Seasonal availability of resources and habitat degradation for the western tree-hole mosquito, Aedes sierrensis

The nutrient base of aquatic tree-hole commu- nities is derived from leaf litter, benthic detritus, and wa- ter flowing down the tree trunk (stemflow water). Previ- ous studies in eastern North America with the mosquito, Aedes triseriatus, have identified leaf litter as a major and stemflow water as a minor source of mosquito nutri- tion, but did not consider the role of the benthic detritus or how the aggregate or relative contribution of these sources of mosquito nutrition changed during the year. We use the leaf litter, benthic detritus, and stemflow wa- ter from tree holes in western Oregon (USA) to deter- mine how these substrates affect mass at metamorphosis, biomass yield, and fitness (cohort replacement rate; R0) of the mosquito, Aedes sierrensis, through both natural and simulated winters, the normal growing season for larvae in tree holes. We found that fresh leaf litter consti- tutes the major determinant of mosquito fitness by a fac- tor of >15:1 over any other substrate taken directly from tree holes in nature. The other substrates, including the benthic detritus, individually make only a meager contri- bution to mosquito fitness but, when added to the leaf lit- ter, can sustain yield and improve fitness at high, limit- ing larval densities. Nutritional quality of tree-hole sub- strates declines by >90% from early (fall) to late (spring) in the larval growing season. At both times of year, the coarse or fine detritus provide minor resources, and stemflow water provides no detectable contribution to mosquito nutrition. The resources in the litter are not transported during the year to the benthic detritus; rather, these resources are either exploited by mosquitoes when they first become available, or they deteriorate and be- come progressively more unavailable to them. Growth and development of A. sierrensis feeding on dried and reconstituted tree-hole contents during a 6-month simu- lated winter in the laboratory showed: (1) the same rela- tive contributions of leaf litter, benthic detritus, and stemflow water to mosquito nutrition, (2) that the winter deterioration of substrate quality is a direct consequence of microbial decomposition, and (3) that pre-emptive competition from pre-existing A. sierrensis greatly in- creases substrate deterioration. We conclude that the pro- gressive winter deterioration of larval resources in com- bination with the dry summers of western North America are the most likely environmental factors that limit spe- cies diversity in tree holes and that have selected for ear- ly recruitment (autumnal hatching) of A. sierrensis and for its univoltine life cycle from Mexico to Canada.