[Ant Control Issue] Picky Eaters

Author’s note: Ants remain one of the most prevalent and widespread urban pests that pest management professionals eliminate for their customers. It seems like every time we turn around a new pest ant species is in the news, especially in warmer, more tropical U.S. climates. When it comes to control measures, each new species emergence brings a new set of rules. For quite some time, baiting for ants has been an effective, although time-consuming, control measure. As with any pest baiting strategy, getting the target pest to “take the bait” is the first step towards effective control. Understanding ant feeding behavior is therefore one of the most important areas of study that developers of ant control products have had to undertake in order to create baits that ants will actually consume and carry back to the colony. Having a general understanding of how ants find, consume and transport food to other colony members involves a series of complex behaviors. What follows is an overview of ant feeding behavior.

From an evolutionary standpoint, ants are one of the more developed and complex pest groups that pest management professionals must deal with on a regular basis. There are a growing number of ant species throughout the United States that have achieved pest status, and it seems like new species are continually being introduced. Without exception, these various ant species exist as colonies in which individuals are capable of communicating with each other in order to search for, find and exploit food sources in their environment. Ants have developed and fine-tuned these survival skills over hundreds of millions of years. As we know, many pest species have developed complex relationships with other organisms in their environments in order to obtain and digest food, for example, honeydew- or aphid-farming ants (such as certain Camponotus species).

Scientists have studied the feeding behavior of ants for decades. E.O. Wilson, the world famous biologist and myrmecologist, aptly described the ant colony as a “super organism,” suggesting that if you step away from observing individual ant behaviors and take a blurred focus of the entire colony, it appears to operate as a single being. Individual worker ants communicate with each other with a complex set of chemical and physical cues that may dictate the location, quality or quantity of a food source, the location of an enemy, or a general warning to be on alert for “danger.”

NOT A MONARCHY. One commonly perpetuated fallacy of the ant colony is that its day-to-day activities are controlled by the queen. Many conditions govern the activity that occurs within a colony. Environmental conditions (temperature and humidity), nutritional status, what foods or substances individual ants are fed, and seasonality, are just a few; however, the queen or queens of most ant colonies are not responsible for controlling or “ruling” over the colony.

The queen or queens are (in most ant species) little more than slaves or prisoners, existing for one primary reason — to produce eggs that will eventually develop into more ants. Needless to say, the level of complexity in behavior, communication and foraging for food makes understanding the basics about ant feeding behavior important.

This social behavior and communication between ants is perhaps the single greatest behavior to exploit with ant baiting programs. The fact that worker ants share food with each other as well as the non-foraging larvae in the colony creates a perfect scenario: Pest management professionals can apply ant baits with active ingredient that acts slow enough to allow ants to feed on it and share with other members of the colony prior to succumbing to the pesticide. In this fashion, one hit on a bait can result in the death of many members of the colony. Some products even claim elimination of the entire colony due to this phenomenon.

It is no easy task to develop a good ant bait that remains stable in the environment until it is located by ants, is palatable to and consumable by the foraging worker ants, and is capable of effectively “carrying” the active ingredient to the feeding worker and subsequently passing this active onto other workers and other members of the colony.

More recently, with a host of non-repellent, highly biologically active insecticide molecules at their disposal, product developers have tried to link what we have learned about ant feeding behavior and food preferences together to create the ant bait that will be acceptable to as many pest species as possible under a variety of conditions. While having just one ant bait for all situations would certainly be desirable from the standpoint of convenience, its not always realistic for those PCOs that offer ant control services for a wide variety of pest species.

FEEDING STRATEGIES. Let’s take a look at the overall variety in ant feeding strategies and the food preferences of common pest ant species in the United States.

The majority of ants are general predators or scavengers, feeding on a wide range of prey, including other arthropods and seeds. Adult ants feed exclusively on liquid foods. They collect these liquids from their prey or while tending (a.k.a., farming) other insects. Solid prey, that which is most often seen being carried by workers, generally is intended as food for larvae located back in the colony nest. Adults that remain in the nest, including the queen, receive much or all of their food directly from returning foragers through a process known as trophallaxis. While foraging, workers collect fluids that are stored in the upper part of their digestive system or crop. Upon returning to the nest, these workers regurgitate a portion of this stored fluid and pass it on to other workers. In some extreme species, this fluid is transferred to special workers, called repletes, which remain permanently in the nest and act as living storage vessels. They store food when available and distribute it to other colony members in times of food shortages.

While most ants will forage for and feed on a wide variety of food items, others specialize on a much narrower range. Some of these feeding preferences result in symbiotic relationships in nature that are quite bizarre. Many ant species prefer to feed primarily on seeds of certain plants. The seeds of many plants have special food bodies (called elaiosomes) that are attractive to ants. These structures are very high in protein and lipid content. Worker ants collect these seeds, eat the food body, and sometimes the seeds as well.

However, many of the seeds remain intact after the food body is removed and are often placed within the ants’ nest or on their midden piles (refuse areas) where they later germinate. It is believed that seeds collected by ants have a higher chance of germinating and surviving when compared with seeds that are not collected. This is because they are less likely to be attacked by seed predators and because they are often placed in sheltered locations near the ants’ nutrient-rich refuse piles.

