Formica obscuripeswestern thatching ant

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Geographic Range

Formica obscuripes, the western thatching ant, is native to the Nearctic region. It is widespread across the western half of Canada and the United States. Its range extends as far south as Arizona and New Mexico, and as far east as Michigan and Missouri. In the southern half of Canada, it can be found from British Columbia to Manitoba. It is especially prevalent in the Pacific Northwest. (Crutsinger and Sanders, 2005; Higgins and Lindgren, 2012; Risch, et al., 2008; Tilman, 1978; Weber, 1935)

Habitat

Formica obscuripes builds its nests in semi-arid regions, such as dry grasslands, including shrub-steppe habitats and sagebrush, prairies, coniferous forests, dunes, and alpine meadows. Nests are also often found in areas of secondary succession. Formica obscuripes can live in a large range of altitudes. Nests have been found as low as 800 m and as high as 3,194 m, though the most common altitudes are between 1,524 to 2,743 m. Nests are built into the ground, often around a structure, such as the main stem of a sagebrush plant or even a fence post. Nests can extend up to 4 feet into the ground and are typically constructed out in the open. The western thatching ant gets its name from a mound of "thatch" that the workers assemble on top of the nest. This thatch consists of twigs, grasses, plant parts, and soil, and can be anywhere from a few centimeters to a meter high. The thermoregulatory abilities of the thatch allow the nests to be exposed to a variety of temperatures, humidity, and weather conditions. Secondary nests are often constructed at the base of plants where Formica obscuripes farms honeydew. (Beattie and Culver, 1977; Conway, 1996a; Crutsinger and Sanders, 2005; Higgins and Lindgren, 2012; McIver and Steen, 1994; Mico, et al., 2000; Risch, et al., 2008; Tilman, 1978; Weber, 1935)

  • Range elevation
    800 to 3194 m
    2624.67 to 10479.00 ft

Physical Description

Formica obscuripes has one petiole and colony members may have a variety of colors and sizes. There is a continuous size distribution in workers, generally ranging from 4.0 to 7.5 mm in length, making it difficult to group the workers by size. Head width ranges from 0.94 to 2.1 mm, showing significant variation. Workers can be grouped into major and minor or major, media, and minor workers. Due to the large variation in size, these ants are likely polymorphic. Workers typically have a reddish-orange head, the thorax can be either reddish-orange or black, and the abdomen is black. Legs and antennae can be reddish-orange or black. Reproductive forms also follow this coloring. Smaller workers can also be all black or dark brown. Eggs are creamy white, and elliptical shaped. They are about 0.6 mm long and 0.31 mm wide. Larvae are the same size as the egg when they hatch and grow to about 6.0 mm in length. Reproductive pupae are 9 mm in length, while worker pupae are 3.5 to 7.0 mm in length. (Billick and Carter, 2007; Fraser, et al., 2001; Herbers, 1979; Weber, 1935)

  • Range length
    4.0 to 7.5 mm
    0.16 to 0.30 in

Development

Formica obscuripes is holometabolous. The first batch of eggs is laid in April. Eggs are laid throughout the summer, until as late as the middle of August. Eggs are laid in a brood chamber, as well as other soil chambers, where they develop and are tended to by adult workers. Eggs hatch after 23 to 53 days. Larvae can be found in brood chambers of the nests from the beginning of June to the end of August, and pupate after 7 to 33 days. Pupae that develop into sexual forms are not present in the nests after June, but those that develop into workers can be found into early September. They remain pupae for 31 to 93 days, before developing into adults. By fall, the brood chamber has emptied. Total time of development from egg to adult takes 61 to 122 days. (Weber, 1935)

Reproduction

Mating takes place during nuptial flights for Formica obscuripes during June and July. Prior to taking flight, reproductive individuals live in the nest. Reproductive females are observed from the end of June to mid-August, while males are present from the end of June to July. When leaving the nest for flight, winged females are often roughed up by workers. Workers tug on the female's wings and antennae with their mandibles. Flights occur in the morning and typically take place on clear, warm, windless days. The participants often climb nearby structures before taking flight. Nuptial flights may involve a few, to several hundred individuals, who fly to nearby swarming grounds. The same swarming ground may be used from year to year. Females land on plants or grass and perch near the top, with their abdomen sticking up in the air. They likely release a pheromone at this point to help males find them. Males fly around and swarm, attempting to find females. While waiting, females may move their bodies and antennae, possibly as a visual indicator of their location. When they find a female, males attach to them, mate and remain motionless for 1 to 5 minutes until one of them moves, then the male drops off. Males fly off and females remain for a little while. If several males find a single female, they fight until one wins and mates, while others drop to the ground. Nuptial flights continue each day until the nest is emptied of all alates. Colonies can have 5 to 16 flights. (Conway, 1996b; Conway, 1996a; Talbot, 1972)

