Pogonomyrmex occidentalis

Pogonomyrmex occidentalis, or the western harvester ant, is a species of ant that inhabits the deserts and arid grasslands of the American West at or below 6,300 feet (1,900 m).[2] Like other harvester ants in the genus Pogonomyrmex, it is so called because of its habit of collecting edible seeds and other food items. The specific epithet "occidentalis", meaning "of the west", refers to the fact that it is characteristic of the interior of the Western United States; its mounds of gravel, surrounded by areas denuded of plant life, are a conspicuous feature of rangeland. When numerous, they may cause such loss of grazing plants and seeds, as to constitute both a severe ecological and economic burden. They have a painful and venomous sting.[3]

Western harvester ant
P. occidentalis worker from California
Scientific classification
Kingdom:
Animalia
Phylum:
Arthropoda
Class:
Insecta
Order:
Hymenoptera
Family:
Formicidae
Subfamily:
Myrmicinae
Tribe:
Myrmicini
Genus:
Pogonomyrmex
Species:
P. occidentalis
Binomial name
Pogonomyrmex occidentalis
(Cresson, 1865) [1]

Colonies

Mature colonies consist of up to 20,000 workers[4] and one queen.[5][6] A queen can live up to 40 years, and many colonies survive for 20 years.[4][7][8] Colonies of the close relative, P. owyheei, contain 1000–3000 brood items [6] and can produce 1000 reproductives, or alates, in a year.[6] A colony inhabits a nest that is up to 5 metres (16 ft) deep.[9] The queen stays at the bottom of the nest, and workers usually relocate themselves and brood within the nest, capturing safe levels of heat.[9][10] A colony's nest is topped by an irregularly conical nest mound[10] that can be more than 89 centimetres (35 in) in diameter.[11] The composition, shape, and size of the mound differ across plant environments.[10] The mounds of most colonies are surrounded by an area devoid of vegetation, and so do not burn during fires.[10] The soil in the mound is drier than that in the surrounding denuded area. Bigger P. occidentalis colonies (in number of workers) have bigger mounds.[12]

Individual queens found colonies on their own, without workers or other queens. Survivorship of colonies in the first year is negatively correlated with increasing density of foundresses.[13] Foraging workers kill queens that they encounter aboveground and occasionally excavate queens.[14] Factors independent of colony density are responsible for >90% of foundress mortality.[14] Queens in some populations found colonies claustrally[15] and in others, semi-claustrally. Many of the eggs laid in the first batch die or are unembryonated.[15] During colony founding, larvae may eat eggs.[15] The first workers produced, known as nanitics, are stunted. They measure 2 mm long (33% shorter than typical workers of established colonies).[15] 2/188 founding queens survived from July to March.[11]

Workers

Workers' bodies are usually dark red and those in a mature colony are on average 6 mm long.[15] Workers vary in size, but are not subdivided into groups of different-sized individuals with special roles. A worker lives for an average of six months, and as it ages, it usually progresses through different roles within the colony. For example, workers forage towards the ends of their lives.[16] In the field, workers are active when the temperature at the surface of the colony's mound is 25–53 °C (77–127 °F).[17]

Nutrition

Workers harvest seeds and pollen directly from plants and gather fallen seeds. Some seeds are sometimes stored in chambers within the nest and are depleted during winter. Workers also gather newly dead insects. Workers generally forage April–September.[17] Workers generally forage throughout the day during cooler months and only 5–11 am and 3–9 pm during the summer.[17] Genetically diverse colonies forage for more hours daily.[17] Foraging times within a day and foraging temperature range vary consistently among colonies.[17] Given a choice, workers select seeds containing more energy.[18] Given a choice, workers select a diversity of seeds or seeds that are new to the colony.[18] Workers usually forage one kind of item each day, but change their specialty daily.[18] Workers usually forage in one direction over and over, even across days.[16] Colonies recruit more workers (from the total worker pool) to forage at a good food source.[16] Colonies lose foragers in encounters with neighboring colonies.[19] Workers defend foraging territories against neighboring colonies. Different plant environments support different densities of colonies. Foragers produce more period mRNA during darkness, the timing of which varies seasonally.[20]

Association with Myrmecocystus mexicanus

Cole et al. surveyed the distribution of Myrmecocystus mexicanus nests, (a species of North American honey ant) relative to the distribution of P. occidentalis nests at a site in western Colorado and found that there was a definite spatial association between the two species. They also observed that M. mexicanus feeds on dead or close-to-death P. occidentalis workers, and believed that P. occidentalis provides the main source of food for the M. mexicanus colonies at that site. Thus, the M. mexicanus situates nests nearby P. occidentalis colonies to collect this source of nutrition. There was no substantial evidence that M. mexicanus actively predates healthy P. occidentalis workers.[21]

