Great Basin Naturalist

Volume 54 Number 4 Article 8

10-25-1994

Use of a secondary nest in Great Basin Desert thatch ( obscuripes Forel)

James D. McIver Eastern Oregon State College, La Grande, Oregon

Trygve Steen Portland State University, Portland, Oregon

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Recommended Citation McIver, James D. and Steen, Trygve (1994) "Use of a secondary nest in Great Basin Desert thatch ants (Formica obscuripes Forel)," Great Basin Naturalist: Vol. 54 : No. 4 , Article 8. Available at: https://scholarsarchive.byu.edu/gbn/vol54/iss4/8

This Article is brought to you for free and open access by the Western North American Naturalist Publications at BYU ScholarsArchive. It has been accepted for inclusion in Great Basin Naturalist by an authorized editor of BYU ScholarsArchive. For more information, please contact [email protected], [email protected]. Great Basin Naturalist 54(4), © 1994, pp. 359--365

USE OF A SECONDARY NEST IN GREAT BASIN DESERT THATCH ANTS (FORMICA OBSCURIPES FOREL)

James D. McIverl and Trygve Steen2

ABsTRACI:-Workers ofGreat Basin Desert thatch ants (Formica obscuripes Forel) dig simple secondary nests at the base ofplants upon which they tend aphids and scales. These secondary nests house only foragers, with the number of foragers occupying each nest positively correlated with the number of worker-tended Homoptera feeding on plant foliage above. Thatch secondary nests are cooler than 25 em below the dome top ofthe primary nest and maintain a sigoificantly more constant temperature than is observed on the ground surface or in the plant canopy. Thatch ant foragers use secondary nests for at least two purposes: as a cool refuge for Homoptera tenders when midday plant canopy tem­ peratures rise dUring the summer months, and as the primary place within which Homoptera tenders transfer honey" dew to larger "honeydew transporters" for ultimate transport back to the primary nest.

Key words: honeydew harvest, thermal refugia, behavioral thermoregulation, red wood ants, desert adaptation, satel­ lite nests.

Although most ant specIes use a nest struc­ STUDY AREA AND SPECIES ture consisting of a single central location (a primary nest), many species also employ "sec­ Thatch ants were studied between June ondary" nests in which a portion ofthe colony 1987 and September 1991 at Pike Creek, 160 population is dispersed among several alter­ Ian southeast ofBurns, Oregon. The Pike Creek nate sites (Wheeler 1910). study site is at 1300 m elevation at the base of Several species ofCamponotus, for example, Steen's Mountain in the northern Great Basin use a secondary nest to which workers transport Desert. Sagebrush (Artemisia tridentata), rabbit­ late-instar larvae and pupae from a centralloca­ brush (Chrysothamnus nauseosus), horsebrush tion occupied by the queen and brood (Hansen (Tetradymia sp.), lupine (Lupinus caudatus and Akre 1987). Similarly, the dolichoderine Kellogg), and cheatgrass (Bromus spp.) are lridomyrmex sanguineus maintains secondary dominant plants at the site, which was grazed nests containing older larvae and ptipae, but moderately by cattle throughout the study workers bring young from several locations period. within oligogynous colonies (McIver 1991). A total offour colonies ofFormica obscuripes Many other ant species (Polyrachis simplex, Forel were observed for various parts of the Lasius niger; L. emarginatus, Formica pratensis, study. E obscuripes is a widespread and abun~ E exsectoides, Crematogaster pilosus) are known dant North American rufa-group species to use secondary nests in which only foraging (Wheeler and Wheeler 1983). Like E rufa­ workers reside (Forel 1921, Andrews 1929, group species elsewhere, E obscuripes builds Ofer 1970). These secondary nests are thought symmetrical, dome-shaped primary nests of to serve as refuges for the workers from the thatch, from which radiate trunk trails that physical environment, as a defense against access foraging territory. In all four study enemies, or as a protected site within which to colonies workers foraged for honeydew on tend Homoptera for honeydew (Wheeler 1910). sagebrush, rabbitbrush, horsebrush, and/or This paper characterizes the secondary nest lupine, and scavenged for in the area used by the thatch ant Formica obscuripes surrounding each nest. Although broodless Forelliving in the Great Basin Desert and dis­ satellite nests were occasionally observed, cusses its possible function within the context there was no evidence of primary nest poly­ ofthe desert environment. domy in any study colonies.

1Deparnnents ofBiology and Edncation, Eastern Oregon State College, La Grande, Oregon 97850, and Blue Mountains Natural Resources Institute, 1401 Gekeler Lane, La Grande, Oregon 97850. 2Deparnnent ofBiolog;; Portland State University, Box 751, Portland, Oregon 97207.