While there has been a large amount of research conducted concerning the nuptial flights of Formica obscuripes, very little is known about what occurs afterwards. Since colonies move nests or grow by budding, it is uncertain where females that have recently mated in the nuptial flights go next. They likely return to already established nests to lay their eggs. Colonies of Formica obscuripes typically have two or more wingless queens that lay eggs. The number of queens present may vary significantly, as one colony was recorded as having 198 queens. The first brood of eggs is laid in the nest in April by queens already present in the nest. The eggs are laid in special brood chambers where they undergo metamorphosis and are cared for by workers. (Conway, 1996b; Conway, 1996a; Herbers, 1979; Weber, 1935)

  • Breeding interval
    Females mate once during the nuptial flight, while males may mate several times.
  • Breeding season
    Nuptial flights occur in June and July.

Colonies of Formica obscuripes exhibit significant brood care. During the summer, a large brood chamber is constructed near the base of the thatch. This chamber is divided by twigs that are poked through at all angles and is well insulated. Pupae can be found in the upper part of the chamber, while eggs and larvae are in the lower part of the chamber, as well as in lower chambers in the soil. The smallest workers remain in the nest to care for and feed the brood. Once they reach adulthood, ants become independent and join the colony as workers or sexual forms. The entire brood has left the chamber by the beginning of fall and the chamber is then filled with thatch. There is also provisioning in the eggs provided by the queen. (Herbers, 1979; Weber, 1935)

  • Parental Investment
  • pre-hatching/birth
    • provisioning
      • female
    • protecting
      • female
  • pre-independence
    • provisioning
      • female
    • protecting
      • female

Lifespan/Longevity

Most workers live 19 to 44 days after reaching adulthood, an average of 31.6 days, though some overwinter and live more than a year. (Conway, 1996a)

  • Range lifespan
    Status: wild
    1+ (high) years
  • Typical lifespan
    Status: wild
    19 to 44 days

Behavior

Like most ant species, Formica obscuripes lives in large colonies. A large colony may have anywhere from 10,000 to 40,000 individuals. It is diurnal and forages during the day. In the warmer parts of its range, F. obscuripes is active year round, while in the northern regions it overwinters in its nest and is active from April until October or November. The sexual males and females are able to fly, while workers are wingless. There are conflicting reports about the role of discrete worker castes in the colony. Some researchers say F. obscuripes lacks discrete castes, while others have observed 2 or 3 distinct castes. In size, workers are on a continuous scale, with a large variety of sizes, though there are no obvious groupings. Workers are often grouped into major, media, and minor workers. Major workers repair the nest and thatch, as well as forage for insect prey to bring back to the nest. Majors often work together to take down large insects. These attacks may last as long as 30 minutes. When attacking prey singly, major workers snatch the insect in their mandibles and immediately return to the nest. Media workers transport broods between nests, forage for vegetation, and farm aphids. Minors are rarely seen outside the nest and likely take part in brood care and tend the queens. All castes clean the nest. None of these tasks are exclusively performed by any one caste and all sizes contribute when necessary, which likely supports the idea that there are no discrete castes. (Billick and Carter, 2007; Conway, 1996b; Conway, 1997; Crutsinger and Sanders, 2005; Heikkinen, 1999; Herbers, 1979; Tilman, 1978; Weber, 1935)

