| Summary: | The only National Inventory of red wood ants in Bulgaria was carried out about 50 years ago (1970–1973). Formica rufa Linnaeus, 1761, F. pratensis Retzius, 1783, F. lugubris Zetterstedt, 1838 and F. polyctena (as F. polyctena x rufa hybrid) were found in a current monitoring programme. This study presents data on their current distribution and nest density, and provides more details about the habitat requirements for conservation purposes. Field studies were carried out by the transect method along the main mountainous areas in Bulgaria. We found 256 nests of red wood ants along 172 transects. The most abundant species was F. lugubris, followed by F. rufa and F. pratensis. Among the environmental variables, the elevation, exposure, ecological groups of plants, stone cover, grass cover, canopy cover and forest age appeared as significantly related to the presence and nest density of red wood ants.
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