Annual Report: West Nile Virus Vector Mosquito Larval ... · Larval mosquito surveillance and...

West Nile Virus Vector Mosquito Larval Monitoring and Surveillance - 2011

February 2012

Watershed Monitoring and Reporting Ecology Division

Acknowledgements

TRCA recognizes the contributions from our staff including Sarah Scharfenberg for her technical

assistance with field sampling and larval rearing; as well as Ming Guo and Jason Tam for their

role in WNV database development and GIS mapping. We would also like to thank our regional

public health partners for providing us with invaluable data and their assistance in larvicide

treatment applications. Art Augenas from the Peel Region Public Health is acknowledged for

providing QA/QC on mosquito larvae identification.

The West Nile Virus Surveillance and Monitoring Program is a part of TRCA’s Regional

Watershed Monitoring Program. It is funded by the following partners:

Report prepared by: Jessica Fang, Aquatic Biologist Watershed Monitoring and Reporting Ecology Division Reviewed by: Scott Jarvie, Manager

Watershed Monitoring and Reporting Ecology Division

This report may be referenced as:

Toronto and Region Conservation Authority (TRCA). 2012. West Nile Virus Vector Mosquito Larval Monitoring and Surveillance – 2011. 18 pages + appendices.

Executive Summary

West Nile virus (WNV) is primarily a bird pathogen that first appeared in Ontario in 2001. Humans

and other mammals are incidental or “dead-end” hosts. Evidence suggests that two key vector

mosquito species, Culex pipiens and Culex restuans, are primarily responsible for spreading the

disease to humans in Ontario (Kilpatrick et al. 2005; Hamer et al. 2009). The level of WNV activity

and risk of exposure depends on the numbers of WNV bird hosts and WNV positive mosquitoes

in a given year. The vector population dynamics change from year to year, jurisdiction to

jurisdiction and are influenced by various biological and environmental factors. WNV

management strategies undertaken collectively by the provincial and regional health agencies in

Ontario focus on prevention and control measures.

The Toronto and Region Conservation Authority (TRCA) established its WNV vector mosquito

larval surveillance and monitoring program in 2003 as a measure of due diligence and at the

request of its regional public health partners. A variety of wetland habitats on TRCA properties

such as marshes, woodland pools and ponds have been considered potential mosquito

breeding habitats. However, monitoring data collected by TRCA since 2003 have shown that

healthy functioning wetlands generally do not support large Culex pipiens and Culex restuans

populations. Occasionally, isolated pockets of water with high densities of vector mosquitoes

have been found on TRCA properties; these findings signify the importance of larval mosquito

monitoring, so that these potential sites of concern can be identified and the appropriate control

measures can be taken.

TRCA’s WNV program has a three-pronged approach including public education and

communication, collaboration with regional public health units, and larval mosquito monitoring.

Public education activities in 2011 continued to focus on WNV related information distribution

and to address public and staff inquires on WNV and standing water complaints. In 2011, TRCA

received seven standing water complaints associated with TRCA properties. The complaints

were addressed according to TRCA’s Standing Water Complaint Procedure; none of the

complaint investigations resulted in larvicide application. Collaboration with regional heath units

in 2011 required TRCA staff to share WNV related information, attend regional health WNV

committee meetings, and provide mosquito larvae identification training to regional health staff.

Larval mosquito surveillance and monitoring was undertaken in 46 sites across TRCA jurisdiction

from May 30th to September 6th, 2011.

