Integrated Mosquito Management
2008 Season Report
Prepared for and in Cooperation with:
Jefferson County Department of Health & Environment
1801 19th Street
Golden, CO 80401
OtterTail Environmental, Inc.
10200 W. 44th Ave.
Wheat Ridge, CO 80033
TABLE OF CONTENTS
EXECUTIVE SUMMARY ......................................................................................................................................... 1
1.0 INTRODUCTION .......................................................................................................................................... 2
2.0 WEST NILE VIRUS (AND OTHER MOSQUITO-BORNE DISEASE) UPDATE ................................ 3
3.0 REGIONAL 2008 CLIMATOLOGICAL DATA AND MOSQUITO ACTIVITY OVERVIEW .......... 5
4.0 LARVAL MOSQUITO SURVEILLANCE AND CONTROL................................................................... 7
LARVAL SURVEILLANCE METHODOLOGY ........................................................................................................ 7
LARVAL CONTROL METHODOLOGY AND APPLICATION METHODS .................................................................. 7
LARVAL SURVEILLANCE AND CONTROL RESULTS AND DISCUSSION ............................................................... 8
5.0 ADULT MOSQUITO AND WNV SURVEILLANCE .............................................................................. 10
ADULT SURVEILLANCE METHODOLOGY ........................................................................................................ 10
ADULT MOSQUITO SURVEILLANCE RESULTS AND DISCUSSION ..................................................................... 11
6.0 PUBLIC OUTREACH AND EDUCATION ............................................................................................ 14
7.0 REFERENCES ............................................................................................................................................. 15
Figure 1 2008 Monthly Mean Air Temperature with the Historical Average ......................................... 6
Figure 2 2008 Monthly Total Precipitation Data with Historical Average ............................................. 6
Figure 3 Site Visit and Treatment Percentages by Area, 2008 ................................................................ 9
Figure 5 Season-Wide Weekly Adult Trap Counts of All Trap Stations, 2008 .................................... 13
Figure 6 Season-Wide Weekly Adult Culex Counts of All Trap Stations, 2005-2008 ......................... 13
Table 1 West Nile Virus Incidence, 2003 - 2008 .................................................................................. 3
Table 2 Colorado WNV Human Cases and WNV Positive Mosquito Pools, 2008 .............................. 4
Table 3 Human WNV Crude Attack Rates1 of Front Range Region Counties, 2008 ........................... 4
Table 4 Larval Surveillance and Control Summary, 2008 .................................................................... 8
Table 5 Larval Surveillance and Control Summary, 2005 - 2008 ........................................................ 8
Table 6 Total Number of Adult Mosquitoes per Trap for the Entire 2008 Season ............................. 11
Table 7 Total Number of Adult Mosquitoes per Trap During the Sentinel Zone Extra Trapnights of
the 2008 Season ..................................................................................................................... 12
Table 8 Number of Site Visits per Municipality from Hotline Requests, 2008 ................................... 14
Table 9 Total Number of Jefferson County Hotline Calls and Site Visits from Hotline Requests, 2005
- 2008 .................................................................................................................................... 14
Appendix A Detailed Larval Surveillance and Site Selection Methodology
Appendix B Adult Mosquito Trap Descriptions
Appendix C 2008 Jefferson County IMM Program Service Area Map
Appendix D Electronic 2008 Site and Site Visit Lists
2008 Jefferson County Mosquito Report i
For the 2008 mosquito season, the Jefferson County Department of Health and Environment contracted
OtterTail Environmental, Inc. to operate an integrated mosquito management (IMM) program to protect
public health from the effects of West Nile Virus (WNV) and other vector-borne diseases. This report
provides a summary of the 2008 program.
West Nile Virus is a disease that was first detected in the United States during the summer of 1999 in
New York City and has since spread westward across the U.S. West Nile Virus is maintained in the bird
population and mosquitoes are usually not carriers of WNV until they bite an infected bird. An infected
mosquito may then pass the virus on to humans, horses and other animals through an additional bite.
While many of the people who contract WNV experience no or very mild symptoms, WNV infection can
result in severe and sometimes fatal illnesses.
There are over 50 species of mosquitoes in Colorado, yet only species from the genus Culex are known to
be effective transmitters of the WNV virus. Mosquitoes and other insects that transmit disease are called
vectors. Following Integrated Pest Management principles, Jefferson County and OtterTail
Environmental focused on controlling and reducing mosquito populations and thereby protecting public
health by decreasing the likelihood of WNV transmission. Through surveillance of potential mosquito
breeding sites, areas identified as containing mosquito larvae were identified and treated with control
agents. This method of control prevents the mosquitoes from developing into adults and transmitting the
virus, and is the most efficient and environmentally friendly way to reduce mosquito populations. In
addition, control of larvae limits the possible future need for nonbiological control of adult mosquitoes
such as ultra-low volume (ULV) pesticide spraying. Adulticiding is not only typically more harmful to
the environment than larviciding; it is also much less effective at reducing mosquito numbers.
Potential mosquito-breeding areas (e.g., wetland areas, water bodies, stream margins, and ditches) were
evaluated for larval control throughout the season. The larval control methods used on the project’s lands
were the biological larvicides Bacillus thuringiensis israelensis (Bti) and Bacillus sphaericus (Bs) and the
monomolecular oil AGNIQUE® MMF.
At the end of the 2008 season, there was a total of 663 potential larval habitat sites in the project area.