Biomass and brood production

Different developmental stages of ants within a colony process different kinds of food; larvae ingest solids, while adults ingest liquids, including larval excretions (The Ants). Immature individuals cannot pass from one larval stage to another or to adulthood without the help of adults; adults help immature individuals remove their old larval and/or pupal skins during ecdysis (molting). As larvae are relatively immobile, they only interact with nutrients as adults bring the nutrients to the larvae or the larvae to the nutrients. Bigger colonies do not necessarily produce a greater total reproductive biomass.[22] Colonies stop producing brood before they overwinter.[23]

Mating

Colonies release alates synchronously.[11][22] Alates mate in hilltop leks [11][15][24] in swarms.[25][26] Major mating swarms are about 1.4 kilometres (0.87 mi) apart,[27] and queens can fly no more than 800 metres (2,600 ft).[13] Gynes mate with 2–11 (an average of 6.3) genetically distinct males.[28] Females always mate multipally.[15] Queens that mate only a few times are less successful.[29] The colonies of queens that mate with more males grow faster.[28] Males sometimes mate multiple times.[15] Females mate nonrandomly.[30] Larger males are more successful at mating (i.e. they are overrepresented among collected maters), but small males can still mate.[24][31] Certain shape characteristics improve male chances of mating success.[24] P. occidentalis populations are effectively small and inbred.[5] Queens pick bare and bright areas to land and then dig where they land.[15] Mating swarms that are consistently present and large determine much of the spatial variability in colony density and emerge over the long term.[27] New colonies are founded in a clumped pattern, around the mating sites.[11][13] The population may self-thin through direct interference competition[11] resulting in a uniformly overdispersed distribution pattern.[11] Long-term colony survival is mediated by proximity to older colonies.[32] Smaller colonies have closer nearest neighbors.[11] Smaller nests are more likely to die.[11] The further a colony is from its nearest neighbor (especially for small colonies), the higher the colony's survival probability.[11] Colony age and size are correlated, especially in young colonies.[22]

