2013 Update on the Mosquito Population
Due to the prolonged period of rainy weather followed by hot, humid weather, mosquito populations in Alexandria dramatically increased in the early part of July. Weekly the Alexandria Health Department traps mosquitoes at thirteen sites around the city. Each site has several types of mosquito traps so that the Health Department can target different species of mosquitoes. During the week beginning July 8th, the Health Department trapped more than 6,000 mosquitoes at its thirteen trap sites. As seen in the graph below, this was more than double the number of mosquitoes the Health Department had trapped in any previous week this year.
Figure 1: Total number of mosquitoes collected weekly at all mosquito trap sites in Alexandria
Much of the increase in the mosquito population (the green segments in the bar chart above) is attributed to increased numbers of Aedes vexans, a species that thrives in wooded areas, especially after flooding or heavy rainfall events. These mosquitoes are most active in the early morning and the early evening hours.
The Health Department has also seen a large increase in Aedes albopictus, the Asian Tiger mosquito, shown in the red segments in the bar chart above. The Asian Tiger mosquito commonly breeds in containers such as flower pots, buckets, and children’s toys that have collected rain water. These mosquitoes are ferocious daytime biters.
As the graph below shows, Alexandria’s mosquito population in early July was over 30% greater than during any week in the last four years.
Figure 2: 2013 mosquito trap counts compared to historical data
Fairfax and Prince William Counties also observed a similar increase in mosquito populations.
How the Health Department is Responding:
- Responding to complaints about mosquitoes by community residents – The number of complaints the Health Department received from residents about mosquitoes spiked in July. Vector Control staff are working to quickly investigate these complaints and show residents how to eliminate the places where mosquitoes are breeding on their properties.
- Community outreach – The Vector Control Program is using mosquito surveillance data and complaints from residents to target specific neighborhoods for community outreach. Staff will distribute flyers in these neighborhoods to provide residents with information on how to eliminate mosquito breeding in their yards and the measures they should take to protect themselves from mosquito bites. Staff also conducts multiple outreach and community events monthly. The Health Department is recruiting Medical Reserve Corps volunteers to help with these tasks.
- Larviciding of mosquito catch basins and other storm water structures –The Health Department’s Vector Control Program staff is applying larvicide to hundreds of storm water catch basins along the curb line on City streets. Retreatment occurs on a monthly basis.
- Continued surveillance of mosquito populations – The Health Department is continuing to monitor the mosquito populations in the City so that we can respond quickly to further changes in population numbers. This surveillance is critical because different mosquito species require different interventions.
- Testing of mosquitoes for West Nile virus – The Health Department tests pools of mosquitoes caught in its surveillance traps for West Nile virus (WNV). The Health Department typically reports the first WNV positive pools of mosquitoes in late July or very early August. Should significant numbers of mosquitoes test positive for WNV, the Health Department would recommend additional appropriate public health interventions.
For more information on the Vector Borne Illness Prevention program or Mosquitoes, visit alexandriava.gov/MosquitoControl.