Since habitat sites can change from week-to-week, sites were deleted and added from the project as
necessary throughout the season. Of these, 379 sites were found to be breeding mosquito larvae.
In addition to larval mosquito surveillance and control treatments, Jefferson County monitored adult
mosquito activity within the area with 25 adult mosquito traps. These trap collections enabled OtterTail
to provide adult mosquito samples which were tested weekly in order to monitor mosquito populations
and any possible WNV activity.
The State of Colorado experienced a significant decrease in WNV activity in 2008. Human WNV case
counts and the number of WNV positive mosquito pools were at their lowest levels since 2002, which
was the first year it was detected in the state. Climate factors including cooler temperatures throughout
the 2008 season, along with drier weather in the spring and early summer, was likely a leading cause for
the decrease of WNV activity throughout the region during 2008. Colorado experienced a statewide
decrease in WNV activity and the levels in Jefferson County remained even lower than many nearby
counties in the Front Range. It is likely that Jefferson County’s intensive larval control program
combined with education and personal protection measures helped further reduce the mosquito
populations and WNV activity levels in the county during 2008.
2008 Jefferson County Mosquito Report 1
Jefferson County continued to contract OtterTail Environmental, Inc. to operate an integrated mosquito
management program in 2008. The County’s primary goal was the same as it was in previous years,
which was to protect public health from the effects of WNV and other similar mosquito-borne viruses.
To accomplish this goal, specific objectives were established for the program. Primarily, larval habitat
surveillance and biological larvicide was used. Habitat with a high potential for breeding was identified,
monitored and treated when necessary to control mosquito larvae. Additionally, they wanted to monitor
adult mosquito populations. This is done by speciation, population counts, and testing for WNV in
trapped vector specimens, to provide an early warning system for the occurrence and severity of WNV
activity in the program area. They also wanted to limit the effect on the environment from control
materials and be as cost-effective as possible. This report explains the methods used in the integrated
mosquito management program and provides a detailed summary of the results of this year’s effort.
2008 Jefferson County Mosquito Report 2
2.0 WEST NILE VIRUS (AND OTHER MOSQUITO-BORNE
Since the introduction of West Nile Virus to the United States in 1999, the virus quickly spread westward
and has been detected in all states except for Alaska and Hawaii. For the 2008 season, there were 1,301
WNV human cases and 34 WNV related deaths in 42 states as of November 18, 2008 (Table 1).
During the 2003 WNV epidemic, Colorado led the U.S. in WNV cases and then experienced a significant
decrease in WNV cases in 2004 and 2005. During the 2006 and 2007 mosquito seasons, Colorado had a
resurgence of cases and once again led the U.S. human case count in 2007. WNV activity decreased
throughout the U.S. and Colorado during the 2008 season. Colorado was fourth in the national case
count, reporting 95 human WNV cases and 1 WNV related death as of November 18, 2008 (CDC 2008).
This season, WNV cases were sporadically located throughout the state with the highest concentrations
along the populous regions of the Front Range. Weld, Boulder and Larimer Counties reported the highest
number of cases (24, 18 and 14 respectively) in Colorado (CDPHE 2008). No mosquito pools, horses,
birds or humans tested positive for St. Louis Encephalitis or Western Equine Encephalitis in Colorado
during the 2008 season (Table 2).
In 2008, the total number of human WNV infections reported in Jefferson County was at the lowest level
since WNV was first detected in the state in 2002. As of November 18, 2008 there were 4 cases and no
deaths reported in Jefferson County (Table 2). Although testing was very widespread and frequent, only
1 of the 43 submitted pools of mosquitoes tested WNV positive in the county, which was lower than
several other counties in Colorado. The decrease of human infections and WNV activity in Colorado may
be attributed to the temperature and precipitation patterns observed during the 2008 mosquito season and
the affect they had on mosquito populations, as discussed further in Section 3.0. There were no deaths in
Jefferson County and the number of Jefferson County cases and WNV positive mosquito pools comprised
approximately 4.7% and 1.6% of the state totals (Table 2). When populations of nearby Front Range
Counties are taken into account, Jefferson County ranked lower in crude attack rates (cases per 100,000
individuals) than several of the other counties (Table 3). The small number of WNV positive mosquito
pools, along with the low WNV crude attack rate, suggests that the viral activity in Jefferson County was
less than in other nearby areas. It is likely that widespread larval control efforts combined with public
education and personal protection measures helped reduce the number of mosquitoes and limit disease
transmission within Jefferson County.
Table 1 West Nile Virus Incidence, 2003 - 2008
2003 2004 2005 2006 2007 2008
Total WNV Human Cases
Cases in the United States1 9,862 2,539 3,000 4,269 3,265 1,301
Deaths in the United States1 264 100 119 177 92 34
Cases in Colorado2 2,947 291 106 345 555 95
Deaths in Colorado2 63 4 2 7 6 1
Cases in Jefferson County2 157 8 6 8 32 4
Deaths in Jefferson County2 3 0 0 1 0 0
Total WNV Positive Results 2003 2004 2005 2006 2007 2008
Mosquito Pools in Jefferson County2 13 0 0 7 2 1
Birds in Jefferson County2 59 0 0 0 0 0
Horses in Jefferson County2 13 0 0 0 2 0
1. Reported by the Center for Disease and Control (CDC); 2008 data reported as of November 18, 2008.
2. Reported by the Colorado Department of Public Health and Environment (CDPHE); 2008 data reported as of November 18, 2008.
