Fitness and habitat segregation of British tree‐hole mosquitoes

.1 Populations of three mosquitoes, Aedes geniculutus (Olivier), Anopheles plumbeus Stephens and Culex torrentium Martini, were found in three types of tree holes: rot holes penetrating the bark of the host tree and exposed, or deeper, more cryptic, bark‐lined buttress holes. Aedes geniculatus occupied all three tree‐hole types but reached their greatest abundance in exposed buttress holes; A.plumbeus predominated in rot holes; C.torrentium occupied buttress holes exclusively. 2 After a 1 month period of winter freezing, larvae of A.plumbeus survived better than larvae of A. genicularus . Freeze resistance of A.plumbeus increased from cryptic buttress to rot holes. Freeze resistance of A.geniculatus declined from exposed buttress to cryptic buttress to rot holes and was markedly higher in more protected than in more exposed macrohabitats. 3 Among manipulated larval cohorts of A.geniculutus and A.plumbeus , survivorship, pupation success, pupal weight, and/or biomass yield did not differ among tree‐hole types or was the reverse of freeze‐related survivorship. Despite seasonal differences in fitness correlates, annual fitness may be similar among tree‐hole types for both species. Manipulations carried out at a single census or during a single season are therefore likely to produce misleading information about fitness variation among sub‐habitats. 4 Both species should exploit a broad range of tree‐hole types but A.plumbeus occupies more restricted types of tree holes than does A.geniculutus . The restricted habitat usage by A.plumbeus does not parallel their fitness among tree‐hole types in southern Britain but does resemble that of other tree‐hole anophelenes. We therefore propose that habitat selection by A.plumbeus is more likely due to their phylogenetically determined physiological tolerances and behaviour than to competition from coexisting A.geniculatus .