[Ant Control Issue] Stuck in the Middle

Ants are recognized as major problems in pest control accounts and proper identification for management is emphasized. Most pest control personnel recognize the "big" ants, such as carpenter ants, and have employed strategies for their control. These large ants excavate in both decayed and sound wood and are important structural pests, particularly in the northern United States, southern Canada and in forested areas. The "tiny" ants, recognized as pest ants, include pharaoh ants, thief ants, little black ants and ghost ants. These are differentiated chiefly by color, are less then 1.5 mm long, and have one or two nodes. Larger ants with two nodes are recognized as pavement, harvester and fire ants.

Overheard among technicians and in discussions with managers, technicians and homeowners is the term "regular" or "household" ants, which are common names used to include all ants larger than the "tiny" ants and smaller than carpenter ants. These ants have a single node and are generally not feared because they lack a stinger. The group "regular" or "household" ants includes several different pest species with a range of behaviors that requires differing control strategies.

Common species that fall into the category of "regular" or "household" ants include the following:

· Argentine ants, Linepithema humile

· Velvety tree ants, Liometopum spp.

· Odorous house ants, Tapinoma sessile

· Forelius pruinosus (no common name)

· Moisture or cornfield ants, Lasius spp.

· Small honey ants, Prenolepis imparis

· Field and thatching ants, Formica spp.

All of these species fall into the size range of 2 to 4 mm in length with the exception of some species of Formica, which are larger. Four of these ants are distinguished by the presence of a slit-like opening on the posterior ventral surface of the gaster and, when crushed, have a distinctive smell of rotten coconut or a musty greasy odor. These ants belong to the subfamily Dolichodorinae and include Argentine ants, odorous house ant, velvety tree ants and F. pruinosus. The three other species are characterized by having an opening at the tip of the abdomen surrounded by a circular fringe of hairs and when disturbed produce an acid-like odor. These ants belong to the subfamily Formicinae and include moisture or cornfield ants, the field and thatching ants and the small honey ant.

A few of these ants can be separated by geography. Argentine ants are more common in the southern states and only occasionally have been reported in northern states. Velvety tree ants are western species and occur from Southern California through British Columbia and eastward through the Rocky Mountains. The other species are found distributed throughout the United States and southern Canada.

Dolichodorinae. Three of these ants when crushed produce a rotten coconut smell: odorous house ants, velvety tree ants and F. pruinosus. These ants cannot be distinguished by odor alone. Looking at morphological features, odorous house ants are characterized by the flattened node on the pedicel, while the other two ants will have a vertical node that is visible in a lateral view. Velvety tree ants can be differentiated from F. pruinosus by the thorax, which is evenly rounded in a lateral view while the latter is not evenly rounded. There are several species of velvety tree ants that range in color from all black or brown to a red mesosoma and black head and gaster. It is the dark colored velvety ant that may be confused with F. pruinosus. The Argentine ant also resembles F. pruinosus and can be differentiated in that the Argentine ant has no erect hairs on the thorax while F. pruinosus will have erect hairs on the thorax and many grayish hairs on the abdomen.

Formicinae. The smaller species of Formica can be distinguished from Lasius in that the ocelli on Formica are conspicuous and the epinotal spiracle is slit-like while the ocelli on Lasius are small or absent and the epinotal spiracle is circular. The other formicine in this group is the small honey ant, which can be differentiated by the pinched appearance of the mesosoma which gives an hourglass appearance when viewed from above. The anterior of the gaster is wider than the mesosoma.

Of the seven groups of ants listed in this article, only one group is a wood-destroying species — velvety tree ants. Moisture or cornfield ants are often classified as wood-destroying organisms, but are generally attracted only to damp wood that is infested with wood decay fungi. They may be responsible, however, for transporting moisture to their nest and increasing the damage.

Not all ants are pests. Many ants are beneficial in that they prey on other insects and many ants have no consequence to people or their homes. Learning to distinguish pest ants can be challenging and is ongoing as ants are transported from one area to another through commerce and the movement of people and their belongings. The ants on the following pages includes the most commonly occurring ants in households. Some additional ants that might be included are pyramid, white-footed, crazy and larger yellow ants.

CONTROL. The first step in management of ants is inspection. This should include identification of the ant and the location of nesting site(s). Identification and knowledge of the pest species will assist in locating ants that may be nesting within the structure. In some cases, entry into the structure can be eliminated by disrupting foraging trails that occur from vegetation in contact with the structure or by removal of infested wood and nesting material.

Foraging habits of these ants are similar in that all will feed on sweet materials, particularly honeydew produced by homopterans. Ants also will enter structures to feed on sweet materials. In addition, all ants require protein for rearing brood and will also feed on insects. Because of the food preferences of these ants, sweet baits are attractive and can be used in management. When possible, the best management strategy is to locate the nest and foraging trails so that baits can be strategically placed to disrupt fidelity to natural foraging sites. Non-repellent pesticides can be used singly or in conjunction with a baiting program. Many of these ants are polygynous and may require monitoring to facilitate management.

