Mowing Grass in Water Detention Basins Increases Mosquito Populations.



During a study of the West Nile Virus (WNV) risk associated with water-detention basins in Central Illinois, researchers noticed a huge rise in populations of the northern house mosquito (Culex pipiens.) This corresponded with mowing of wetland plants in basins that failed to drain properly. This was highly important to the study as this mosquito is renowned for transmitting West Nile. A paper describing their findings was published in the journal, “Ecological Applications.”

Plant Species Debris Which May Be Indirectly Responsible for Increasing West Nile Disease.

Mosquito populations increase when grass is mown in water detention basins*

Recent research into the ecology of infectious diseases indicates that the establishment of invasive plant species can trigger a knock-on effect, altering the transmission dynamics of mosquitoes that put human health in danger. This latest study, led by University of Illinois postdoctoral researcher Andrew Mackay, examined whether the presence of cattails (Typha spp.) and phragmites (Phragmites australis), (two invasive plants that tend to permeate stormwater basins,) alter the local distribution of vectors, avian hosts, or West Nile Virus (WNV) transmission risk in urban residential settings. The study found that the mowing of cattails and phragmites adds a lot of plant debris to the water.


This resulted in a significant and sustained increase in the abundance of WNV-infected mosquitoes, causing an increase in risk extended to adjacent residential sites.


“We suspect bacteria quickly colonize the waterborne debris, and mosquito larvae feed on the bacteria,” said Illinois entomology professor Brian Allan, a co-author on the study with Mackay, Illinois Natural History Survey entomologist Ephantus Muturi and University of Illinois, natural resources and environmental sciences professor, Michael Ward.


“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,” Mackay 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.”


Why Bird Roosts among Phragmites are Not Considered Factors in WNV Spread.

The researchers had observed communal bird roosts (European Starlings, Red-winged Blackbirds and Common Grackles), in the basins containing unmanaged stands of phragmites. It was believed that mowing these invasive plants would disperse the bird roosts, according to Allan.


“We had observed that these phragmite-invaded basins would become colonized by large communal roosts of birds,” he said. “And we thought that was important because birds are the natural reservoir hosts of WNV.”


The researchers suspected that a bird roost near a mosquito nursery might increase the West Nile virus risk to people living nearby.


“Instead, we found that the presence of a communal bird roost actually decreased WNV 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.



These findings indicate that mowing of invasive plants in DDBs (Dry Detention Basins) during the growing season can increase, while presence of communal bird roosts can decrease, WNV transmission risk.


“You might think you’re helping by mowing, but you’re creating another problem,” Muturi said. “It’s all a matter of good planning and coordination to be sure that the kind of activities we do, either for aesthetic or for any other reason, don’t increase public health risk.”

*Image courtesy of  Free Digital Images