Culex tarsalis

Larval Habitat of Culex tarsalis

Current attempts to control Cx. tarsalis populations in the Northern Plains through classic larval control approaches have been based largely upon recommendations used for Cx. pipiens. Since it is clear that Cx. tarsalis utilizes breeding habitats that are very different from those used by Cx. pipiens, there is great need to identify natural habitats used by Cx. tarsalis even under drought condition in order to develop larval control strategies that are focused on the correct species and regional specific. Very little is known of the locations used by Cx. tarsalis for larval development within the Northern Great Plains.

Edmunds (1955) describes the types of larval habitats used by Cx. tarsalis larvae in Scotts Bluff County Nebraska. He found that the breeding habitats which produced the greatest number of larvae were permanent or semipermanent seepage areas and surface pools associated with irrigated pastures. He measured that egg hatching occurred in 1-3 days and hatching to adult emergence ranged from 14 to 31 days in this location. Fourth-instar larvae first appeared in these habitats on May 25, the population peaked July 11, and larvae couldn’t be found after October 29. Flood irrigation continues to take place within the Northern Plains, but there are still vast areas that are not irrigated and yet these is still large numbers of Cx. tarsalis present and plenty of WNV cases.

Because very few recent studies have focused on the ecology of Cx. tarsalis in non-irrigated areas of the Northern Great Plains, it is very difficult to provide regionally-specific recommendations to public health personnel in regards to mosquito-related risk factors for WMV (i.e. when and where are risks highest), and to approaches that can be taken to minimize exposure by controlling Culex populations through larval control measures.

Based upon various Californian studies, Cx. tarsalis appears to exhibit several ecological characteristics that are different from those of Cx. pipiens. Culex tarsalis is a dispersive colonizing species in which its larval stages are most frequently found in freshly created aquatic habitats with high primary bacterial, protozoan and invertebrate productivity associated with vegetative decay (Beeha and Mulla, 1995; Fanara and Mulla, 1974). These larvae are often one of the first macroinvertebrates to colonize newly-created, sunlit surface water pools generally surrounded by grasses and annual vegetation. Once productivity has reach its plateau, oviposition decreases. Waters that are highly polluted with organic material are not tolerated, and abundant populations are not generally found in permanent water with fixed depths. Development during these stages is also known to depend strongly on water temperature, food availability and refugia from exposure to physical disturbance and predators. These conditions are most notably associated with irrigated pastures, and for this reason, Cx. tarsalis is often referred to as the irrigation mosquito. From these larval habitats, adults will become randomly dispersed into the environment at a rate of about 1km/day (Reisen and Lothrop, 1995, Reisen et al., 2003).

During the 2003 WNV epidemic in South Dakota, our lab made several attempts to locate breeding habitats for Cx. tarsalis in untreated areas of Brookings County, but were largely unsuccessful. For this study, a total of 33 potential mosquito breeding sites were selected during the early spring of 2003 from both natural and urban settings within Brookings County, SD. These sites represented the various classifications of habitats potentially used by various mosquito species for larval development including: flowing streams, ponded streams, lake edges, swamps and marshes, shallow-permanent ponds, shallow-temporary pools, intermittent-ephemeral puddles, natural containers and artificial containers. The water condition of these sites were monitored weekly during the summer and sampled for the presence of mosquito larvae. A standard collection of 10 “dips” using a 13 cm mosquito dipper was made from each location. Collected larvae were preserved (70% ethanol), dehydrated in an ethanol series, mounted on glass slides, identified and enumerated. Focus was given to Culex larvae. Moderate numbers of larvae were recovered from many of the sites during the summer, but by August 6 only 21 of the sites contained water, and therefore, still able to support larval development. Culex tarsalis larvae were not found in any natural site; Cx. tarsalis larvae was eventually found in 3 artificial containers (plastic barrels), but only after August 5. These 30 gallon barrels had been filled with water at the beginning of the summer, and mosquito larvae collected throughout the summer. Topsoil (approximately 1 ft sq, 4 in deep, without vegetation) was added to each barrel at the beginning of the summer.

2003 Barrel in Full Sunlight 2003 Barrel in Full Shade

As described in other studies, (e.g. Brust, 1990), the barrels in full sunlight contained more Cx. tarsalis larvae (up to 15-20 larvae/dip) than those in full shade. Other sites outside the original study were also surveyed, and Cx. tarsalis larvae were found in 2 locations: 1) an artificial barrel mesocosm within Oak Lake Field Station during the early summer, and 2) in a water-filled, shallow tire-imprint located in a hay-field in Minnehaha County, South Dakota.

During the summer of 2004, we continued this Cx. tarsalis larvae study from within Brookings County in an attempt to define the types of natural habitats used by this species especially during the population build-up phase of the mosquito season (mid-June to early August). We focused primarily on temporary pools left after sporadic rain events. Cx. tarasalis larvae were occasionally found in a few natural sites in areas surrounding Brookings, SD. All of these natural sites were classified as temporary or semi-permanent pools, generally associated with considerable vegetation. During a single trip to Eagle Butte, SD (approximately 10 days following a 5 inch rain in that area), many larvae were found in temporary or semi-permanent pools formed by this rain. These sites were generally associated with considerable vegetation. Numerous Cx tarsalis larvae were also found in 3 permanent aquatic sites. Two of these sites were seepage areas/permanent-tiny-ponds formed by the spillage from artesian wells. The third site was a permanent seepage area/creek that appeared to be coming from a sewage lagoon. During this summer, 3 artificial containers (plastic barrels) were again filled with water and monitored as described above. These barrels were located in full sunlight, but the conditions of each barrel varied as described below:

The top of one of the barrels was cut off, leaving a shorten container about 12 inches tall. Sod containing tall grass was added to this "barrel" and the barrel was filled with water. The water was allowed to evaporate down to about 50% of its full volume, and then it was again filled up with water.

Larvae were periodically dipped from this container, and identified to species and counted.

Sod containing tall grass was also added to this "full-size barrel" and the barrel was filled with water (about 30 gallons). Water was also added to this barrel at the same times as the shorter barrel.