[Ant Control] Uncooperative Ants

Baits are a key component of many pest ant management programs. Sometimes baits provide dramatic control while at other times, and for reasons quite unclear to pest management professionals, results are less successful. What are the possible reasons behind the failure of ants to cooperate with this normally effective control technique?

INCORRECT IDENTIFICATION. First, you must consider the possibility that the species you’re dealing with is not the same one that you’ve previously had success with. Many sources have stated that one key to successful ant baiting is proper species identification. This is not always as easy to do as one would like. Even if one is skilled at keying out ants, the taxonomic status (number of species, distribution) of many North American pest ant groups remains unclear. Notable examples of this include the thief ant group (Solenopsis molesta), little black ants (Monomorium minimum) and rover ants (Brachymyrmex spp.). All of these are probably complexes of many, very small species that are poorly known and difficult to identify. We cannot assume that species within a group all have the same feeding preferences.

INVASIVE SPECIES. Another phenomenon to keep in mind is that many of our worst ant pests have entered the United States from abroad. These biological invasions will continue so we can expect to see the number of exotic pest ants within the United States to increase. For example, the widespread pavement ant (Tetramorium caespitum) is an exotic species native to Europe and Asia. However, in the St. Louis area, T. caespitum has recently been replaced by yet another exotic species, the Asian Tetramorium tsushimae which invaded within the last 20 or 30 years. These two species of Tetramorium are very difficult to tell apart. Another good example is the invasion of Maine by the exotic European ant, Myrmica rubra. This species is displacing native ants and ground-dwelling arthropod fauna, while appearing to enhance populations of honeydew-excreting insects in the order Homoptera.