Western thatching ants get their name from the piles of thatch they constructs to cover their nest mounds. The thatch is created from seeds, twigs, plant stems, grass, and soil. Piles of thatch can be several centimeters to a meter deep and several centimeters to 1.5 m across. This thatch keeps nests at a constant temperature throughout the day, even though nests are typically constructed out in the open in dry, warm regions. Workers are constantly repairing and adding to the thatch. Many mounds are also built around the main stem of plants such as sagebrush. Ants chew the bark on the stem and spray formic acid at it until it dies, at which point it can be removed, creating a central passage in the center of the nest. Formica obscuripes is polydomous, with one colony typically living in several mounds. Colonies often switch primary nests. New colonies are formed by budding. Secondary mounds may become primary nests, or new mounds may be created nearby, with workers moving between the mounds. (Conway, 1996a; Conway, 1997; Herbers, 1979; McIver and Steen, 1994; Mico, et al., 2000; Weber, 1935)

Mound usage by the colony can change throughout the day and year. A nest can have anywhere from 1 to 52 entrances, and these entrances constantly change. During warmer parts of the day, ants use entrances that are under cover and use entrances that are in the sun during the morning and evening. The same principle applies to trail usage. Most trails are constructed under cover of vegetation, rarely veering out into the open. Ants will also remain in the nest or secondary nest during the hottest part of the day, with most foraging taking place during the morning and evening. Many colonies have secondary nests. These nests are typically constructed at the base of the plants where workers farm aphids. Workers use this secondary nest throughout the day, the largest number take shelter in the mid-afternoon during the warmest temperatures. There are two types of workers involved in honeydew farming, tenders and transporters. Transporters spend much of the day in the secondary nest, while the tenders farm honeydew and bring the honeydew to the transporters. The transporters collect the honeydew in their crop from the tenders and return to the nest with the honeydew. (Conway, 1996a; Conway, 1997; Herbers, 1979; McIver and Steen, 1994; Mico, et al., 2000; Weber, 1935)

Home Range

Formica obscuripes tends to stay close to the nest; one study showed that plants farther than 20 m away from the nest were not visited by the ants. Mound density in some areas is 115 mounds/ha. In one study, the closest mounds were 2.36 m, while another study showed nests were usually separated by more than 100 m. (Conway, 1996a; Higgins and Lindgren, 2012; Tilman, 1978)

Communication and Perception

Antennae are one of their most important sensory organs, used for olfaction, chemical detection, and tactile perception. Formica obscuripes communicates with other workers by antennation and also perceives their environment with their antennae. Ant crickets (Myrmecophilus manni) live in Formica obscuripes nests and have learned to mimic their antennae movements, which allows crickets to antennate with ants and remain undetected as non-colony members. When foraging at extra floral nectaries, ants can communicate with other foragers. If an ant finds a depleted nectary, it leaves a drop of liquid at the junction of the main stem and the stem to the depleted nectary. When another ant moves along the stem, it will antennate the drop of liquid and move past the depleted nectary without having to investigate itself. Females release pheromones into the air during mating to attract males. As males swarm over the plants where the females wait, females also move their body and antennae to signal their location, indicating that vision is important in perceiving other individuals. (Conway, 1996b; Henderson and Akre, 1986; Talbot, 1972; Tilman, 1978)

Food Habits

Formica obscuripes is an omnivore. It preys on many species of insects, particularly orthopterans, beetles, terrestrial isopods, hemipterans, lepidopteran larvae, dipterans, and other ants. It also scavenges dead insects and other invertebrates. Foraging ants bring both living and dead insects back to the nest. Formica obscuripes also eats organic matter, nectar from extra floral nectaries, and plant tissues including leaves, galls, and flowers. It has been recorded scavenging seeds, eating the edible part and storing the rest in the nest. Occasionally, these ants also feed on carrion, such as dead rattlesnakes, birds, and small mammals. Ants typically collect liquid from the carcasses and store it in their crops, returning to the nest and regurgitating the liquid via trophallaxis. Formica obscuripes also farms honeydew from honeydew-producing insects, including aphids and membracids. Honeydew is an important component of their diet, as a significant source of amino acids, carbohydrates, and water. It provides energy for the workers, and nutrients for the brood and queen. These ants occasionally also prey on the insects that they tend. (Beattie and Culver, 1977; Billick and Carter, 2007; Clark and Blom, 1991; Conway, 1997; Erickson, et al., 2012; Heikkinen, 1999; McIver and Yandell, 1998; McIver, et al., 1997; Tilman, 1978)

  • Animal Foods
  • carrion
  • insects
  • terrestrial non-insect arthropods
  • Plant Foods
  • leaves
  • seeds, grains, and nuts
  • nectar
  • flowers

