A recent University of Illinois study on the risk of West Nile virus with “dry” water-detention basins in Central Illinois indicated that mowing these dry basins worsens mosquito problems.
The researchers said mowing wetland plants inside basins that have not properly drained can cause a rapid increase in Culex pipiens mosquito populations, which can carry and transmit West Nile virus.
“We suspect bacteria quickly colonize the waterborne debris, and mosquito larvae feed on the bacteria,” Brian Allan, a University of Illinois entomology professor and co-author of the study, said.
“After aquatic plants were mowed in the basins, we saw a large increase in the number of Culex pipiens mosquito larvae in the basins, which had relatively few before mowing,” postdoctoral researcher Andrew Mackay, another author of the study, said. “And perhaps more importantly, we caught about twice as many adult Culex mosquitoes in traps at basins after these plants were mowed, compared with basins where the aquatic vegetation was left intact.
“We had observed that these phragmites-invaded basins would become colonized by large communal roosts of birds, and we thought that was important because birds are the natural reservoir hosts of West Nile virus,” Mackay said.
“Instead, we found that the presence of a communal bird roost actually decreased West Nile virus risk,” Allan said. “That may be because these wetland roosts include a variety of bird species, many of which are not good reservoirs of the virus. They don’t amplify the virus like other bird species more associated with residential areas do — the American robin, for example.
“We measured mosquito abundance, and we measured West Nile virus prevalence in the mosquitoes we collected in this field study, and we were able to show that it’s these mowed areas where you actually get the highest West Nile virus risk to people in the surrounding landscape,” Allan said.