359 360 GREAT BASIN NATURALIST [Volume 54

METHODS AND MATERIALS tended Homoptera at Pike Creek during the study period. Viewed from above, secondary Secondary Nest Charactelistics nests were simple openings in the ground The aboveground structure of the sec­ adjacent to plant trunks (Fig. 1). Ground ondary nest is portrayed by a photograph taken around an opening was typically littered with from colony 5 at Pike Creek, August 1988. The thatch material, fallen from the plant canopy, belowground structure was investigated by blown in, or excavated from the gallery pouring a measured quantity ofdentallabstone beneath. down 10 different secondary nest entrances of Volume of10 secondary nests beneath active two colonies (colonies 4 and 26) during August tending groups of workers ranged from 35 to 1991. Quantity oflabstone required to fill each ' 125 cc. Secondary nest volume was not signifi­ secondmy nest was then correlated with basal cantly correlated with basal plant dimneter (R2 plant dimneter and number ofworkers tending = .02, P > .05, N == 10) but was significantly Homoptera in the plant canopy. Actual struc­ cOlTelated with number of tenders (R2 = .33, ture of the secondary nest interior was deter­ P < .05, Y = .54X + 43.3, N = 10). mined by excavating two nests, photographing Excavations of secondary nests into which the labstone "plug" in place, and drawing one labstone had been poured revealed that cavities ofthese to scale using the photograph as refer­ essentially conformed to morphology of the ence. plant trunk itself (Fig. 2). Thatch ant workers Temperature at 6 cm depth in a typical sec­ typically removed dirt, small stones, and other ondary nest (plant A, colony 2) was measured debris from within 5-20 mm of the plant during summer 1987 and compared to mea­ trunk, leaving a cavity punctuated with large surements for tending localities in the plant stones and roots. The nest represented in canopy, ground surface, and 25 cm below the Figure 2 was 10.8 cm deep and consisted of top ofthe primary nest dome. three separate chmnbers totaling 175 cc in vol­ Secondary Nest Use ume. Temperature within the secondary nest dif­ Use of the secondary nest by thatch ant fered considerably from temperatures record­ workers was explored by conducting intensive ed Simultaneously on the ground swface, in the observations on a selected sagebrush plant plant canopy, or deep within the primary nest (plant 13) at the Pike Creek study site during (Fig. 3). Over the l-wkperiod 13-19 June 1987, July 1987. Beginning 1 July 1987, thatch ants for exmnple, the secondary nest we measured working in the vicinity of plant 13 were indi­ vidually marked with "beenumbers" (Charles was an average ofabout 1°C cooler than 25 cm Graz Co., Frankfurt, ) so that the from the dome top ofthe primary nest (18.8 ° vs. activity pattern of each could be determined. 26.1°), with a little over twice the variance over By 23 July, a total of 66 workers had visited time (12.6 vs. 5.7). Compared to ground surface, plant 13 and been marked, 30 of which were the secondary nest was slightly cooler (18.8 ° still using the plant daily. At noon on 23 July, vs. 19.2°) but much less variable, exhibiting a we began a 24-h continuous period ofobserva­ variance of about one-ninth the ground sur~ tion ofworker behavior on plant 13. We record­ face (12.6 vs. 112.4). Compared to the canopy of ed the location and task of each worker at 15­ the smne plant, the secondary nest was slightly min intervals throughout the 24-h period and warmer on average (18.8° vs. 18.0°) but about noted its interaction with other workers. The one-fifth as variable (12.6 vs. 67.1). Temperature result was a time budget for 30 different work~ trends over the entire summer were similar to ers that frequented plant 13 during the 24-h those measured in this l-wk smnple period in period, from which we could infer how work­ lnid-June. ers ofvarious task specializations used the sec­ Secondary Nest Use ondary nest at the plant base. Observations of individually marked work­ RESULTS ers on plant 13 of colony 2 clearly show that the secondary nest is used throughout the day Secondary Nest Characteristics (Fig. 4). The greatest percentage of workers Thatch ant secondary nests were found at was found in the secondary nest during lnid­ the base of each plant upon which workers afternoon, cOlTesponding to highest daily tem- 1994] SECONDARY NESTS IN THATCH ANTS 361