2008 Jefferson County Mosquito Report 3
Table 2 Colorado WNV Human Cases and WNV Positive Mosquito Pools, 2008
1 1 1
Human Cases Human Deaths Pools
County Number % of State Number % of State Number % of State
Adams 7 7.4% 0 0.00% 0 0.0%
Arapahoe 2 2.1% 0 0.00% 0 0.0%
Bent 0 0.0% 0 0.00% 1 1.6%
Boulder 18 18.9% 0 0.00% 3 4.9%
Costilla 1 1.1% 0 0.00% 0 0.0%
Delta 1 1.1% 0 0.00% 1 1.6%
Denver 7 7.4% 0 0.00% 0 0.0%
El Paso 0 0.0% 0 0.00% 2 3.3%
Gunnison 1 1.1% 0 0.00% 0 0.0%
Jefferson 4 4.2% 0 0.00% 1 1.6%
Kiowa 1 1.1% 0 0.00% 0 0.0%
Larimer 14 14.7% 1 100.00% 25 41.0%
Logan 2 2.1% 0 0.00% 1 1.6%
Mesa 1 1.1% 0 0.00% 1 1.6%
Morgan 2 2.1% 0 0.00% 2 3.3%
Otero 3 3.2% 0 0.00% 3 4.9%
Phillips 1 1.1% 0 0.00% 0 0.0%
Prowers 1 1.1% 0 0.00% 4 6.6%
Pueblo 4 4.2% 0 0.00% 2 3.3%
Weld 24 25.3% 0 0.00% 15 24.6%
Yuma 1 1.1% 0 0.00% 0 0.0%
Totals 95 1 61
1. Reported by CDPHE as of November18, 2008
Table 3 Human WNV Crude Attack Rates1 of Front Range Region Counties, 2008
% of Front 2008 Crude % of Front Crude
# of WNV Range Cases by # Attack Rates Range Cases by Attack
County Population Cases of Cases (per 100,000) Population Rank
Adams 414,338 7 8.8% 1.69 6.0% 5
Arapahoe 537,197 2 2.5% 0.37 1.3% 8
Boulder 282,304 18 22.5% 6.38 22.6% 2
Broomfield 45,116 0 0.0% 0.00 0.0% 9
Denver 566,974 7 8.8% 1.23 4.4% 6
Douglas 263,621 0 0.0% 0.00 0.0% 10
El Paso 576,884 0 0.0% 0.00 0.0% 11
Jefferson 526,994 4 5.0% 0.76 2.7% 7
Larimer 276,253 14 17.5% 5.07 17.9% 3
Pueblo 152,912 4 5.0% 2.62 9.3% 4
Weld 236,857 24 30.0% 10.13 35.9% 1
1. WNV human case information used for Crude Attack Rate calculations was obtained from CDPHE (CDPHE 2008); Population information
for Crude Attack Rate calculations was obtained from U.S. Census Bureau ‘s 2006 population estimates (USCB 2008); Crude Attack Rates are
listed as cases per 100,000 people.
2008 Jefferson County Mosquito Report 4
3.0 REGIONAL 2008 CLIMATOLOGICAL DATA AND
MOSQUITO ACTIVITY OVERVIEW
The weather patterns leading into and during the mosquito breeding season are important factors that
influence mosquito abundance and WNV activity. The following section describes the regional climate,
the weather during the season, and how that may have affected the mosquito populations.
Jefferson County is located in a semi-arid environment with elevations in the project area varying from
approximately 5,300 feet to 5,700 feet. The mosquito season for the Jefferson County program area is
from April to October. Current and historical climate data from the High Plains Regional Climate
Center’s Denver International Airport weather station was used for regional temperature and precipitation
patterns. Historical records for the mean monthly temperature at the station show that temperatures
usually have a steady increase from April to July, making July, on average, the hottest month of the year.
There is then typically a steady decrease into September. Monthly mean precipitation for the same time
period indicates that April, May and July are usually the wettest months of the year. Figure 1 and Figure
2 graphically represent the average temperature and precipitation for the period of record and for the year
2008 (HPRCC 2008).
In 2008, temperatures were below average during five of the seven months of the mosquito season. Every
month from March through September had below normal temperatures except June and July. The month
of July had the highest variation from normal with its monthly mean temperature approximately 2.8
degrees above normal.
During 2008, the accumulated precipitation from January through September was below the historical
average of accumulated precipitation for the same period. From January to September of 2008 there was
an accumulation of 8.4 inches. This is approximately 76.4% of the normal accumulation for this time
period when compared to the historical average, which is 11 inches. Seven of the nine months received
precipitation amounts lower than their normal averages. Of these, the most significant variations were the
months of March and April, which each received only 17% of their average amounts. August received
225% more precipitation than average, making it the wettest month of the 2008 mosquito season (HPRCC
Temperatures and precipitation amounts varied greatly throughout the 2008 mosquito season. The low
amounts of rainfall in the spring and early summer led to many mosquito habitats not being inundated
with water early in the season. This decrease in early season breeding habitats, along with cooler-than-
normal temperatures, led to lower Culex populations in most areas of the region throughout the season.
The especially low abundance of Culex mosquitoes in the early part of the season, along with cooler
temperatures during the majority of the 2008 season, was likely a leading cause of the lower levels of
WNV activity throughout the region during 2008. The low temperatures and Culex populations caused
the WNV cycle to start later and magnify a much lower rate than during a year with higher Culex
populations and temperatures.