Predation

There are a variety of predators of Formica obscuripes. As predators themselves, they are aggressive and defend their brood and the aphids that they tend. They can spray formic acid when threatened or attacked. Other insects that gain entry to the nest can pose a threat to the brood. Ant crickets usually live peacefully in the nest, but have been observed attacking larvae. Another ant species, Leptothorax hirticornis, may eat larvae if it gets in the nests and will also attack and eat isolated workers. Many species of spiders are also predators. Many bird species, including eastern kingbirds, western kingbirds, flickers, and American crows feed on Formica obscuripes. Toads are also predators, including Canadian toads and Woodhouse's toads. Bears have also been known to dig open nests to feed. (Conway, 1996a; Conway, 1997; Heikkinen, 1999; Henderson and Akre, 1986; McIver, et al., 1997; Weber, 1935)

Ecosystem Roles

Formica obscuripes is a mutualist with many species. Honeydew plays a significant role in this species' diet. In exchange for collecting and eating honeydew from the insects that it tends, it protects the insects from other predators and parasitoids. It also destroys insects that have been parasitized before the parasitoid completes development. The aphids that it tend includes Pleotrichophorus utensis, Uroleucon escalantii, and Lachnus allegheniensis, as well as members of the following genera Aphis, Macrosiphum, Pleotrichophorus, and many more. It tends honeydew producing membracids, as well as some jumping plant lice, scale insects, mealybugs, and gall wasps. Thatch nests of Formica obscuripes provide shelter to many species of insects and other Arthropods without directly impacting the ants. One such insect is the beetle Euphoriaspis hirtipes. All life stages of this beetle can be found within the thatch. The exact relationship is uncertain, as the ants do not seem to get anything out of the beetles presence and do not even seem to notice them. The beetles can also survive in the thatch without the ants. Other Arthropods found in the thatch and nest of F. obscuripes include pseudoscorpians, springtails, hemipterans, dipterans, and other beetle species. Larvae of these Arthropods often use the thatch or chambers in the nest for hibernation or development and feed on decaying matter. The ants largely ignore them. (Conway, 1997; Erickson, et al., 2012; Grinath, et al., 2012; Henderson and Akre, 1986; Mico, et al., 2000; Risch, et al., 2008; Seibert, 1992; Seibert, 1993)

Ant crickets are one notable species that live in Formica obscuripes nests. Ants are aggressive toward the crickets and will attack if they realize the crickets are there. However, crickets have learned to imitate the way ants use their antennae to identify other individuals and trick the ants into allowing the crickets to stay. Crickets even participate in trophallaxis with the ants. The ants do not seem to gain any benefits from the crickets' presence, while the crickets get shelter, food, and will even attempt to eat larvae if given access. Other ant species have also been documented living in the nests of Formica obscuripes. Formica dakotensis has been found living peacefully in the same mounds as Formica obscuripes in Yellowstone National Park. Leptothorax hirticornis may eat larvae and isolated workers given the opportunity, and Tapinoma sessile often steals honeydew from within the nest, but F. obscuripes does not act particularly aggressive to these other ant species. (Conway, 1996a; Henderson and Akre, 1986; Mico, et al., 2000; Risch, et al., 2008)

Formica obscuripes plays a variety of other roles in the ecosystem. It is prey to a variety of insects and bird species. It also feeds on a large number of other insect species. Ectoparasitic mites of genus Uropoda are often found on both workers and sexuals, often in the joints of the legs. The wasp Elasmosoma michaeli is a parasitoid of F. obscuripes workers. The wasp lays eggs in the abdomens of worker ants, killing the ants upon hatching. As a significant aphid-tending ant species, Formica obscuripes can play a role in determining the density of other arthropods and herbivores in their habitat. In some habitats, such as a coastal dune habitat, Formica obscuripes is a keystone species. It reduces competing herbivores on the aphid-infested plants, while also increasing arthropod density by creating new shelters by rolling leaves on which the aphids live. Formica obscuripes is also known to collect seeds and bring them back to the nest. It eats the edible part and stores the rest of the seed in chambers of the nest. These chambers can often be a good habitat for the plant to grow and develop, allowing the ant to aid in seed dispersal. (Berg-Binder and Suarez, 2012; Conway, 1996a; Crutsinger and Sanders, 2005; Shaw, 2007)

Mutualist Species
Commensal/Parasitic Species

Economic Importance for Humans: Positive

Formica obscuripes colonies likely eat insects and other arthropods that can be pests to their habitat (particularly forest defoliators), as well as insects that could be pests to humans. (McIver, et al., 1997)

  • Positive Impacts
  • controls pest population

Economic Importance for Humans: Negative

There are no known adverse effects of Formica obscuripes on humans.