A total of 37 wetlands and 9 stormwater management ponds (SWMPs) were sampled using a

standard mosquito dipper. Larval mosquito sampling in 2011 yielded a high number of larvae

(n=8233) of which 6784 larvae were collected from the wetlands and 1449 were collected from

the SWMPs. Approximately 10% of larvae were not identified due to premature mortality during

the rearing process. Less than 25 % of mosquito species found in Ontario are considered WNV

vector species. WNV vector mosquito species refer to the species that are capable of carrying

and transmitting the virus from one host to another. The species that do not transmit the virus

are referred to as non-vector species. A total of 13 mosquito species were found during the 2011

sampling season including 5 non-vector species (Anopheles earlei, Culex territans, Culiseta

inornata, Psorophora ferox, and Uranotaenia sapphirina) and 8 WNV vector species (Aedes

vexans, Anopheles punctipennis, Anopheles quadrimaculatus, Culex pipiens, Culex restuans,

Culex salinarius, Ochlerotatus japonicus, and Ochlerotatus trivittatus). The most abundant

species in terms of presence was Culex territans which inhabited 39 of the 46 (85%) sampling

sites. The two key vector, Culex pipiens and Culex restuans, were found in 17 sites and 10 sites

respectively.

In the wetlands, 53.9% of larvae were non-vector species and 46.1% were vector species. The

non-vector, Culex territans (n= 3175; 53.1%) was the dominant species in terms of abundance

and presence. The two key WNV vectors Culex pipiens and Culex restuans represented 31.4%

and 2.6 % of the larvae collected respectively. Culex pipiens was the predominant vector species

found in the wetlands. Grenadier Pond in High Park, Lacey’s Pond in Altona Forest, and Goldfish

Pond in Tommy Thompson Park were identified as hotspots for Culex pipiens in 2011.

A total of 1449 larvae were collected in the SWMPs representing 17.5% of all larvae collected in

2011. Similar to the results from previous years, vector species comprised 83.5% of the

mosquito larvae collected, while Culex territans, the only non-vector species made up the

remaining 16.5%. Culex pipiens was the predominant species (n=1015; 73.6%) found in the

SWMPs. The other key vector species Culex restuans represented 3.8% (n=52) of the larvae

collected in the SWMPs. SWMP 279.1 in Durham and L’Amoreaux Park North Pond in Toronto

were identified as hotspots for Culex pipiens, and these two sites were larvicided by the Durham

Region Public Health and the City of Toronto Public Health respectively.

In conclusion, the results from the 2011 sampling season supported the findings from the

previous TRCA studies. It showed that wetlands do not generally support high densities of WNV

vector larvae, and SWMP sampling yielded low numbers of mosquito larvae overall. Only 6 of the

167 (3.6%) collected samples had high densities of vector mosquito larvae that warranted

control measures. However, WNV “hotspots” continue to occur and the locations vary from year

to year within TRCA’s jurisdiction. The most important objective of TRCA’s WNV monitoring

program is to reduce WNV risk to residents and conservation area user. In 2011, this objective

was met through identifying WNV hotspots and following-up with appropriate intervention

measures, through public education and communication, and through collaboration with TRCA’s

regional public health partners.

Table of Contents

1. Introduction ................................................................................ 1

2. Larval Mosquitoes Monitoring Methods .................................. 2

2.1 Larval Collection and Identification ............................................................2

2.2 WNV Risk Assessment ................................................................................3

3. Public Education and Communication ..................................... 5

3.1 Distribution of WNV Educational Materials ................................................5

3.2 Standing Water Complaint Procedure .......................................................5

4. Collaboration with Regional Health Units ................................ 7

5. Results and Discussion............................................................. 7

5.1 Mosquito Larval Density .............................................................................7

5.1.1 Wetlands ...............................................................................................8

5.1.2 Stormwater Management Ponds ...................................................... 10

5.1.3 Interspecies competition between key vector species .................... 12

5.2 Risk Assessment and WNV Vector Hotspot Identification ..................... 12

5.2.1 Wetlands ............................................................................................ 12

5.2.2 Stormwater Management Ponds ...................................................... 13

5.2.3 Standing Water Complaint Sites ....................................................... 13

5.3 Mosquito species composition and in-situ water quality........................ 15

6. Conclusions and Recommendations ..................................... 15

7. References ............................................................................... 17

List of Figures

Figure 1. Location of wetland and stormwater management pond monitoring

sites, 2011. .................................................................................................... 4

