CLBMON-13: Inventory of Mosquito Populations in the Revelstoke Area

Author: Culex Environmental Ltd

The key objective of this study is to determine whether there are water management strategies and operating alternatives that could be implemented to minimize potential impacts on mosquito production in the Revelstoke area. The general approach of this study is to collect data on mosquito populations in the Revelstoke area to resolve current gaps in the understanding of how dam and reservoir operations affect the distribution, abundance and productivity of these mosquito populations. In particular, surveys have been designed to address uncertainties related to: -The species of mosquito that inhabit the Revelstoke town site, and which species are likely to be a nuisance to humans/livestock. -Habitat and vegetation types occupied by mosquitoes in relation to the zone of influence of BC Hydro operations. -Areas that constitute prime mosquito breeding grounds. -Critical discharges and durations of flooding at which mosquito hatching occurs. -Other environmental and biotic factors affecting egg hatching and larval survival (e.g., seasonal temperatures, precipitation, predators). -Effects of vegetation management within the drawdown zone on mosquito production (species and abundance).

Mosquito populations in the Revelstoke area were very low in 2009. Despite intensive sampling effort (108 larval site visits, 3400 larval dips, 56 adult trap nights), only 200 mosquito larvae and 133 adult mosquitoes were sampled. A combination of factors likely contributed to this yearŸs low mosquito numbers, including unusually hot and dry weather, lower than average reservoir levels, and widespread larvicide treatment.

Mosquito populations are highly dynamic, often fluctuating greatly from one month to the next. There is even more deviation in mosquito populations from year to year, as the result of varying environmental conditions such as snow pack, rainfall, and temperature. Predator-prey relationships also commonly operate on alternating interannual cycles. Hydrological and typographical questions, such as the effect of river flows and dam discharges on groundwater levels in different marshes, are also liable to be complex and might require extensive investigation over a period of several years to resolve. The saturation level of the soil may be particularly important and high groundwater levels caused by flooding in the previous year might be significant.

The risk of running a single year study on mosquitoes is that the amount of data available to be collected is subject to those conditions as well. In 2009, these factors were not well-suited for generating large mosquito populations, and as a result relative little data could be collected. Only with a multi-year study can scientists begin to gather enough data to minimize effects of annual fluctuations in mosquito populations, water conditions, temperature and precipitation on study results

Management Questions:
Does operation of Revelstoke Dam and Arrow Lakes Reservoir influence mosquito production through flooding/reflooding of habitats within the drawdown zone?

Revelstoke Dam controls the inflow of water into the reservoir, but it is the Hugh L. Keenleyside Dam which controls the outflow. When Hugh L. Keenleyside Dam is not discharging water, the water levels rise in the reservoir as Revelstoke Dam daily discharge is fairly constant. This flooding event increases the amount of habitat available for mosquitoes to lay eggs, and the number of eggs that are likely to hatch in a season. Conclusions about a re-flooding effect could not be made in 2009 because there was no significant increase in late-season discharge from Revelstoke Dam.

Are there opportunities to minimize nuisance and potential WNv vector mosquito populations through modifyingoperations of Revelstoke Dam or Arrow Lakes Reservoir?

The low mosquito population observed in 2009 makes conclusions difficult to make after only one year of research. However, it appeared as though most of the mosquito habitat seepage areas eventually dried out when the Hugh L. Keenleyside Dam began to increase discharge levels. The amount of mosquito development habitat is also largely dependent on environmental conditions (temperature, precipitation, snowpack), which are outside the influence of the operations of the Dam.

Are there areas where revegetation (CLBWORKS-2) or wildlife physical work (CLBWORK 29/30) of the drawdown zone would exacerbate mosquito production?

The larval sampling site in the revegetation area turned out to be part of a flooded area that was very unlikely to contain mosquito larvae. The water became very deep, and remained continuous with the Columbia River, flowing and cold. The likelihood that revegetation would affect mosquito production in this particular area is very low. The proposed wildlife physical works projects as discussed above can have a positive or negative effect on mosquito production depending on whether more or less mosquito habitat is created.

Our surveys in 2009 have laid the groundwork and improved our understanding of the composition and structure of the mosquito population in the Revelstoke area. Some of the key findings from 2009 include:

Mosquitoes were associated with Reed Canary Grass-Lenticular Sedge, black cottonwood-Riparian, and Reed Canary Grass-Horsetail vegetation communities. Other vegetation classes in the region were both rare and provided unsuitable mosquito habitat.