What follows on this page and pages 38 and 39 is a brief description of each "regular" or "household" ant.

Argentine ants: Linepithema humile

Workers of these ants are monomorphic (one size) with an uneven thorax, a vertical node and no erect hairs on the thorax. The head has a rounded shape. This ant is competitive and often drives out native species. It nests in diverse habitats in both urban and rural areas and produces a stale musty greasy odor. It tends aphids and homopterans for honeydew and feeds on floral and extrafloral secretions and buds of some fruit trees. It will also feed on sweets, meats, fruits, eggs, animal fats, vegetable oils and dairy products. This ant will also steal seeds from seedbeds and attack honeybee colonies. This introduced species is found chiefly in southern states but has been transported to sites in northern states. The colonies are large with many queens. Color varies from light to dark brown.

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Forelius pruinosus (no common name)

These monomorphic ants have a thorax with an uneven dorsal profile and an erect node. Erect hairs on the thorax are sparse but the body is covered with a dense grayish pubescence. It can be distinguished from Argentine ants by the rectangular shape of the head. This ant feeds on honeydew and dead insects. The colonies are small to moderate in size with a single queen. Foragers will invade from outdoor nesting sites into structures to feed on sweets and meats. The color is dark brown to black with a lighter color on the gaster.

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Moisture or cornfield ants: Lasius spp.

Workers of these ants have an epinotal spiracle that is round in shape. The ocelli are small or absent on workers and the antennal scape barely surpasses the edge of the head. Workers of these ants are monomorphic, feed on sweet materials, and tend aphids. Reproductives swarm in late summer to early autumn and often these winged forms are the first indication of an infestation. Although the colony usually starts in decayed wood, they may transport moisture into wood structures and increase the damage. They excavate the dark-colored sawdust of the decayed wood as they enlarge their nests. Some will produce a carton-like material produced by cementing wood and soil particles with mandibular gland secretions. The colonies are moderate in size with a single queen. The color varies from yellow to dark brown.

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Small (or false) honey ants: Prenolepis imparis

Small honey ants are monomorphic and have a thorax with a strong constriction at the middle of the mesosoma; an antennal scape extending beyond the posterior border of the head by at least 1⅓ its length; and a broad gaster at the anterior edge. These ants nest in soil and swarm in early spring although mating occurs on the ground. Workers forage at night and on cool days, often working at low temperatures between 40° and 65° F. These nonaggressive ants tend homopterans for honeydew and will also feed on live and dead insects, fruit juices, and sap or fluids from flower buds. They may invade houses in early spring and feed on sweets, fruits and meats. The colonies are small to moderate in size with a single queen. The color is variable from brown to blackish with a darker gaster.

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Field and thatching ants: Formica spp.

Workers of these ants are polymorphic and have distinct ocelli and an epinotal spiracle that appears as a narrow slit. The antennal scape barely surpasses the edge of the head. Many thatching ants are larger (4 to 8 mm), but some ground nesting field ants, also known as silky ants, are smaller (3 to 5 mm) and may be confused with other "household" species. These ants tend aphids and feed on other insects. They may also be injurious to seedling trees, plants near their nests and buds on some fruit trees. These ants have a painful bite and spray formic acid when disturbed. Some species nest in decayed logs or landscaping timbers but are seldom found in structures. They are not wood-destroying ants but can be serious nuisance pests in yards because of their aggressive biting habits when disturbed. The colonies may become very large and may have either a single or many queens. The color varies from all dark brown or black to combinations of red and black.

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Velvety tree ants: Liometopum spp.

The dorsal profile of these polymorphic (many sizes) workers includes a smooth convex thorax that resembles the characteristic outline for carpenter ants. They are smaller than carpenter ants and the tip of the abdomen is slit-like rather than circular fringed with hairs. These ants have an erect node on the pedicel. The common name for this group is derived from the fine pubescence that covers the gaster. These ants excavate wood and insulation in their nesting habits. Their excavations are much finer than the coarse excavations of carpenter ants. Velvety tree ants feed on honeydew and insects. They are not often seen feeding on food items within the structures, but are attracted to sweet baits. These ants have a painful/irritating bite when disturbed. Colonies are large with many queens. Color varies from all blackish brown in L. luctuosum to a red mesosoma with a black head and gaster in L. occidentale.

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Odorous house ants: Tapinoma sessile

These monomorphic ants have an uneven thorax and are characterized by having a flattened node on the pedicel. They have a preference for sweets and tend aphids and feed on honeydew. They invade houses to feed on sweets, meats and dairy products. Odorous house ants have diverse nesting sites and can be found in woodlands, on beaches, and under the bark of logs, under rocks and vegetation. Colonies are large with many queens. They are dark brown.

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The author is an instructor in biology at Spokane Falls Community College, Spokane, Wash. She can be reached at lhansen@giemedia.com.