Red wood ants, Formica aquilonia and F. rufa (Hymenoptera: Formicidae), are very common in forests of Sweden. In fact, species of the genus Formica are often the dominant ants of forests throughout the northern hemisphere. F. aquilonia is the most prevalent species in Norway spruce forests of middle Sweden (e.g., Värmland) where they may build nest mounds of dried spruce needles and twigs of a meter or more in diameter and height. The workers vary in size from about 0.5 cm to 1 cm in length with a red thorax, black abdomen, and red and black marked head. The ants are both scavengers and general predators of insects, carrying many soft- bodied caterpillars, flies, and sawflies along their several major trails back to the nest (McNeil et al. 1978; Vielma et al. 1982; Mabelis 1984; Ito & Higashi, 1991; Mahdi & Whitaker 1993). In addition, a major food source is the honeydew of aphids in the genus (Cinara) which are tended and protected by the ants (Holopainen & Soikkeli, 1984). Red wood ants are undoubtedly keystone species (i.e., without them the ecosystem changes fundamentally). When red ants are removed by certain forest practices, it has been found that many herbivorous insects become damaging to forest trees (Skinner & Whittaker 1981; Gösswald 1984; Whittaker & Warrington, 1985). In forests weakened by pollution and acid rain in central Europe, red wood ant populations are often endangered for little known reasons which in turn causes further imbalances in predator-prey dynamics and the ecosystem (Gösswald 1984, 1985).
Gösswald, K. 1984. The relocation of Formica lugubris. Z. Angew.
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Gösswald, K. 1985. Wood ants as indicators of forest diseases with
     special reference to mass breeding of females. Z. Angew. Zool.

Holopainen, J.K., and Soikkeli, S. 1984. Occurrence of Cinara pini
     (Homoptera: Lachnidae) in Scotch pine (Pinus sylvestris)
     seedlings with disturbed growth. Ann. Entomol. Fenn.

Ito, F., & Higashi, S. 1991. An indirect mutualism between oaks and
     wood ants via aphids. J. Anim. Ecol. 60:463-470.

Mabelis, A.A. 1984. Interference between wood ants and other ant
     species (Hymenoptera: Formicidae). Neth. J. Zool. 34:1-20.

Mahdi, T., & Whittaker, J.B. 1993. Do birch trees Betula pendula
     grow better if foraged by wood ants?. J. Anim. Ecol.

McNeil, J.N., Delisle, J., & Finnegan, R.J. 1978. Seasonal
     predatory activity of the introduced red wood ant Formica
     lugubris (Hymenoptera: Formicidae) at Valcartier, Quebec
     Canada, in 1976. Can. Entomol. 110:85-90.

Skinner, G.J., & Whittaker, J.B. 1981. An experimental
     investigation of interrelationships between the wood ant
     Formica rufa and some tree canopy herbivores. J. Anim. Ecol.

Vielma, Orozco, P.E., Mackay, W.P., & Mackay, E.E. 1982. Evaluation
     of Formica haemorrhoidalis as an agent of biological control.
     Southwest. Entomol. 7:263-268.

Whittaker, J.B., & Warrington, S. 1985. An experimental field study
     of different levels of insect herbivory induced by Formica
     rufa predation on sycamore maple (Acer pseudoplatanus) III.
     Effects on tree growth. J. Appl. Ecol. 22:797-812.

image (C) 1995 by John A. Byers