Conservation Status

Formica obscuripes has no special conservation status.

Contributors

Angela Miner (author), Animal Diversity Web Staff, Leila Siciliano Martina (editor), Animal Diversity Web Staff.

Glossary

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carrion

flesh of dead animals.

chemical

uses smells or other chemicals to communicate

colonial

used loosely to describe any group of organisms living together or in close proximity to each other - for example nesting shorebirds that live in large colonies. More specifically refers to a group of organisms in which members act as specialized subunits (a continuous, modular society) - as in clonal organisms.

desert or dunes

in deserts low (less than 30 cm per year) and unpredictable rainfall results in landscapes dominated by plants and animals adapted to aridity. Vegetation is typically sparse, though spectacular blooms may occur following rain. Deserts can be cold or warm and daily temperates typically fluctuate. In dune areas vegetation is also sparse and conditions are dry. This is because sand does not hold water well so little is available to plants. In dunes near seas and oceans this is compounded by the influence of salt in the air and soil. Salt limits the ability of plants to take up water through their roots.

detritus

particles of organic material from dead and decomposing organisms. Detritus is the result of the activity of decomposers (organisms that decompose organic material).

diurnal
  1. active during the day, 2. lasting for one day.
ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

eusocial

the condition in which individuals in a group display each of the following three traits: cooperative care of young; some individuals in the group give up reproduction and specialize in care of young; overlap of at least two generations of life stages capable of contributing to colony labor

fertilization

union of egg and spermatozoan

forest

forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.

heterothermic

having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

hibernation

the state that some animals enter during winter in which normal physiological processes are significantly reduced, thus lowering the animal's energy requirements. The act or condition of passing winter in a torpid or resting state, typically involving the abandonment of homoiothermy in mammals.

internal fertilization

fertilization takes place within the female's body

keystone species

a species whose presence or absence strongly affects populations of other species in that area such that the extirpation of the keystone species in an area will result in the ultimate extirpation of many more species in that area (Example: sea otter).

metamorphosis

A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

omnivore

an animal that mainly eats all kinds of things, including plants and animals

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

pheromones

chemicals released into air or water that are detected by and responded to by other animals of the same species

polymorphic

"many forms." A species is polymorphic if its individuals can be divided into two or more easily recognized groups, based on structure, color, or other similar characteristics. The term only applies when the distinct groups can be found in the same area; graded or clinal variation throughout the range of a species (e.g. a north-to-south decrease in size) is not polymorphism. Polymorphic characteristics may be inherited because the differences have a genetic basis, or they may be the result of environmental influences. We do not consider sexual differences (i.e. sexual dimorphism), seasonal changes (e.g. change in fur color), or age-related changes to be polymorphic. Polymorphism in a local population can be an adaptation to prevent density-dependent predation, where predators preferentially prey on the most common morph.

seasonal breeding

breeding is confined to a particular season

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

tactile

uses touch to communicate

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

terrestrial

Living on the ground.

tropical savanna and grassland

A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.

savanna

A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.

temperate grassland

A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.

visual

uses sight to communicate

References

Beattie, A., D. Culver. 1977. Effects of the Mound Nests of the Ant, Formica obscuripes, on the Surrounding Vegetation. American Midland Naturalist, 97/2: 390-399.

Berg-Binder, M., A. Suarez. 2012. Testing the directed dispersal hypothesis: are native ant mounds (Formica sp.) favorable microhabitats for an invasive plant?. Oecologia, 169/2: 763-772.

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Shaw, S. 2007. A new species of Elasmosoma ruthe (Hymenoptera : Braconidae : Neoneurtnae) from the northwestern United States associated with the western thatching ants, Formica obscuripes Forel and Formica obscuriventris clivia Creighton (Hymenoptera : Formicidae). Proceedings on the Entomological Society of Washington, 109/1: 1-8.

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