2008 Jefferson County Mosquito Report 5
Figure 1 2008 Monthly Mean Air Temperature with the Historical Average*
Figure 2 2008 Monthly Total Precipitation Data with Historical Average*
2008 Jefferson County Mosquito Report 6
4.0 LARVAL MOSQUITO SURVEILLANCE AND CONTROL
LARVAL SURVEILLANCE METHODOLOGY
Ottertail staff began the season by inspecting, prioritizing and reclassifying the larval habitat sites from
the 2007 season. Habitats were again classified as either “Target” or “Non-breeding”. Target habitat
sites included the areas that were most suitable for breeding mosquito larvae. Many of the target habitats
were those with stagnant water high in nutrients and organic matter including: cattail marshes, non-
flowing irrigation and drainage ditches, flood irrigated fields, and lake and river shores. Non-breeding
habitat included those areas with a lower potential to become breeding habitat. The two most common
types of non-breeding habitat sites were: 1) areas that would become suitable larval habitat if the water
level dropped (e.g., stagnant areas might develop in a currently flowing stream); and, 2) areas that would
become suitable larval habitat if the water level rose (e.g., low-lying grassy areas surrounding ponds).
Target sites were generally inspected once a week and non-breeding sites were inspected once each
month to monitor any possible changes in their habitat. Habitat sites were added, refined or deleted
throughout the field season as needed. A detailed explanation of the larval surveillance methodology
used during the 2008 season can be found in Appendix A and a map of the larval surveillance area can be
found in Appendix C.
To ensure a comprehensive larval control program, OtterTail continued to operate a toll-free telephone
hotline so that citizens could report areas of concern. The hotline number was advertised by many of the
municipalities in the project area throughout the season. OtterTail provided site inspections and any
consequential treatments to potential larval habitat as necessary. If landowners gave permission, then
suitable areas were added to the project and checked regularly. OtterTail field technicians only accessed
private properties if permission was granted by the owner.
LARVAL CONTROL METHODOLOGY AND APPLICATION METHODS
Larval mosquito control methods employed by OtterTail were aimed at reducing the potential of West
Nile Virus and other similar mosquito-borne diseases while also minimizing the annoyance level of
mosquitoes to local residents. In an effort to save on time and labor costs, OtterTail no longer
distinguished vector larvae from nonvector larvae during the 2008 season; therefore the threshold for
larval control was the presence of any type of mosquito larvae.
The application of Bacillus thuringiensis israelensis (Bti), Bacillus sphaericus (Bs) and the
monomolecular oil AGNIQUE® MMF were the methods used for larval mosquito control. Other larval
control materials were available but were not needed during the 2008 season. Control materials were
applied at the recommended rates, thereby minimizing any potential adverse impacts to areas being
treated. Routine post-treatment checks were conducted in 48-72 hours to assure the larval population was
controlled. If any larvae were found, a second application of the control material was applied.
In balancing environmental resources, cost effectiveness, and public health needs, Bti was chosen as the
primary treatment method. Bti is a larvicide that is a naturally occurring protein which is toxic to
mosquito larvae upon its ingestion. It provides a residual treatment that lasts for approximately two days.
Since new mosquito larvae may hatch after the product dissipates, the sites must be inspected for
mosquito larvae every 1 to 2 weeks. The existence of mosquito larvae between monitoring periods has
the added side benefit of allowing these larvae to still be part of the aquatic food chain, but be eliminated
before they can emerge as adults. This helps protect the public from WNV, while still providing an
important food source for many animals.
2008 Jefferson County Mosquito Report 7
B. sphaericus is a larvicide that is very similar to Bti, but has a longer residual time. The protein in B.
sphaericus products is able to provide continuous treatment of mosquito larvae for up to 4 weeks. It was
typically used on sites that were found to be breeding mosquitoes on a continuous basis. The longer
residual time required fewer site checks, saving labor and travel costs to those sites, but costs substantially
more making it practical in only certain situations.
AGNIQUE® MMF is a monomolecular oil that was used if mosquito pupae were present during a site
check. Mosquitoes do not feed during their pupal stage, therefore the usage of Bti and Bs is ineffective
against mosquito pupae since they must be ingested. Monomolecular oil’s method of control is by
interrupting the air and water interface during the mosquito’s larval and pupal development stages.
LARVAL SURVEILLANCE AND CONTROL RESULTS AND DISCUSSION
The 2008 larval surveillance season started on June 1 and ended September 30. During the season, a total
of 8,987 individual larval site visits were performed within the program area (Table 4). Out of the
season-end-total of 663 potential breeding sites, 379 sites bred mosquito larvae at some point during the
2008 season. Approximately 560 lbs of Bti, 48 lbs of Bs, and 108 ounces of AGNIQUE® MMF were used
in combination to treat approximately 96 acres of actively breeding habitat. In comparison to previous
years, 2008 had the second highest site visit and treatment totals of the last four seasons. (Table 5)
Table 4 Larval Surveillance and Control Summary, 2008
Surveillance Hotline Site Site Acres Treatment Site Visit
Area Inspections Evaluations Treatments Treated Inspections Total
Unincorporated 2471 5 818 44.9 288 3577
Arvada 2139 1 453 19.6 207 2799
Golden 777 2 96 6.7 45 918
Lakewood 651 1 119 6.2 57 827
Littleton 88 0 64 5.6 0 152
Morrison 16 0 2 0.4 1 19
Wheat Ridge 490 0 176 12.4 29 695
Project Total 6632 9 1728 95.8 627 8987
Table 5 Larval Surveillance and Control Summary, 2005 - 2008
Surveillance Hotline Site Treatment Site Acres Site Visit
Year Inspections Evaluations Inspections Treatments Treated Total
2005 6892 47 766 984 164.8 8689
2006 6746 56 673 1464 111.4 8939
2007 5754 28 1228 2077 137.9 9087
2008 6632 9 627 1728 95.8 8987
Many of the habitat sites bred mosquito larvae multiple times during the season causing the treated acres
at certain sites to be counted multiple times for the season total. As the season progressed, the sites were
categorized according to larval abundance and occurrence. Low priority breeding sites consisted of sites
with poor habitat or had the presence of aquatic predators. High priority breeding sites had larvae when
sampled and consistently bred mosquitoes every 7 to 10 days during the peak season.