More mosquitoes were detected in seepage sites than in flood sites in 2009. Many of the flood sites contained water that was too deep and cold to provide suitable mosquito development sites.

Eleven mosquito species were detected in the Revelstoke area in 2009, including nuisance species from the Anopheles genus, and potential West Nile virus vectors, Culex pipiens and Culex tarsalis.

Historical analysis suggests that the worst mosquito years (many nuisance mosquitoes) in the past have been associated with a combination of above average Arrow Lakes Reservoir levels, above average precipitation, and reduced area and amount of larvicide treatment.

Mosquito populations in the Revelstoke area were very low in 2009. Despite intensive sampling effort (108 larval site visits, 3400 larval dips, 56 adult trap nights), only 200 mosquito larvae and 133 adult mosquitoes were sampled. A combination of factors likely contributed to this yearŸs low mosquito numbers, including unusually hot and dry weather, lower than average reservoir levels, and widespread larvicide treatment.

Mosquito populations are highly dynamic, often fluctuating greatly from one month to the next. There is even more deviation in mosquito populations from year to year, as the result of varying environmental conditions such as snow pack, rainfall, and temperature. Predator-prey relationships also commonly operate on alternating interannual cycles. Hydrological and typographical questions, such as the effect of river flows and dam discharges on groundwater levels in different marshes, are also liable to be complex and might require extensive investigation over a period of several years to resolve. The saturation level of the soil may be particularly important and high groundwater levels caused by flooding in the previous year might be significant.

The risk of running a single year study on mosquitoes is that the amount of data available to be collected is subject to those conditions as well. In 2009, these factors were not well-suited for generating large mosquito populations, and as a result relative little data could be collected. Only with a multi-year study can scientists begin to gather enough data to minimize effects of annual fluctuations in mosquito populations, water conditions, temperature and precipitation on study results

Management Questions:
Does operation of Revelstoke Dam and Arrow Lakes Reservoir influence mosquito production through flooding/reflooding of habitats within the drawdown zone?

Revelstoke Dam controls the inflow of water into the reservoir, but it is the Hugh L. Keenleyside Dam which controls the outflow. When Hugh L. Keenleyside Dam is not discharging water, the water levels rise in the reservoir as Revelstoke Dam daily discharge is fairly constant. This flooding event increases the amount of habitat available for mosquitoes to lay eggs, and the number of eggs that are likely to hatch in a season. Conclusions about a re-flooding effect could not be made in 2009 because there was no significant increase in late-season discharge from Revelstoke Dam.

Are there opportunities to minimize nuisance and potential WNv vector mosquito populations through modifyingoperations of Revelstoke Dam or Arrow Lakes Reservoir?

The low mosquito population observed in 2009 makes conclusions difficult to make after only one year of research. However, it appeared as though most of the mosquito habitat seepage areas eventually dried out when the Hugh L. Keenleyside Dam began to increase discharge levels. The amount of mosquito development habitat is also largely dependent on environmental conditions (temperature, precipitation, snowpack), which are outside the influence of the operations of the Dam.

Are there areas where revegetation (CLBWORKS-2) or wildlife physical work (CLBWORK 29/30) of the drawdown zone would exacerbate mosquito production?

The larval sampling site in the revegetation area turned out to be part of a flooded area that was very unlikely to contain mosquito larvae. The water became very deep, and remained continuous with the Columbia River, flowing and cold. The likelihood that revegetation would affect mosquito production in this particular area is very low. The proposed wildlife physical works projects as discussed above can have a positive or negative effect on mosquito production depending on whether more or less mosquito habitat is created.

Our surveys in 2009 have laid the groundwork and improved our understanding of the composition and structure of the mosquito population in the Revelstoke area. Some of the key findings from 2009 include:

Mosquitoes were associated with Reed Canary Grass-Lenticular Sedge, black cottonwood-Riparian, and Reed Canary Grass-Horsetail vegetation communities. Other vegetation classes in the region were both rare and provided unsuitable mosquito habitat.

More mosquitoes were detected in seepage sites than in flood sites in 2009. Many of the flood sites contained water that was too deep and cold to provide suitable mosquito development sites.

Eleven mosquito species were detected in the Revelstoke area in 2009, including nuisance species from the Anopheles genus, and potential West Nile virus vectors, Culex pipiens and Culex tarsalis.

Historical analysis suggests that the worst mosquito years (many nuisance mosquitoes) in the past have been associated with a combination of above average Arrow Lakes Reservoir levels, above average precipitation, and reduced area and amount of larvicide treatment.