Fig. 1. Aboveground appearance ofsecondary nest at base ofsagebrush plant, Pike Creek, southeastern Oregon, June 1994 (photograph by Trygve Steen). peratures. Secondary nest population was low­ mary nest, worker 13 often had a distended est between 1700 and 2000, and between 0600 gaster, indicating a crop swollen with honey­ and 0900, during principal times when work­ dew. Typically, workers like #13 were scaven­ ers deliver honeydew to the primary nest. gers, secondary nest excavators, and/or honey­ Two typical patterns of activity were dew transporters, receiving the majority of observed for plant-associated workers (Fig. 5). their honeydew from workers that concentrat­ Tenders spent the majority oftheir time tending ed on tending Homoptera in the plant canopy. Homoptera for honeydew in the plant canopy. Of the 30 workers associated with plant 13 Worker 84, for example, spent 54% ofher time during the intensive observation period, 19 tending aphids, with each visit to the plant were classified as tenders, 6 as honeydew transporters/scavengers, 2 had behavior inter­ canopy lasting between 2 min and about 3 h. mediate between tender and b'ansporter/scav­ Her visits to the plant canopy were inter­ enger, and 3 were not observed often enough spersed with frequent visits to the secondary to classify. nest at the plant base, where it is likely she transferred honeydew to larger nontending DISCUSSION individuals like worker 13 (chain transport). Twice per day she returned to the primary Great Basin Desert thatch ants use sec­ nest: once in the early evening and once in the ondary nests as a refuge from high midday morning. temperatures and as a site within which honey­ Honeydew transporters spend the majority dew is transferred from workers who collect it of their time in the secondary nest itself. in the plant canopy to those who help trans­ Worker 13, for example, spent 66% ofher time port it back to the primary nest. Ground tem­ in the secondary nest, 23% scavenging on the peratures above 50 0 C have been reported as ground surface, and 9% on twice-daily returns lethal to F. obsGuripes (O'Neill and Kemp to the primary nest. On her returns to the pri- 1990), and Mackay and Mackay (1984) 362 GREAT BASIN NATURALIST [Volume 54

.~

Q o

o Fig. 2. Scale drawing ofsecondary nest, taken from photograph of labstone plug, Pike Creek, southeast­ ern Oregon, August 1991.

observed that E haemorrhoidalis workers hide Thatch ants living at other sites in the Great under pine cones or retreat to shady places Basin also use secondary nests of this kind during midday heat. Chain transport appears (McIver personal observation); Weber (1935) to be an effective way to increase delivery of described secondary nests in his study of honeydew to the primary nest (McIver and South Dakota thatch ants. However, Weber Yandell 1994); thus, it is not surprising that reported that the function of these nests was honeydew transfer occurs at a site offering to serve as (1) an arborescent chamber within refuge from midday heat. which to tend Homoptera and (2) a potential The use of cool midday refugia by workers site for development into primary nests. may also reduce metabolic costs and increase Certainly, colonies of -group worker longevity. In a study on fire ant thermal species often reproduce by budding (Mabelis preferences, Porter and Tschinkel (1993) report~ 1979; E polyctena), and the site of a new pri­ ed that fire ant workers consistently choose mary nest is very often a secondary nest cooler temperatures than those selected for (Scherba 1959, McIver personal observation). the brood. They postulate that this tendency It is not known whether E rofa-group species increases longevity of workers not directly living in other habitats employ secondary associated with brood care. This idea is sup­ nests for these or other reasons. ported by Calabi and Porter (1989), who Other Formica species are also known to demonstrated that because temperature and employ secondary nests. The mound-building metabolic rate are highly corrrelated, fire ants ant E exsectoides (exsectoides-group) uses sec­ reared and maintained under high tempera­ ondary nests as shelters for b:eehoppers and as ture regimes have lower longevity. sites for food exchange (Andrews 1929). 1994] SECONDARY NESTS IN THATCH ANTS 363

GROUND Pike Creek Colony 2 -- 1987 50 / o 40 o w a: PRIMARY, 25cm below dometop ::> 30 ,.... ~ , w ,. a.. , :lE 20 .: •...... \ w ...... -'. I-

10 SECONDARY

o 8 16 0 8 16 0 8 16 0 8 1$ 0 8 1$ 0 8 16 0 a 1$ 13 June 14 June 15Jun~ 16June 17 June 18 June t9June

TIME

Fig. 3. Temperature (OC) during week of 13-19 June 1987, on ground surface, in sagebrush canopy, 25 cm below dome top ofprimary nest, and in secondary nest ofcolony 2, Pike Creek, Oregon.