2008 Jefferson County Mosquito Report 8
As shown Figure 3, sites in Unincorporated Jefferson County received the most treatments followed by
Arvada, Wheat Ridge, Lakewood, Golden, Littleton and Morrison. It is likely that Jefferson County’s
larval control program helped reduce the adult mosquito population levels within the county during 2008.
The low (relative to other nearby counties) human WNV case numbers and low WNV positive mosquito
pools also suggest that the larval control program was effective in reducing WNV activity.
Figure 3 Site Visit and Treatment Percentages by Area, 2008
2008 Jefferson County Mosquito Report 9
5.0 ADULT MOSQUITO AND WNV SURVEILLANCE
ADULT SURVEILLANCE METHODOLOGY
Adult mosquito population surveillance is a crucial component of any successful IMM program. Adult
surveillance can provide information on what types of mosquito species are in an area as well as
information on their abundance. Adult mosquito surveillance is also critical to disease surveillance.
Mosquitoes collected from the mosquito traps can be tested for a variety of mosquito-borne diseases,
OtterTail uses two different types of mosquito traps to monitor adult mosquito populations, the gravid
trap and the carbon dioxide (CO2) light trap. The CO2 light traps are based on the principle that most
adult mosquitoes are attracted to light, CO2 (via respiration), and heat. The CO2 light trap collects adult
female mosquitoes that are seeking a blood meal. The gravid trap mimics sources of mosquito breeding
habitat and attracts gravid female mosquitoes that are seeking a spot to lay their eggs. Both types of traps
are set overnight and on the following morning the nets are collected and returned to OtterTail’s lab to be
identified and counted. Once identified, the mosquitoes were then sorted by species and the vector
mosquitoes were submitted to the Colorado Department of Public Health and Environment (CDPHE) for
WNV testing. A detailed explanation of each type of trap used during the 2008 season can be found in
To maximize the efficiency of their WNV testing resources, CDPHE developed a new protocol for the
sentinel surveillance program in 2007, which it continued during the 2008 season. The first part of the
protocol was to change each county’s sentinel testing location from a single sentinel sampling site into a
sentinel zone. The sentinel zone is a system of five light traps at five different locations within a five mile
radius. Culex mosquitoes collected from all five traps within the zone are then pooled together each week
for WNV testing. It is thought that the use of five traps within the sentinel zone will give more reliable
data over time as well as cut down on weeks with lost data (such as if there was a trap malfunction when
only a single trap site was being used).
The second major change to the CDPHE protocol was the monitoring schedule for the sentinel zones.
After analyzing data from previous years, CDPHE found that the majority of all WNV activity occurred
between the months of June and August. In an effort to perform most of its WNV testing during this
heightened transmission period, CDPHE requested that counties no longer perform early (May) or late
(September) season monitoring for the sentinel program. Instead, they requested that counties run their
sentinel zone traps once a week starting in the first week of June, then run their sentinel zone traps for a
second night each week from the last week of June until the first week of August, and run them once a
week again until the end of August. This monitoring schedule would allow the vector mosquitoes
collected from the traps within the sentinel zone to be submitted for WNV testing throughout the season,
with extra samples being collected and tested during the height of the transmission period.
Beginning in June, sixteen light traps and nine gravid traps were set at sixteen locations with suitable
harborage for adult mosquitoes. These traps were all part of JCDHE’s county-wide adult mosquito
surveillance program. Five of the traps were also used in Jefferson County’s CHPHE WNV sentinel
surveillance program. The traps were placed at historical locations that were used during previous years
of mosquito surveillance in Jefferson County (Appendix C). All 25 traps were set on weekly basis
through the end of August, with the five sentinel zone traps having a second trap night from the last week
of June until the first week of August.
OtterTail used the adult mosquito data to calculate infection rates and to help county officials determine
local areas of concern for public awareness and safety. This data could also be used to guide any
2008 Jefferson County Mosquito Report 10
potential adulticide efforts within the county, which was an important health and environmental issue to
county officials and residents.
