, are the basis for the remainder of this article.

Ant species that tend to have relatively small forager populations with small home ranges should be baited when colonies are inaccessible or their whereabouts unknown. Ant species characterized by large, diffuse super colonies with long foraging trails and home ranges should be controlled primarily with liquid spray products and secondarily with baits.

Additionally, when PMPs encounter large colony ants (LCAs), it is important they communicate expectations to property owners that, for example, it may take several monthly trips at first before services can be moved to quarterly.

Generally, products intended to eliminate ants do so orally (baits), topically (spraying live ants directly with acute toxicants) or by contact with dry residues (again, acute toxicants delivered as a spray or dust). Baits take advantage of the social nature of ants because ants routinely share food and exchange other important behavior-modifying chemicals, which is known as trophallaxis (Figure 1).

The best baits are highly palatable, attractive and effective over a range of doses. Neither the active ingredient, nor the bait food base itself, should be a feeding deterrent or kill too fast. If the bait kills workers too fast, the chance for trophallaxis, and ultimately queen intoxication, is minimized, resulting in eventual recovery.

Today, the industry is fortunate to have a diversity of effective ant baits containing the active ingredients indoxacarb, thiamethoxam and fipronil, among others. Several decades ago, the only active ingredient in ant baits was boric acid. Over the past years, though, aided by the development of new active ingredients, product manufacturers have delivered numerous attractive, palatable and effective ant baits.

Some of these same active ingredients are also contained in myriad insecticidal spray products that kill when applied topically to live ants and/or by worker ant contact with the dry residues of the applied product. Most of these active ingredients are effective at very low concentrations.

RESIDUALS FOR LARGE COLONY ANTS. The Argentine ant, Linepithema humile; the tawny crazy ant (TCA), Nylanderia fulva; and the odorous house ant (OHA), Tapinoma sessile, are examples of pest ant species characterized by extremely large colonies (numerous workers) whose foragers move long distances, encompass a large home range and may originate from nest sites that are far away, diffuse and not obvious. I once measured an Argentine ant trail that was at least 350 feet long.

TCA also form large colonies that consist of numerous nesting sites encompassing large foraging areas (often multiple properties). In areas where they become established, TCA dominate food resources and nest locations of native ants. Infestations of Argentine, TCA and OHA are best controlled first by eliminating nesting materials such as leaf litter, rotted logs, piles of trash and other household debris.

These sanitation practices, however, are usually restricted to the infested property and rarely reduce ant populations below pest levels, especially when considering the intrusion of ants from off the property.

In numerous field research trials of Argentine ant-infested structures over several decades, the only long-term (8-10 weeks) relief we have documented has been with the focused application of no more than 1-2 gallons of labeled rates (concentration and volume) of residual spray products applied directly to trailing ants on and around the infested structure and to central nest sites characterized by an accumulation of eggs, pupae and queens. This skinny approach to Argentine ant control was well documented, more than a decade ago, in a series of peer- reviewed research articles authored by Dr. John Klotz, Dr. Les Greenberg and others at the University of California, Riverside.

Operating proactively, it is important to inspect and maybe treat for LCAs early in the spring before ant populations have a chance to grow. Generally, LCA populations are smaller and less active then. As a result, proactive treatments may reduce the density of ants later in the season.

Because of the biology of super colony ants, gel, liquid and granular baits rarely reduce ant activity below pest levels even after multiple, sustained bait applications. For this reason, baits should be considered supplemental, or secondary, to the use of residual sprays to control LCAs.

Residual sprays routinely outperform baits when a product application is deemed necessary to provide relief from invading ants (Figure 2).

Figure 2. Numbers below the dotted line (negative numbers) indicate that ant populations were more active post-treatment than they were prior. For instance, four and 19 days post-treatment, ants provided only gel bait were about 20 percent more active than they were prior to that. In structures treated with the liquid spray, activity was reduced 100 percent after four days, about 95 percent after 19 days and 80 percent after 41 days. 

When sprays are used, it is important that PMPs read and strictly follow all product label instructions. Some insecticides are harmful to fish and other aquatic life. It is, therefore, important to avoid the application of any pest control product that might inadvertently contaminate local waterways. Never apply liquid spray products to flat, hard surfaces, especially if they are routinely exposed to irrigation or rain. Check with your distributor or product manufacturer for recommendations and additional information.

BAITS FOR SMALL COLONY ANTS. Black carpenter ant (Camponotus pennsylvanicus) colonies are small (up to a few thousand workers), and worker foraging distances and home ranges are typically confined to the offended property. Colonies may be located wholly inside the structure, such as under insulation in the crawlspace, or they might be living in a large hardwood tree next to the structure and foraging into the structure at night.

Black carpenter ants are active at night from spring through early fall. Peak activity, of course, is mid-summer (July and August). In areas where a few ants might be seen soon after sunset, carpenter ant foragers leave their nest in small packs in search of food, returning to and leaving from the nest all night. At dawn the next morning, all the foragers return to the nest where they spend the daylight hours.

I have been involved in screening baits for ant control, using the black carpenter ant as a model species for nearly 30 years. I can remember when the industry was clamoring for effective ant baits, especially for carpenter ants, beyond the few rudimentary baits in existence then.

Once again, because of the evolution and delivery of better active ingredients, there are several gel baits available today that, when properly placed, will eliminate a carpenter ant colony with the amount of bait that can be placed on a quarter. We do this routinely in our field trials when screening new baits for efficacy.

The key, however, is bait placement. Carpenter ants tend to not deviate from their established foraging path, and even if baits are placed close to this path, the foragers may not find it. Inspection, therefore, is the key to proper bait placement.

For carpenter ants, at least, baits should be placed under their nose. I have used carpenter ants as an example here, but this same concept holds true for other pest ant species where colonies are small (few workers), and home ranges confined. Considering the plethora of baits available today, there is likely one that every species will find palatable and that will be effective.

SUMMARY. Ant problems in residential environments vary widely from coast to coast and border to border regarding intensity of infestation, diversity of problematic species and longevity of ant season. Ant services are widely offered by licensed PMPs who offer pest control services in their state. Although ant control services are common, ants can be difficult to control. Due to the various biological traits (large colonies with large home ranges) of some ant species, it is recommended that the first choice involving control attempts of these species involve the targeted, responsible application of liquid residual spray products at labeled rates, while ant species characterized by small, distinct colony size and limited home ranges be baited. Regardless of the target species, treatment should only be conducted following a thorough inspection to determine the species of offending pest, the extent of its distribution and most likely location of its nesting site(s). In aggregate, these inspection principles are the core of IPM.

The author is an extension entomologist in the University of Georgia’s Department of Entomology.