Lasius subumbratus

This species exhibits temporary social parasitism. Queens found new colonies by infiltrating an established nest of Lasius neoniger or Lasius pallitarsis, killing the queen and using host workers to care for her initial brood.

Identification
A close relative of Lasius umbratus sympatric with that species over most of its range and best distinguished from it by differences in body pilosity.

Mackay and Mackay (2002) - These ants are yellow or pale brown, with small eyes (about 0.13 - 0.15 mm in greatest diameter, with about 50 ommatidia). The longest hairs on the posterior half of the first gastral tergite are at least 0.60 times as long as the maximum width of the posterior tibia at mid length. The erect hairs are dense enough that their length is less than the distance between their tips. The apex of the petiole is flat or slightly concave, as seen from the front.

Key to Lasius workers, queens, males

Distribution
From the maritime provinces of Canada to western North America.

Distribution based on Regional Taxon Lists
Nearctic Region: Canada, United States. Neotropical Region: Mexico.



Biology
Wilson (1955) - Cole's New Mexican collections were made under stones in a variety of habitats, including a dry, open slope, an alpine meadow, and spruce-aspen, spruce-pine, and pine-aspen forests. Wheeler (1917b) found it common at Cloudcroft, New Mexico, under stones in pine forest. I found two colonies in pine-fir forest on a southern slope of the San Francisco Mountains of Arizona, one under a stone and one under a rotting log. In the southern Rocky Mountains Lasius subumbratus is clearly a high-elevation species; thus far it has been found only well above the lower elevational limits of sympatric populations of Lasius pallitarsis, Lasius neoniger, Lasius crypticus, and Lasius sitiens, and it ranges at least to the upper elevational limits of these species. In New Mexico Cole encountered it between 7400 and 10,000 feet, and at Cloudcroft Wheeler was unable to find it below 9000 feet. My Arizona colonies were found at about 8000 feet.

Wheeler's studies at Cloudcroft (1917b) leave no doubt that subumbratus is a temporary social parasite of pallitarsis. The alternate host, Lasius neoniger, which he found nesting in open, dry situations, may be pallitarsis also. I have found nothing but pallitarsis in his collections from this locality, and this species was the only member of the subgenus encountered during my own brief visit there. According to Wheeler, subumbratus is abundant enough at Cloudcroft to flood the pallitarsis nests with queens at the time of the nuptial flight. At one spot after a nuptial flight (occurring July 6 or 7) he found dealate subumbratus queens in nearly every nest of the host species uncovered. He observed that these queens approach the pallitarsis workers in a conciliatory manner, that they are often rebuffed at first, and that they sometimes hide in the vicinity of brood piles prior to adoption. He saw one queen in the act of stealing a host pupa and another carrying an uninjured host worker. The latter incident is reminiscent of the conditioning behavior of Lasius umbratus more recently described by several European authors (q.v.). Several pallitarsis colonies with recently adopted parasite queens were found, as well as two colonies containing workers of both species. From these observations Wheeler drew the conclusion that the subumbratus queens are by nature conciliatory, but still find it necessary to acquire some amount of the host nest odor in order to secure final adoption.

Additional evidence is available to indicate that pallitarsis is the principal, if not the only, host. Cole has found mixed subumbratus-pallitarsis colonies at Tesuque Canyon and Eagle Nest, New Mexico; the parasite workers from the latter locality are minimas and much smaller than the associated host workers. In another nest at the second locality a single dealate subumbratus queen was found with pallitarsis workers. Finally, there is in the Museum of Comparative Zoology a series of 6 dealate subumbratus queens associated with pallitarsis (Bedford, Nova Scotia; W. Reig leg.; MCZ).

In New Mexico (Mackay and Mackay 2002) - Ranging from meadows and dry open slopes through mixed forest into ponderosa pine - riparian up to spruce forests. This species nests under stones or logs in loam soils with rocks. It is a temporary social parasite of Lasius pallitarsis. Two dealate females were found in one nest, suggesting that this species may be polygynous. Brood was found in nests in March and August, reproductives were found in nests in July and August. Dealate females were collected in July and August. It tends mealybugs.

Castes

 * Worker

Nomenclature

 *  subumbratus. Lasius umbratus subsp. subumbratus Viereck, 1903: 73 (q.) U.S.A. Wheeler, W.M. 1910e: 238 (w.m.). Combination in L. (Chthonolasius): Emery, 1925b: 235. Raised to species: Creighton, 1950a: 424. See also: Wheeler, W.M. 1917h: 167; Wilson, 1955a: 175.

Worker
Wilson (1955) - (1) Gastric pilosity longer and denser than in Lasius umbratus. The hairs-on the first tergite with a maximum length of about 0.10 mm., or about 0.6 X the maximum width of the hind tibia at its midlength, mostly decumbent-subdecumbent (occasionally tending to suberect), and dense enough for the individual hairs to overlap one another widely. Scapes, femora, and tibiae with dense, predominantly decumbent pubescence and occasional standing hairs (umbratus in all populations with sparser pubescence, often appressed, and the North American population always lacking standing hairs). Gaster with very sparse pubescence, not obscuring in any way the shining cuticular surface.

(2) Eyes smaller than in North American umbratus, the HW-EW regression zones of the two species well separated, although no single absolute measurement will suffice to separate all the series. The Eurasian population of umbratus connects and overlaps the two. (3) Size averaging larger than in umbratus, PW range 0.55-0.85 mm.

(4) Body and appendages uniformly medium yellow, lighter than most North American umbratus and Lasius minutus.

Queen
Wilson (1955) - (1) Pilosity on anterior three gastric tergites very long (maximum length 0.24-0.27 mm., or approximately the maximum width of the hind tibia at midlength), silvery yellow, and predominantly decumbent-subdecumbent. Similar erect hairs form a fringe around the entire dorsal and lateral margins of the petiole; these are often curved toward their tips. Pilosity of alitrunk mostly limited to the dorsal surface, from the posterior margin of the pronotum to the dorsal face of the propodeum, averaging shorter than on the gaster and petiole, maximum length about 0.24 mm., subdecumbent to erect, often curved or sinuate. Pilosity of head mostly limited to the occipital zone, averaging shorter than on the alitrunk, maximum length about 0.18 mm., predominantly subdecumbent-suberect and often curved.

(2) Averaging larger and with proportionately longer appendages than the sympatric North American population of umbratus. Extreme range of HW with attendant SI of series examined 1.56 mm., 85; 1.74 mm., 83.

(3) Color averaging lighter than in umbratus. Body typically medium yellowish brown, head somewhat lighter; appendages light yellowish brown.

Male
(1) Gastric pilosity similar to that of worker in form and inclination, but sparser, more often subdecumbent, and showing only limited overlap between individual hairs; maximum length of hairs about 0.13 mm., or 0.9-1.1 X the maximum width of the hind tibia at midlength. Maximum length of the hairs of the posterior two-thirds of the clypeus 0.12 mm., or slightly less than 0.10 X the HW.

(2) Averaging larger than in umbratus; maximum range in all series studied 1.05-1.17 mm.

(3) Genitalia similar in all respects to those of umbratus.

Type Material
HOLOTYPE. A winged queen in the Academy of Natural Sciences, Philadelphia, in good condition and showing all of the characters used in the diagnosis above. A paratopotype queen, recently compared with the holotype, is in the Museum of Comparative Zoology.