ANT NUTRITION. In temperate zones such as most of North America, the majority of ant species are generalist feeders and a wide range of foods are consumed. Most ants, including pest species, feed on animal protein (largely invertebrate in origin), plant exudates, fruit, and seeds in various proportions. However, even the most omnivorous of ant species are selective of foods with high protein and carbohydrate contents allowing exploitation of the most nutritious foods available. Furthermore, omnivorous species are opportunistic feeders, focusing on different food resources whose availability changes in both time and space.
Bait consumption is driven by the nutritional needs of the ant colony, and consideration of how ants meet these needs may help explain some of the variation in bait efficacy. Nutrient management within an ant colony is influenced by the presence of multiple worker castes, such as brood tending individuals and foragers, by larvae that rely on workers for food procurement, and by the multi-year longevity of queens and colonies. These characteristics of an ant colony are then further coordinated with seasonal colony cycles, such as the production of reproductives, mating flights, and environmental cycles that affect food availability and colony activity (e.g., cold weather may slow activity). Red imported fire ants (Solenopsis invicta) illustrate this coordination of the flow of nutrients through a colony. Worker populations decline from January to midyear, which coincides with the production of the reproductive caste and their major mating flights in early summer. Worker fat content declines gradually through winter, with the lowest fat levels occurring in June after the spring production of reproductives. Hence, worker fat is expended for overwintering and the production of reproductives, while from July through the fall worker fat content increases as lipid resources become available and are accumulated (Tschinkel 1993). Other factors relating to nutritional needs include:
Short term changes in ant food preferences – satiation and starvation effects. One way to think about changes in ant food preferences is to examine them in the context of a concept called “optimal foraging theory.” Optimal foraging theory predicts that animals selecting among alternate resources (foods) will show stronger preferences for items that are relatively less available. This concept is directly related to the phenomenon of satiation-related behavior that has largely been supported by laboratory and field studies on ants. For example, Kay (2004) demonstrated that colonies of Dorymyrmex smithi (a pyramid ant) that had first been allowed access to proteins alone and subsequently offered a choice between proteins and carbohydrates preferred carbohydrates. Similarly, worker pharaoh ants (Monomorium pharaonis) show distinct preferences for certain foods such as honey and peanut butter. However, when given only one of these foods continuously for several weeks, workers show a marked preference for other foods when offered a choice. Workers also prefer to alternate between carbohydrate foods and protein foods (Edwards and Abraham 1990). Similarly, the tropical pest ants, Monomorium floricola and Monomorium destructor as well as M. pharaonis, all preferred the alternate food type (protein or carbohydrate) to that food upon which they had fed to satiation (Eow et al. 2005). 
Starvation effects on feeding preferences can also occur. Howard and Tschinkel (1981) showed that severely starved workers of the red imported fire ant preferred aqueous solutions of sugars and proteins instead of lipids. Eow et al. (2005) demonstrated that M. floricola workers starved of carbohydrates, proteins or lipids consistently favored these food types when they were again offered. Workers from sugar-starved colonies of a carpenter ant (Camponotus mus) were much more likely to accept sugar solutions and the feeding time increased in a manner proportional to the starvation time (Josens and Roces, 2000).
Seasonal changes in food preferences. Seasonal changes in food types preferred by ant colonies are documented both for temperate as well as tropical species. In both regions, the choice of foods taken is usually associated with the level of brood production in the colony. At those times of the year when the nutritional needs of the colony, reflected by the amount of larval brood present, are greatest, preferences for protein-based foods are also likely to reach their peak. Preferences for carbohydrates, necessary as an energy source for workers, can also change seasonally. Some examples will help to illustrate the complex nature of these switches.
The red imported fire ant prefers carbohydrates during the cooler months in Texas and protein during warmer months of the year when most colony growth occurs (Stein et al. 1990). The pavement ant (Tetramorium tsushimae) prefers protein foods during most of the growth season but preferences for lipids, sugars and proteins are similar in the fall corresponding to a probable reduction in brood rearing activity (Sims 2006). Carpenter ants (Camponotus pennsylvanicus) prefer protein (mealworms) early in the season and then prefer carbohydrate (honey) from mid-summer onward (Tripp et al. 2000).  Foraging by Argentine ants (Linepithema humile) in California is seasonal with peak activity occurring in the warmer months of June through October. During these months, 25-60% of the food foraged by workers was protein but this dropped to 16-40% during the cooler winter months. (Rust et al. 2000). The authors noted that an optimal baiting program might include protein baits in the early summer and liquid baits in summer and early fall. Allegheny mound ants (Formica exsectoides) preferred high sugar honeydews early in the season, but shifted in mid-season to a preference for high amino acid honeydews. This switch coincided with an increase in the amount of actively growing brood (Bristow and Yanity 1999).  Sudd and Sudd (1985) presented interesting results on the seasonal acceptability of different sucrose solution concentrations to wood-ants (Formica lugubris) in northern England. In spring and early summer, ants preferred 0.15 Molar sucrose but during a brief period in late June, the preferred concentration increased dramatically to 0.90 Molar. The preference change favoring the more concentrated solutions was correlated to the stage of aphids tended (apterae) which increased the “quality” of the natural food resource so that low sucrose concentrations could no longer compete for ant attendance.
Recognition of food and hunger status. One consequence of adopting a generalist diet is the necessity for recognizing a wide range of foods. Many species of ants can undoubtedly do this, but surprisingly little is known of the cues involved. Like many insects, ants are faced with the problem of recognizing and discriminating between adult and larval food requirements which may differ widely. Larval food must induce workers either to pick up and carry or to ingest materials they may not themselves require, and later donate them to larvae. The reward of larval secretions may provide an adequate incentive for this service, but the stimulus to forage will derive directly from the hunger state of other individuals. Traniello (1977) found that the foraging activity of Camponotus pennsylvanicus was related to the degree of colony starvation, and Brian and Abbott (1977) demonstrated that stimuli from larvae induce workers of Myrmica rubra to forage. 

BAIT IMPLICATIONS. Food preference studies rank high on industry’s priorities for research on bait development. There has been considerable research on the sugar preferences of various pest ants, but less work on other dietary components. To formulate good combination baits, more information is needed, for specific pest species, on dietary requirements and preferences for proteins and fats. To optimize collection, transport and sharing of baits by ants, formulations and delivery systems must continue to improve. Granular baits are becoming more popular but their application may not always exploit the natural behavior of some ant species. Liquids continue to have potential because they capitalize not only on the natural behavior of ants feeding on honeydew and nectar but also on the specialization of the digestive system of some ants to handle these liquid foods.

CONCLUSIONS. Correct ant species identification remains a “core” part of successful baiting because it allows access to published literature on food preferences and other aspects of ant biology that can be manipulated to suppress populations.
• Ant feeding preferences, even within a single species, are clearly not simple and they can be difficult to characterize. Within individual ant colonies, food preferences can change on a daily or seasonal basis for reasons that will usually not be apparent to the pest management professional.
• Sometime the best baiting strategy will be to offer several baits simultaneously, including baits offering a variety of food groups (proteins, lipids, sugars) and formulations (granules, liquids) to determine which food(s) the ants are interested in at that one point in time.
The authors are senior research entomologist, Whitmire Micro-Gen and extension entomologist at the University of Georgia’s Griffin, Ga., campus.

References Cited
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