ADULT MOSQUITO SURVEILLANCE RESULTS AND DISCUSSION
Over the season, from all 25 traps, there was an average of 18 total adult mosquitoes per trap per night
and an average of 8 adult vector mosquitoes per trap per night. The total adults collected during the
season resulted in Aedes and Ochlerotatus species (48.7 percent) being the most abundant, followed by
Culex (vector) (45.8 percent), Culiseta species (5.0 percent) and Anopheles species (0.5 percent) as
shown in Table 6. This results in 54% non-vector vs. 46% vector adults being collected over the entire
Table 6 Total Number of Adult Mosquitoes per Trap for the Entire 2008 Season1
Trap Name Culex spp. Ae./Oc. Spp. Anopheles spp. Coquillettidia spp. Culiseta spp. Trap Avg Per
and Location # %RA # %RA # %RA # %RA # %RA Total Trapnight Trap %RA
Westminster 18 31.6% 39 68.4% 0 0.0% 0 0.0% 0 0.0% 57 4.4 1.0%
Cameo Estates 128 43.0% 140 47.0% 16 5.4% 0 0.0% 14 4.7% 298 22.9 5.0%
Johnson Park 22 25.9% 46 54.1% 11 12.9% 0 0.0% 6 7.1% 85 6.5 1.4%
Johnson Park 179 95.7% 8 4.3% 0 0.0% 0 0.0% 0 0.0% 187 14.4 3.1%
Willow Creek Park 38 12.7% 261 87.3% 0 0.0% 0 0.0% 0 0.0% 299 23.0 5.0%
Morrison 16 28.6% 39 69.6% 0 0.0% 0 0.0% 1 1.8% 56 4.3 0.9%
Prospect Park 62 60.8% 37 36.3% 1 1.0% 0 0.0% 2 2.0% 102 7.8 1.7%
Prospect Park 215 88.1% 24 9.8% 0 0.0% 0 0.0% 5 2.0% 244 18.8 4.1%
Meadowbrook 65 38.9% 78 46.7% 1 0.6% 0 0.0% 23 13.8% 167 12.8 2.8%
Meadowbrook 198 93.4% 14 6.6% 0 0.0% 0 0.0% 0 0.0% 212 16.3 3.5%
Ken Caryl Ranch 2 16.7% 8 66.7% 0 0.0% 0 0.0% 2 16.7% 12 0.9 0.2%
Weaver Creek 11 16.2% 52 76.5% 0 0.0% 0 0.0% 5 7.4% 68 5.2 1.1%
Arvada 98 32.9% 188 63.1% 0 0.0% 0 0.0% 12 4.0% 298 22.9 5.0%
West Lake Park
Arvada 78 98.7% 1 1.3% 0 0.0% 0 0.0% 0 0.0% 79 6.1 1.3%
West Lake Park
Arvada 53 42.4% 61 48.8% 0 0.0% 0 0.0% 11 8.8% 125 9.6 2.1%
Arvada 224 98.2% 4 1.8% 0 0.0% 0 0.0% 0 0.0% 228 17.5 3.8%
Arvada 403 49.5% 366 45.0% 2 0.2% 0 0.0% 43 5.3% 814 62.6 13.6%
Arvada 26 76.5% 4 11.8% 0 0.0% 0 0.0% 4 11.8% 34 2.6 0.6%
Golden Fossil 552 37.4% 783 53.1% 0 0.0% 0 0.0% 140 9.5% 1475 113.5 24.6%
Trace Golf Crs.
Golden Fossil 146 93.0% 11 7.0% 0 0.0% 0 0.0% 0 0.0% 157 12.1 2.6%
Trace Golf Crs.
Lakewood 33 17.3% 147 77.0% 0 0.0% 0 0.0% 11 5.8% 191 14.7 3.2%
Lakewood 106 83% 15 11.7% 0 0.0% 0 0.0% 7 5.5% 128 9.8 2.1%
Lakewood Fox 13 5.9% 204 93.2% 0 0.0% 0 0.0% 2 0.9% 219 16.8 3.7%
Hollow Golf Crs.
Lakewood Fox 23 95.8% 0 0.0% 0 0.0% 0 0.0% 1 4.2% 24 1.8 0.4%
Hollow Golf Crs.
Lakewood 32 7.5% 383 89.5% 0 0.0% 0 0.0% 13 3.0% 428 32.9 7.1%
Total 2741 45.8% 2,913 48.7% 31 0.5% 0 0.00% 302 5.0% 5,987 460.5 100.0%
Average 109.6 116.5 1.2 0.0 12.1 239.5
Notes: 1. Season includes only the regular trapnights (1 night per week) from June 5 to August 28, 2008 for a total of 325 trapnights.
%RA= Percent Relative Abundance
2008 Jefferson County Mosquito Report 11
Table 7 Total Number of Adult Mosquitoes per Trap During the Sentinel Zone Extra
Trapnights of the 2008 Season1
Trap Name Culex spp. Ae./Oc. Spp. Anopheles spp. Coquillettidia spp. Culiseta spp. Trap Avg Per
and Location # %RA # %RA # %RA # %RA # %RA Total Trapnight Trap %RA
Johnson Park 7 22.6% 19 61.3% 0 0.0% 2 6.5% 3 9.7% 31 4.4 2.2%
31 70.5% 12 27.3% 0 0.0% 0 0.0% 1 2.3% 44 6.3 3.1%
Arvada 99 30.9% 183 57.2% 0 0.0% 0 0.0% 38 11.9% 320 45.7 22.2%
Golden Fossil 309 34.2% 481 53.3% 0 0.0% 0 0.0% 113 12.5% 903 129.0 62.7%
Trace Golf Crs.
Lakewood 25 17.5% 110 76.9% 0 0.0% 0 0.0% 8 5.6% 143 20.4 9.9%
Total 471 32.7% 805 55.9% 0 0.0% 2 0.14% 163 11.3% 1,441 205.9 100.0%
Average 94.2 161.0 0.0 0.4 32.6 288.2
Notes: 1. Sentinel zone trapnights includes one (extra) trapnight a week from July 1 to August 5, 2008 for a total of 30 trapnights.
