Mosquito larval development in container habitats: the role of rotting Scirpus californicus.

Larvae of. Culex tarsalis Coquillett and Culiseta inornata Williston are abundant in duck ponds within the San Joaquin freshwater marsh. Culex tarsalis are found year-round and Culiseta inornata are found during the winter months. Each duck pond has an area ofopen water ringed with emergent Scirpus californicus (bulrush) and Typha latifolia and Typha anpstifolia (cattails). Low densities of larvae are found in the areas of emergent vegetation, with species composition changing with season. The ponds dry seasonally and the emergent vegetation lodges and forms rafts of decaying vegetation when the ponds are reflooded. In these rafts. Iarval densities can be quite high. Rafts of decaying Scirpus are common in the leeward corners of the duck ponds and provide good larval habitats, with larval densities reaching 40 per dip with a mean value of 12 per dip (n = 17 sites) (Bradley et al. 1986). In manipulative field experiments, we were able to increase Iarval densities of unproductive corners of two duck ponds by introducing rafts of decaying Scirpus. Our ponds are stocked with Gambusin affinis, which reproduce and build up sizeable populations in summer and fall. The high densities of mosquito larvae found in the rotting Scirpus mats may reflect a failure of mosquitofrsh to achieve control in this microhabitat. An analogous effect of abundant aquatic vegetation on control by Gambusia has been observed for Arwphcles occi.dentalis Dyar and Knab larvae by sago pondweed (Potamogeton) (Collins et al. 1983, Collins and Resh 1984, Feminella and Resh 1986). Schaefer and Miura (1985), found high densities of Culcx spp. and Aedes melanimonDyat in a habitat similar to our duck ponds, i.e., ponds containing lodged TWlw. This study examined the reasons mosquito Iarvae are found in gxeater abundance in decaying Scirpus mats than in open water. Previous work in similar habitats. as well as our own observations, suggests that Scirpus mats provide a refuge from mosquitofish predation. This is a difficult hypothesis to test by field observations. We designed an experiment using containerized field habitats to specifically test this hypothesis.