Surface Conditions Limiting Larval Sites of Certain Marsh Mosquitoes1

Abstract

Literature on the bionomics of mosquitoes that develop in shallow bodies of ground water, while replete with accounts of gross distribution of larvae, leaves much in doubt about the minute aspects of developmental sites. Among the more detailed works dealing with species in the United States are those based on observations in certain large reservoirs and natural lakes of the South where large areas of fairly uniform conditions obtain. Bang et al. (1943) showed that larvae of Anopheles walkeri (Theob.) occurred in Reelfoot Lake in Tennessee wherever the water was densely shaded by emergent vegetation and were especially abundant in communities of cutgrass (Zizaniopsis) shaded by willow or buttonbush. Eyles (1948) demonstrated that larvae of both A. walkeri and A. quadrimaculatus Say were abundant in areas shaded by cutgrass or willow wherever the mean maximum surface temperature of the water did not exceed 31° C. These species were absent in all water densely shaded by water lily and cow lily. Furthermore, no larvae of these species were found in locations where the mean maximum surface temperature reached 40° C. Relatively few larvae of either A. punctipennis (Say) or A. crucians Wied. occurred even in the cooler water in the areas observed. Hess & Hall (1945) reported that the most important aspects of plants in relation to A. quadrimaculatus were (1) absence of canopy species and (2) abundance of subsurface plants intruding into the water surface in the reservoirs of the Tennessee River. Larvae of this species were rare in communities of plants with naked emergent stems. An opportunity to explain some of the apparent discrepancies between the observations cited above came during 1950. A marsh near Muncie, Illinois, located in the eastern part of the state at about 40° north latitude provides a wide range of temperature and plant communities. The situation permitted establishment of groups of stations of uniform size and representing different surface conditions. The marsh covers an area roughly 2 acres in extent with a soft mud bottom some 30 cm. deep overlaid by water of varying depth to about 20 cm. The water level remained nearly constant during the summer as losses from all causes were balanced by inflow from marginal seepage and springs. Runoff from the scant watershed caused only a slight temporary elevation of the surface even after heavy rains. Temperature changes , in the surface water showed certain consistent features. At night minimum temperatures were much the same in all parts. Maximum surface temperatures were lowest on the side of the principal affluent