Formica integra of North America and F. ported by the National Geographic Society pratensis of Europe construct secondary nests and the Systematic Entomology Laboratory of along covered paths (Wheeler 1910, Forel Oregon State University (Dr. John Lattin), 1921). where voucher specimens are held. Other Homoptera~tendingants, including the formicines Lasius niger (Forel 1921), L. LITERATURE CITED emarginatus (ForeI1921), L. flavus (Soulie ANDREWS, E. A. 1929. The mound_building ant, Fonnica 1961), and Polyrachis simplex (Ofer 1970), and exsectoides E, associated with treehoppers. Annals of the myrmicines Crematogaster pilosus (Fore! the Entomological Society ofAmerica 22: 369-391. 1921) and C. auberti (Soulie 1961), use sec~ CALABI, P., AND S. D. PORTER. 1989. Worker longevity in ondary nests as shelters for their homopteran the fire ant Solenopsis invicta: ergonomic considera­ tions ofcorrelations between body size and metabol­ symbiotes. ic rates, Journal of Physiology 35: 643-649. FOREL, A. 1921. Le monde social des fourmis du globe. ACKNOWLEDGMENTS Vols. 1-3. Geneve, Libraire Kundig. 1921-1923. HANSEN, L. D., AND R. D. AlmE. 1987. Biology of carpen­ ter ants. Pages 274-280 in R. K. Vander Meer, K. Bryce Kimberling drew the secondary nest Jaffe, and A. Cedeno, eds., Applied myrmecology-a from photographs. We thank Courtney Loomis, world perspective. Westview Press, Boulder, Deborah Coffey, Joseph Furnish, and Bill Colorado. Clark for assistance in the field. Jeffrey C. MABELIS, A. A. 1979. Nest splitting by the red wood ant (E polyctena Foerster). Journal of Zool­ Miller provided the datapod for temperature ogy 29: 109-125. recordings. Andre Francour kindly identified MACKAY, E. E., AND W P. MACKAY. 1984. Biology of the Formica obscuripes Fore!. Research was sup- thatching ant Formica haemorrhoidalis Emery 364 GREAT BASIN NATURALIST [Volume 54

• SI;CONDARY NEST D PLANT CANOPY !ill] GROUND 1]1] PRIMARY NEST

...... I- 30 c::i fi) 15 ~ 6 Cf) Cf) ....J W r:r:w o ~ o a: -..- o 1-25 ~ ~ 10 ::> o ..... w a:

o I 1200 1400 1600 1800 2000 2200 2400 200 400 600 800 1000 1200 TIME OF DAY

Fig. 4. Activity ofmarked workers ofplant 13, Pike Creek colony 2, 23-24 July 1981. Number ofworkers observed in secondary nest, in plant canopy, on ground, at primary nest, and temperature in degrees Celsius over 24-h period.

% TlMEi LOCATION Worker 84: tender, Honeydew Transporter 54% PLANT CANOPY -

2% GROI,IND

36% SECONDARY NEST '- '---- '-- '-

~ ~ ~ ~ ~ 12 13 14 15 18 17 18 18 1 2 3 4 5 e 7 8 10 11 12 mJdnlQI't

8% PRIMARY NEST

Worker 13: Scavenger, Honeydew Transporter 2% PLANT CANOPY

23% GROUND

66% SECONDARY NEST

12 13 14 15 18 17 18 1; ~ ~ ~ ~ ~ 1 2 3 4 5 e 7 8 a 10 11 12 mIdnIott

9% PRIMARY NEST

Fig. 5. Activity over 24-h period ofworkers 84 and 13 onplant 13, colony 2, Pike Creek, Oregon, 23-24 July 1987. 1994] SECONDARY NESTS IN THATCH ANTS 365

(: Fonnicidae). Pan-Pacific Entomology SOULIE, J. 1961. Les nids et Ie comportem~nt nidificateur 60: 79-83. des Fourmis du genre Crematogaster d'Europe, McIvER, J. 1991. Dispersed central-place foraging in d'Afrique du nord et d'Asie du sud-est. Insectes Australian meat ants. Insectes Sociaux 38: 129-137. Sociaux 8: 213-297. ' McIVER, J., AND K. YANDELL. 1995. Honeydew harvest in WEBER, N. A. 1935. The biology of the thatching ant, Great Basin Desert thatch ants. National Geographic Formica rufa Obscllripes Fore!, in North Dakota. Research 11: In press. Ecology 5: 166-206. l OFER, J. 1970. Polyrachis simplex, the weaver ant ofIsrael. WHEELER, w: M. 1910. Ants. Their structure, develop­ Insectes Sociaux 17: 49-82. ment and behavior. Columbia University Press, New O'NEILL, K. M., AND w: P. KEMP. 1990. Worker response York. 663 pp. to thermal contraints in the ant Formica obscllripes WHEELER, G. C., AND J. N. WHEELER. 1983. The ants of (H:F). Journal ofThermal Biology 15: 133-140. Nevada. Natural History Museum of Los Angeles PORTER, S. D., AND w: R. TSCHINKEL. 1993. Fire ant ther­ County, Los Angeles, California. 138 pp. mal preferences: behavioral control of growth and metabolism. Behavioral Ecology and Sociobiology Received 24 August 1993 32: 321-329. Accepted26January 1994 SCHERBA, G. 1965. Analysis of inter-nest movements by workers of the ant Formica opaciventris Emery. Behavior 12: 508--512.