%RA= Percent Relative Abundance
As shown in Table 7, an additional 1,441 mosquitoes were captured during the extra sentinel zone
trapnights. Over the course of six weeks, from all 5 sentinel traps, there was an average of 48 total adult
mosquitoes per trap per night and an average of 16 adult vector mosquitoes per trap per night. The total
adults collected during the sentinel trap nights resulted in a trend similar to the regular trap nights with
Aedes and Ochlerotatus (55.9 percent) being the most abundant, followed by Culex (vector) species (32.7
percent), Culiseta species (11.3 percent), and Coquillettida species (0.14 percent). This results in 67%
non-vector vs. 33% vector adults being collected over the six weeks of additional trapnights.
As shown in Figure 5, vector species adult mosquito populations peaked during the first week of July and
again in late August. As shown in Section 3, after a very dry early season, the month of May received a
moderate amount of precipitation which led to many of the previously dry mosquito habitats to become
inundated with water. During the months of June and July, precipitation levels were again much lower
than normal while temperatures remained warm. During this period, many of the habitats that were filled
during the May rains began to stagnate from the warm temperatures and became much more conducive to
breeding Culex mosquitoes. This led to a rise in Culex populations in early July in most areas of the
region. Culex populations then fluctuated through the end of the season based on the availability of
stagnant water that developed after localized rainfalls. Non-vector species adult mosquito populations
peaked in late July after localized rainfalls and had another peak after above normal precipitation levels in
August. Vector and non-vector mosquito populations then decreased sharply in September as the night
and day time temperatures decreased.
2008 Jefferson County Mosquito Report 12
Figure 5 Season-Wide Weekly Adult Trap Counts of All Trap Stations, 2008
The early season low abundance of Culex mosquitoes, along with the low precipitation throughout the
2008 season, was likely a leading cause of the reduction in WNV activity throughout the region during
2008. In Figure 6, the 2008 adult Culex population trends are compared to those of 2005 and 2007. In a
year with more normal precipitation patterns, such as 2005, Culex populations will slowly build during
the early season, peak in mid-summer and then decrease into the fall. During the 2007 season, which had
the highest WNV levels since the epidemic year of 2003, Culex populations were at much higher levels
early in the season. In 2008, the lower temperatures and Culex populations caused the WNV cycle to
start later and magnify at a much lower rate than during a year with higher Culex populations and
Figure 6 Season-Wide Weekly Adult Culex Counts of All Trap Stations, 2005-2008
2005 2006 2007 2008
Avg. # of Culex Mosquitoes per Trap per Trapnight
6/7 6/14 6/21 6/28 7/5 7/12 7/19 7/26 8/2 8/9 8/16 8/23 8/30
2008 Jefferson County Mosquito Report 13
6.0 PUBLIC OUTREACH AND EDUCATION
Public education is an important component to any mosquito control program and is vital in combating
West Nile Virus. Jefferson County continued to provide valuable education materials to residents and the
general public through local media outlets, bulletins, and pamphlets and offered extensive information on
their internet website. The educational materials stressed the importance of actions that residents could
take to aid in the effort to combat WNV, including the topics of personal protection, property
maintenance and source reduction, and general information related to the WNV disease cycle. OtterTail
also provided local (303-273-2878) and toll-free (1-888-774-2161) telephone hotlines so that Jefferson
County residents could call and get answers on any specific mosquito related issues or to report potential
mosquito breeding areas. The hotline numbers were advertised by many of the municipalities in the
project area. There were 9 site investigations that originated from 21 hotline calls in 2008 (Table 8 and
Educating residents on the need for property maintenance, source reduction, and the use of personal
protection measures continued to be crucial in the fight against WNV in 2008. The resulting actions
taken by the public was another likely reason that Jefferson County experienced relatively lower WNV
activity, when compared to other nearby counties, during the 2008 season.
Table 8 Number of Site Visits per Municipality from Hotline Requests, 2008
Entity # Visits
JeffCo Unincorporated 4
Wheat Ridge 0
Total Visits from Hotline Requests 9
Table 9 Total Number of Jefferson County Hotline Calls and Site Visits from Hotline
Requests, 2005 – 2008*
Year # Hotline Calls # Hotline Visits
2005 No Data 47
2006 83 56
2007 39 28
2008 21 9
*No data available for total number of calls during the 2005 season
2008 Jefferson County Mosquito Report 14
CDC 2008. Centers for Disease Control and Prevention (CDC). West Nile Virus, 2008. Centers for
Disease Control and Prevention, Atlanta, Georgia. [Web page]. Located at
Accessed November 20, 2008.
CDPHE 2008. Colorado Department of Public Health and the Environment (CDPHE). West Nile Virus,
2008. [Web page]. Located at : http://www.cdphe.state.co.us/dc/zoonosis/wnv/wnvhom.html.
Accessed November 20, 2008.
HPRCC 2008. High Plains Regional Climate Center (HPRCC). [Web page]. Located at
Accessed November 18, 2008.
USCB 2009. United States Census Bureau. [Web Page]. Located at
Accessed November 20, 2008
2008 Jefferson County Mosquito Report 15
APPENDIX A - DETAILED LARVAL SURVIELLANCE
AND SITE SELECTION METHODOLOGY
Larval Surveillance Methodology
The following is a summary of the procedures used by OtterTail staff during larval surveillance. To
inspect a project site, a plastic dipper cup with a 3-foot wooden handle was used to collect water from the
site. Each sample (dip) was closely examined for mosquito larvae presence. The majority of the sites had
poor open water habitat in the center and good mosquito breeding habitat around the perimeter of the site.
