Detritus that forms the basis for mosquito production in tree hole ecosystems can vary in type and timing of input. Although plant-derived material usually dominates the detrital pool, the importance of animal-derived detritus, usually invertebrate carcasses, has recently been recognized as an important contributor to mosquito growth because of its relatively high nutritional value. We investigated the contributions of plant- and animal-derived detritus to the biomass of Ochlerotatus triseriatus (Say) pupae and adults by using stable isotope techniques (¹⁵N, ¹³C) in lab experiments and field collections. We found that lab-reared mosquito isotope signals were characteristic of their detrital resource base, providing a clear distinction between mosquitoes reared on plant or animal detritus. Isotope signals from field-collected pupae were intermediate between what would be expected if a single (either plant or animal) detrital source dominated the resource base. However, values clustered most closely with plant-derived signals and would suggest that plant detritus formed the bulk of the resource base. Pupae collected in the latter part of the sampling period tended to have isotope signals that were consistent with an increased contribution from animal detritus, suggesting this resource became more nutritionally important later in the season.