Week 33 – Aug 13 – 14

2008 Mosquito Report: Week 33

Report based on data collected from traps deployed on Wednesday, August 13, 2008 and retrieved on Thursday, August 14, 2008. Report prepared by John Smith, Chan Suom, M.S., and Nate Boonisar.

1. Current mosquito activity/trend

CDC Light Traps:
CDC activity reflected a mix of reflood/floodwater species including Aedes vexans, Aedes cinereus, Psorophora ferox and Ochlerotatus canadensis. Aedes vexans numbers, for the most part, fell off their peak of week 32 whereas Ochlerotatus canadensis numbers increased at several locations. NCMCP anticipates that these levels will be maintained or increase in the next few weeks due to continuing heavy rain events scattered across the county. An outbreak of salt marsh mosquitoes (Ochlerotatus sollicitans) continues in coastal areas and although the numbers of salt marsh mosquitoes is declining, these populations are mixing with inland species which is causing calls to continue at high levels in these coastal areas.

Gravid Traps:
Gravid trap collections, in general, seem to be declining across the county. Since these declines have now been seen for a period of three consecutive weeks at most trap sites it does appear that the populations of Culex pipiens/restuans are in fact declining rather than the lower numbers reflecting poor trap night weather conditions. Weather conditions during the week 33 trap night were conducive to good collections yet the numbers continued to decline. Ochlerotatus japonicus populations are remaining at about the same levels as last week.

2. Current Predominant Species

The most predominant mosquito in CDC traps this week was again Aedes vexans. due to continuing rain events that have occurred over the past several weeks. Culex spp.s were the second most common species in most CDC traps, followed by Ochlerotatus canadensis. The Culiseta melanura collection spiked at the Holbrook CDC site.

3. Comparison to previous season

CDC collections, in general, are showing an active period for reflood/floodwater species. Not the worst we have seen but in some areas the numbers are impressive. Psorophora ferox is now showing up in some traps which usually indicates significant local flooding. Ochlerotatus canadensis is showing a second late season population peak which is not unusual for this species during wet summers. The good news is that many of the gravid trap collections are at or below their multi year average and have been for several weeks now with the exception of Bellingham, Foxboro, Medway and Milton.

4. Weather Summary

Showers and thunderstorms continued this week, with the action mainly centered on the 10th, and especially the 15th. On Aug 10th, nearly stationary thunderstorms in the Needham area produced localized 2-2.5 inches of rain while the remainder of the county received under a half inch. Between the 10th and the 15th, lighter rain showers generally produced under a half inch county wide. A short term (few hours) excessive rainfall event occurred on Friday the 15thdue to a stationary line of training thunderstorms. The most intense rain fell over Wrentham, where up to 6.3 inches of rain fell (radar estimated; a ground report to the National Weather Service somewhere in Wrentham confirmed 5.50”). See attached precipitation map for rainfall amounts on the 15th.

Some of this heavy rain fell along the headwaters and mainline of the Neponset River causing sharp rises. Due to the localized nature of these rainfall events, weekly rain was highly variable across the county with as little as a half inch across the Millis and Medfield areas to 1-1.5 inches across eastern sections. The heaviest amounts (more than 2 inches) fell in a band from the southwest corner of the county northeast to Walpole, and across parts of Needham.

Total weekly rainfall: 1.74 inches (+0.98 in.)
Total Monthly rainfall: 4.89 inches (+3.19 in.)
Total Yearly rainfall: 32.98 inches (+6.95 in.)

5. Number of requests for service

Year to date NCMCP has received 3,455 calls for service as of 8/16/08. This represents a total of 289 calls for the week. The City of Quincy showed the most increase in calls representing 70 of the weekly total. This increase is no doubt due to a large emergence of salt marsh mosquitoes. Numbers of calls per week are approach the higher weekly call numbers of the past two seasons. Last year at this time the calls for service (year to date) were 4,978. Year to date for 2006 NCMCP had received 5,977 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 areas active for WNv. Coordination between NCMCP and the Needham/Dedham/Weymouth Boards of Health is ongoing relative to control/surveillance options in the vicinity of WNv positive birds/mosquito pools.


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