At these sites, the dipping effort was completed using a linear approach (walking around the perimeter
and sampling the margins). Some sites were small (1 acre or less) and had good habitat throughout the
site. At these sites the dipping effort was completed using surface area guidelines where the entire site
was methodically sampled.
Using the linear approach, sites 1 acre or less in size were dipped approximately every 10 to 20 feet; sites
1 to 10 acres were dipped approximately every 50 to 100 feet and sites greater than 10 acres were dipped
approximately every 200 to 500 feet. Using the surface area approach, sites 1 acre in size and less were
dipped approximately every 10 to 20 square feet. Since each project site’s characteristics could vary as
the season progressed (e.g., become drier, wetter, increased vegetation), there were field adjustments
made during the season concerning the appropriate number of dips.
Larval Surveillance Site Selection/Characterization Methods
OtterTail used a series of maps, generated with Geographic Information Systems (GIS) technology, for
identifying and monitoring the larval habitat areas within the county that could support mosquito larvae.
Mosquito larvae require stagnant water and will thrive in areas where the water is high in nutrients,
organic matter, or other organic pollutants. Common habitats include: wetlands, riparian groundwater
sinks, non flowing irrigation ditches, flood irrigated fields, floodwater retention ponds, lake and river
shores, and a wide array of man-made habitats including pools, tires, pots, buckets, eves troughs, bird
baths and other similar containers. Since habitat sites can drastically change over time, the sites were re-
evaluated and classified, based upon their breeding potential, at the beginning of the season.
All habitat areas were classified as either “Target” or “Non-Breeding”. Target habitat included all areas
that were most suitable for mosquito larvae. Non-breeding habitat included areas with the potential to
become breeding habitat. The two most common types of non-breeding habitats were: 1) areas that
would become suitable larval habitat if the water level dropped (e.g., stagnant areas might develop in a
currently flowing stream); and, 2) areas that would become suitable larval habitat if the water level rose
(e.g., low-lying grassy areas surrounding ponds). Habitat sites were refined throughout the field season.
Additional habitat was mapped in response to calls from citizens and when technicians found additional
areas of concern in the field.
To ensure a comprehensive larval control program, OtterTail operated a toll-free telephone hotline so that
citizens could report areas of concern. OtterTail provided site inspections and any consequential
treatments to property owners with lands containing potential larval habitat. If landowners gave
permission, then suitable areas were added to the surveillance maps and checked regularly.
2008 Jefferson County Mosquito Report Appendix A
APPENDIX B - ADULT MOSQUITO TRAP DESCRIPTIONS
There are typically two different types of mosquito traps used to monitor adult mosquito populations, the
gravid trap and the carbon dioxide (CO2) light trap. For the season, both types of traps were incorporated
into Jefferson County’s adult mosquito surveillance system.
CO2 Light Trap
To capture the most representative sample of adult mosquitoes in an area, adult CO2 light traps were set
overnight, in adult mosquito harborage areas, to collect live adult mosquitoes throughout the season. The
traps are based on the principle that most adult mosquitoes are attracted to light, CO2 (via respiration), and
heat. The CO2 light trap often collects adult female mosquitoes that are attracted to mammals. Therefore,
the CO2 trap indicates when the vector mosquito species are no longer feeding solely on birds and has
turned to mammals for an alternate bloodmeal. This is one of the first indicators that WNV is likely to be
transmitted to people through the vector mosquito (assuming the trapped vector mosquitoes are found to
be carrying WNV).
The trap consisted of a plastic insulated bucket with a light and fan unit, with a finely meshed net attached
to it, hanging below it. The entire device was placed on a tree branch approximately 5 to 7 feet off the
ground by a small chain or rope letting the bucket and net hang free. The bucket was filled with 2 to 3
pounds of commercial dry ice (CO2). Holes at the base of the bucket allowed the dissipating CO2 to be
emitted as an attractant. A battery ran the small fan and light positioned above the net. The light
provided further attraction and once the mosquitoes were near the light, they were pulled down into the
net and trapped by the downward force of the fan. In the morning, the mosquitoes were removed and
then frozen to aid in identification. Once identified, the mosquitoes were then sorted by species and
vector mosquitoes were submitted to the Colorado Department of Public Health and Environment
(CDPHE) lab for WNV testing as needed.
The gravid trap mimics sources of mosquito breeding habitat and collects gravid female mosquitoes. The
trap consists of a 1 gallon plastic tub with a fan unit and net above it. The device is placed on level
ground in a mosquito harborage area. The plastic tub is filled approximately halfway with an odorous
liquid attractant that is obtained from a hay infusion. The attractant is a highly organic solution that
attracts gravid female mosquitoes. Above the tub is a fan unit with a finely meshed net attached to it. As
the mosquito flies along the surface of the attractant to lay its eggs, it is pulled up by the fan and into the
net. The net is collected the next day and the mosquitoes are frozen to aid in identification. Once
identified, the mosquitoes were then sorted by species and vector mosquitoes were submitted to the
CDPHE lab for WNV testing as needed.
2008 Jefferson County Mosquito Report Appendix B
APPENDIX C – 2008 JEFFERSON COUNTY IMM SERVICE
2008 Jefferson County Mosquito Report Appendix C
2008 Jefferson County Mosquito Report Appendix C
2008 Jefferson County Mosquito Report Appendix C