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References
  1. "Pogonomyrmex occidentalis (Cresson, 1865)". Integrated Taxonomic Information System. Retrieved February 11, 2011.
  2. Cole
  3. Capinera, John L. Encyclopedia of Entomology. Publisher: Springer 2008. ISBN 978-1402062421
  4. Kathleen H. Keeler (1993). "Fifteen years of colony survivorship in the western harvester ant Pogonomyrmex occidentalis". The Southwestern Naturalist. 38 (3): 286–289. doi:10.2307/3671438. JSTOR 3671438.
  5. Blaine J. Cole & Diane C. Wiernasz (1997). "Inbreeding in a lek-mating ant species, Pogonomyrmex occidentalis". Behavioral Ecology and Sociobiology. 40 (2): 79–86. doi:10.1007/s002650050318. JSTOR 4601301.
  6. R. J. Lavigne (1969). "Bionomics and nest structure of Pogonomyrmex occidentalis (Hymenoptera: Formicidae)". Annals of the Entomological Society of America. 62 (5): 1166–1175. doi:10.1093/aesa/62.5.1166.
  7. Sanford D. Porter & Clive D. Jorgensen (1988). "Longevity of harvester ant colonies in southern Idaho". Journal of Range Management. 41 (2): 104–107. doi:10.2307/3898942. JSTOR 3898942.
  8. Keeler, 1982
  9. Cole, 1994
  10. Cole,1932
  11. Diane C. Wiernasz & Blaine J. Cole (1995). "Spatial distribution of Pogonomyrmex occidentalis: recruitment, mortality and overdispersion". Journal of Animal Ecology. 64 (4): 519–527. doi:10.2307/5654. JSTOR 5654.
  12. Cole and Wiernasz, 1995
  13. Blaine J. Cole & Diane C. Wiernasz (2002). "Recruitment limitation and population density in the harvester ant, Pogonomyrmex occidentalis" (PDF). Ecology. 83 (5): 1433–1442. doi:10.1890/0012-9658(2002)083[1433:RLAPDI]2.0.CO;2. ISSN 0012-9658.
  14. Ian Billick, Diane C. Wiernasz & Blaine J. Cole (2001). "Recruitment in the harvester ant, Pogonomyrmex occidentalis : effects of experimental removal". Oecologia. 129 (2): 228–233. doi:10.1007/s004420100721.
  15. Harold G. Nagel & Carl W. Rettenmeyer (1973). "Nuptial flights, reproductive behavior and colony founding of the western harvester ant, Pogonomyrmex occidentalis (Hymenoptera: Formicidae)". Journal of the Kansas Entomological Society. 46 (1): 82–101. JSTOR 25082548.
  16. Jennifer H. Fewell (1990). "Directional fidelity as a foraging constraint in the western harvester ant, Pogonomyrmex occidentalis". Oecologia. 82 (1): 45–51. doi:10.1007/BF00318532. JSTOR 4219200.
  17. Blaine J. Cole; Adrian A. Smith; Zachary J. Huber; Diane C. Wiernasz (2010). "The structure of foraging activity in colonies of the harvester ant, Pogonomyrmex occidentalis". Behavioral Ecology. 21 (2): 337–342. doi:10.1093/beheco/arp193.
  18. Jennifer H. Fewell & Jon F. Harrison (1991). "Flexible seed selection by individual harvester ants, Pogonomyrmex occidentalis". Behavioral Ecology and Sociobiology. 28 (6): 377–384. doi:10.1007/bf00164118. JSTOR 4600566.
  19. Joseph De Vita (1979). "Mechanisms of interference and foraging among colonies of the harvester ant Pogonomyrmex californicus in the Mojave desert". Ecology. 60 (4): 729–737. doi:10.2307/1936610. JSTOR 1936610.
  20. Krista K. Ingram, Scott Krummey & Michelle LeRoux (2009). "Expression patterns of a circadian clock gene are associated with age-related polyethism in harvester ants, Pogonomyrmex occidentalis". BMC Ecology. 9: 7. doi:10.1186/1472-6785-9-7. PMC 2676274. PMID 19374755.
  21. Cole, Blaine J., Kevin Haight, and Diane C. Wiernasz. "Distribution of Myrmecocystus mexicanus (Hymenoptera: Formicidae): Association with Pogonomyrmex occidentalis (Hymenoptera: Formicidae)." Annals of the Entomological Society of America. Vol. 94, No. 1 (2001), pp. 59-63.
  22. Blaine J. Cole & Diane C. Wiernasz (2000). "Colony size and reproduction in the western harvester ant, Pogonomyrmex occidentalis". Insectes Sociaux. 47 (3): 249–255. doi:10.1007/PL00001711.
  23. A. Cole, Jr. (1934). "The relation of the ant, Pogonomyrmex occidentalis Cr., to its habitat". Ohio Journal of Science. 32 (2): 133–146. hdl:1811/2557.
  24. Allison J. Abell; Blaine J. Cole; Ruth Reyes; Diane C. Wiernasz (1999). "Sexual selection on body size and shape in the western harvester ant, Pogonomyrmex occidentalis Cresson" (PDF). Evolution. 53 (2): 535–545. doi:10.2307/2640789. JSTOR 2640789.
  25. J. F. Mull & Thomas O. Crist (1993). "Timing of mating flights in the western harvester ant, Pogonomyrmex occidentalis Cresson (Hymenoptera: Formicidae)". Journal of the Kansas Entomological Society. 66: 372–374.
  26. Wiernasz, 1995
  27. Ian Billick, Blaine J. Cole & Diane C. Wiernasz (2004). "Scale of recruitment limitation in the western harvester ant (Hymenoptera: Formicidae)". Annals of the Entomological Society of America. 97 (4): 738–742. doi:10.1603/0013-8746(2004)097[0738:SORLIT]2.0.CO;2. ISSN 0013-8746.
  28. Diane C. Wiernasz, Christina L. Perroni & Blaine J. Cole (2004). "Polyandry and fitness in the western harvester ant, Pogonomyrmex occidentalis". Molecular Ecology. 13 (6): 1601–1606. CiteSeerX 10.1.1.131.8368. doi:10.1111/j.1365-294X.2004.02153.x. PMID 15140102.
  29. Cole and Wiernasz, 1999
  30. Wiernasz et al., 1995
  31. Diane C. Wiernasz; Amy K. Sater; Allison J. Abell; Blaine J. Cole (2001). "Male size, sperm transfer, and colony fitness in the western harvester ant, Pogonomyrmex occidentalis". Evolution. 55 (2): 324–329. doi:10.1554/0014-3820(2001)055[0324:msstac]2.0.co;2. JSTOR 2640754. PMID 11308090.
  32. Ryti, R.T. and T.J. Case. 1988. The regeneration niche of desert ants: effects of established colonies. Oecologia 75:303-306.
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