Week 32 – Aug 5 – 6

2008 Mosquito Report: Week 32

Report based on data collected from traps deployed on Tuesday, August 5, 2008 and retrieved on Wednesday, August 6, 2008. Report prepared by John Smith, Chan Suom, M.S., and Nate Boonisar.

1. Current mosquito activity/trend

CDC Light Traps:
CDC activity in the following towns increased dramatically compared to last year’s and the previous week’s data: Avon, Bellingham, Canton, Dedham, Foxboro, Franklin, Holbrook, Medfield, Medway, Millis, Milton, Needham, Norfolk, Plainville, Walpole, Weymouth, and Wrentham. This was due to an increase in collections of Aedes vexans in most of the CDC traps. NCMCP anticipates that these levels will be maintained or increase in next weeks collections. As reflected in trap data, service requests and site visits to the City of Quincy and North Weymouth, a rather large outbreak of salt marsh mosquitoes(Ochlerotatus sollicitans) has occurred in coastal areas which appears to have been rain induced rather than tidal.

Gravid Traps:
Gravid trap collections increased in the following towns: Foxboro, Medfield, Medway, Needham (WNv+ Epi Week 29), Norfolk, Randolph, Quincy, and Wrentham. In general gravid trap collects seem to be declining with most collections at or below the multi-year average with the exception of a few trap sites. The gravid traps in general (especially in the eastern third of the county) have come down from their seasonal highs set back between Epi weeks 27 and 30. It is still unclear whether this decline is an actual population decline or poor weather conditions during the trap nights. Although, considering these declines have occurred over the past two weekly trapping sessions, it appears there may be a decline in population occurring. Ochlerotatus japonicus populations are slowly increasing compared to last week’s and last year’s numbers.

2. Current Predominant Species

The most predominant mosquito in CDC traps this week was Aedes vexans. This can be attributed to the myriad rain events that have occurred over the past several weeks. Many traps were inundated with Ae. vexans, especially in Canton along the Neponsett River floodplain. Culex pipiens/restuans were the second most common species in most CDC traps, followed by Coquillettidia perturbans.

3. Comparison to previous season

CDC collections have increased dramatically this week compared to last year’s and last week’s data as noted in the above towns due to a fairly substantial emergence of summer reflood mosquitoes, especially Aedes vexans. There have been numerous thunderstorm/rain events in past weeks that have resulted in what appears to be a county wide emergence of Aedes vexans. Gravid trap activity has declined somewhat in many of the trap sites with the exception of a few sites. Unfortunately collections at both trap sites (Dedham and Needham) which have produced positive WNv mosquito pools increased. The good news is that many of the gravid traps have gone below their multi year average for several weeks now.

4. Weather Summary

The wet weather pattern continued this week with a highly anomalous trough across the northeast and very cold temperatures aloft, some of which translated to the surface causing below normal temperatures all week. Rounds of showers and thunderstorms occurred nearly every day bringing variable amounts of rain to the county, with the most rainfall occurring on August 8th. For the entire week, the heaviest rainfall occurred in central and western areas, where a widespread 2-3 inches fell, with local amounts nearing 4 inches. Otherwise, eastern and extreme southern portions received 1-2 inches of rain for the week. Ground water levels are running high for this time of year. The Charles and Neponset Rivers are running high, but probably below levels that would inundate much of the floodplains. However, with each heavy rain event, brief spikes in river level may inundate portions of the flood plain and the cumulative effect of these rains continues to slowly raise the longer term river level making the flood plains more susceptible to inundation with each event. Higher water tables may also allow any ponding of water on the flood plain to remain longer than it otherwise would.

Total weekly rainfall: 2.64 inches (+1.90 in.)
Total Monthly rainfall: 3.15 inches (+2.21 in.)
Total Yearly rainfall: 31.24 inches (+5.97 in.)

5. Number of requests for service

Year to date NCMCP has received 3,166 calls for service as of 8/9/08. This represents a total of 297 calls for the week. The City of Quincy showed the most increase in calls representing 86 of the weekly total. This increase is no doubt due to a large emergence of salt marsh mosquitoes. Last year at this time the calls for service (year to date) were 4,775. Year to date for 2006 NCMCP had received 5,687 calls for service by the same date.

6. MCP/Commission response

In general our response has been to continue ground ULV applications targeting areas of high public demand and areas of positive WNv mosquito pools. NCMCP anticipates increases in public requests for ULV applications in areas where summer reflood activity has been identified. NCMCP has completed treatments of rain basins in all urban areas. Additional trapping of gravid Culex pipiens/restuans has taken place in the vicinity of WNv positive bird locations which resulted in the positive WNv pool in Needham collected by SLI on August 5, 2008. Coordination between NCMCP and the Needham/Dedham Boards of Health is ongoing relative to control/surveillance options in the vicinity of WNv positive birds/mosquito pools.


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