HUNTERS AND GATHERERS. Individual species differ widely in their diets and may be carnivorous, herbivorous or omnivorous. Harvester ants eat and store seeds; these sometimes sprout around the nest, leading to the erroneous belief that these ants cultivate their food. However, cultivation is practiced by certain ants that feed on fungi grown in the nest. Some of these, called leaf-cutter, or parasol ants, carry large pieces of leaves to the nest, where the macerated leaf tissue is used as a growth medium for the fungus. Most leaf-cutters are tropical, but the Texas leaf-cutting ant is a serious crop pest in North America. The army ants of New World tropical regions and the driver ants of tropical Africa are carnivorous, nomadic species with no permanent nests. They travel like armies in long columns, overrunning and devouring animals that cannot flee their path; the African species are even capable of consuming large mammals.

Although they are not pests or even prevalent in North America, the following demonstrate some of the more specialized feeding preferences that certain ant species have developed. For instance, a number of species, especially those in the genera Pyramicia and Sturmigenys, show a strong preference for insects in the Order Collembola (springtails). Others, for example species of the genus Discothyrea, prefer the eggs of assorted arthropods found in the detritus of the forest floor. Worker ants of the genus Cerapachys and Sphinctomyrmex raid the nests of other ants to capture their larvae and pupae that they then carry back to their own nest and in turn feed to members of their own colony. Many of these groups with specialized feeding requirements also possess unusual morphological or body-shape adaptations. For example, the mandibles in some of the highly predacious groups are much more elongated and are armed with large teeth, especially at their tips.

In general, ants show a preference for foraging either during the day or at night. In some groups, foraging will occur both during the day and at night, although there may be peaks of activity with fewer foragers active during other periods. In the arid zone, the foraging activities of many species are highly dependent on temperature. Some species, for example most Tetramorium species (pavement ants), are only active during the cool morning and evening hours, while others are active only during the hottest parts of the day. On cool or heavily overcast days, species that normally are only seen at night may be active during the day while high-temperature-loving species may remain in their nests all day.

COMMON FEEDING PREFERENCES. Now let’s talk about food preferences for urban pest ants in a general way. While we’ve pointed out that ants are a diverse group, they do have behaviors and general food preferences in common. By using this knowledge, baits can be developed that work to exploit the common biological attributes and behaviors. As previously stated, the queen does not necessarily “rule” or control colony activity, she ultimately remains the main target and most important individual in the colony. She produces all the eggs that grow into larvae, pupae, adult workers and the “reproductives” that will be necessary to expand or start new colonies. In general, worker ants are usually sterile females that maintain the colony, care for the immature ants and the queen and forage for food.

The ant colony is an efficient place. The oldest, most expendable workers are sent out to forage for food. This is, unfortunately for baits, usually only about 10 percent of the workers. These foraging ants will travel around the surrounding area of the nest until they locate a food source. Once a food source is located, they will take some of that food back to the colony or share with other foraging worker ants to recruit them to help harvest that particular food source. In doing so these worker ants may leave chemical trails (short-lived pheromones) to help other workers find their way to the new food resource.

Ants can be very selective in the foods they bring back to the colony. Most ants will feed on carbohydrates most of the year, some ants prefer protein, while still others prefer fats. However, note that feeding preferences can change based on the time of year or the current needs of the colony. It is even possible for feeding preferences to change from one time of the day to another, as this has been demonstrated in a number of our better-studied pest species. This fact points out that developing a single bait that will be acceptable at all times can prove difficult. Ant control strategies that rely on baits will use the ant’s natural behaviors of harvesting food in the environment and bringing it back to the colony to help destroy the colony. But this strategy only is effective if the toxic agent in the bait works slowly enough to make it to other workers to effect transfer of the toxicant throughout the colony.

CONCLUSION. In the past, some baits didn’t work well because they were targeting only one species of ant, which may have had feeding preferences different from the other pest ant species in a given area. Another reason for poor bait activity is that the bait uses a toxicant that does not allow enough time for the worker ants to share food through the colony; worker ants then die before they reach the nest and total colony control cannot be achieved. Finally, baits may not be offered in suitable quantities to work their way into large colonies in order to obtain colony collapse. If sufficient workers or larvae — or the queen — are not killed, the colony can recover and continue to thrive.

So the next time you are brushing up on (or teaching new service technicians) proper baiting strategies, be sure to review how ants forage, feed and share food with the rest of the colony. Understanding the basics behind these behaviors leads to a better understanding of the “how and why” of ant food preferences — why they change and how you can adapt your own baiting strategies to meet the customers’ needs.

The author is an entomologist who lives in Racine, Wis.

References
Hedges, S.A. 1998. Field Guide for the Management of Structure-Infesting Ants, G.I.E. Inc., Publishers. Cleveland, Ohio.
Holldobler, B. & Wilson, E.O. 1991. The Ants. Harvard University Press. Cambridge, Massachusetts.
Mallis, A. 1990. Handbook of Pest Control 7th Edition. Franzak & Foster Co. Cleveland, Ohio

Feeding Preferences of Common Ant Species

Carbohydrates (sugars, honey-dew, plant nectars)

• Argentine ant (Lepithema humile)
• Carpenter ant (Camponotus spp.)
• Ghost ant (Tapinoma melanocephalum)
• Odorous house ant (Tapinoma sessile)
• White-footed ant (Technomyrmex albipes)

Proteins (meats, seeds)

• Carpenter ant (Camponotus spp.)
• Crazy ant (Paratrechina longicornis)
• Fire ant (Solenopsis invicta)
• Thief ant (Solenopsis molesta)

Lipids (fats, grease, oils)

• Big-headed ant (Pheidole megacephala)
• Fire ant (Solenopsis invicta)
• Pavement ant (Tetramorium caespitum)
• Pharaoh ant (Monomorium pharaonis)