Winter cold influences the spatial and age distributions of the North American treehole mosquito Anopheles barberi.

We conducted experiments to assess the importance of winter cold and photoperiod as factors affecting the spatial and age distributions of overwintering larvae of the treehole mosquito Anopheles barberi. Larval dormancy in A. barberi was induced by photoperiods with 14.75 h of light or less per 24 h cycle. About 75% of the larvae entering dormancy were in the second instar regardless of photoperiod. Dormant second instar larvae survived freezing at-15° C for 24 h better than dormant third instar larvae. Larvae were more likely to survive freezing at-15° C in water from treeholes in which they were commonly found in nature than in water from treeholes in which they were unlikely to occur. Female oviposition was significantly higher into water from treeholes in which larvae were likely to be found than in either water from treeholes in which larvae were not commonly found or distilled water. These findings suggest that, in the northern part of its range, the distribution of A. barberi and the age structure of overwintering cohorts are influenced by extreme winter cold. The mechanisms responsible for the distribution of larvae and the overwintering age structure are, respectively, female oviposition behavior and larval photoperiodism.