Figure 2. TRCA Standing Water Complaint Procedure .......................................... 6

Figure 3. Vector and non-vector mosquito larvae composition in wetlands from

2006 – 2011. ................................................................................................. 8

Figure 4. Mosquito species composition in wetlands, 2011. ................................. 9

Figure 5. Vector and non-vector mosquito composition in stormwater

management ponds, 2006 – 2011. ............................................................ 10

Figure 6. Mosquito species composition in stormwater management ponds,

2011. ........................................................................................................... 11

List of Tables

Table 1. List of WNV vector hotspots in wetlands, 2011. ..................................... 12

Table 2. List of WNV vector hotspots in stormwater management ponds, 2011. 13

Table 3. Results of standing water complaint investigation, 2011. ...................... 14

Appendices

Appendix A. Total Reported West Nile Virus Clinical Cases in Canada

Appendix B. WNV vector species of concern in Ontario

Appendix C. Overall summary the 2011 WNV monitoring season results

Appendix D. Risk ranking of wetland sites in 2011

Appendix E. Risk ranking of stormwater management pond sites in 2011

West Nile Virus Vector Monitoring and Surveillance – 2011 February 2012

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1. Introduction

West Nile virus (WNV) is primarily a bird pathogen that first appeared in Ontario in 2001. The

disease is transmitted to humans by mosquitoes that become infected by feeding on infected

birds. Humans are considered to be incidental or “dead-end” hosts whereby although humans

can be infected by the virus, humans do not spread the disease. For those people who become

infected, the majority will have no symptoms or only mild flu-like symptoms. Severe cases of

WNV, including the development of meningitis and encephalitis, are extremely rare but can be

fatal. In Canada, the numbers of human WNV cases fluctuate from year to year (Appendix 1),

driven by environmental and biological factors. In 2011, a total of 101 human cases were

reported in Canada; Ontario reported 64 cases, and Quebec reported 37 cases. There were 30

human WNV cases reported within TRCA’s jurisdiction: 24 cases in the City of Toronto, 3 cases

in Peel Region, 2 cases in Durham Region and 1 case in York Region (Public Health Agency of

Canada 2011).

In Ontario, since the discovery of WNV in 2001, the provincial and regional health agencies have

launched various WNV monitoring and surveillance programs involving dead birds, adult

mosquitoes, larval mosquitoes and human cases. Surveillance information from birds and adult

mosquitoes is crucial for determining the immediate risk of humans contracting WNV in an area.

Larval mosquito surveillance provides important information in identifying breeding sites that

support large mosquito populations. Once the sites of concern have been identified, the regional

health units are able to take mosquito control intervention actions such as source reduction.

Source reduction, which involves eliminating favourable breeding sites and reducing mosquito

larvae through larvicide application, is an effective means of reducing the risk of WNV.

The Toronto and Region Conservation Authority (TRCA) is the largest landowner in the Toronto

region managing over 13,377 hectares of land. TRCA properties include natural and constructed

wetlands, woodland pools, reservoirs, and ponds. These aquatic ecosystems have been

considered “mosquito friendly” as a result of permanent availability of standing water (Knight et

al. 2003; Gingrich et al. 2006; Rey et al. 2006). TRCA has initiated its WNV Surveillance and

Monitoring Program since 2003 as a measure of due diligence, and also at the request of

TRCA’s regional public health partners (Regions of Peel, York, Durham and the City of Toronto).

Since the launch of the program, TRCA has been monitoring larval mosquito populations in

selected natural habitats (collectively referred to as “wetlands” in this report) and stormwater

management ponds (SWMPs). The data collected are essential to identify the sites of potential

concern or vector “hotspots” and to follow up with appropriate management actions.

There are approximately 57 mosquito species in Ontario, of which 13 species are considered

WNV vectors (Appendix 2). WNV vector mosquito species refer to the species that are capable of

carrying and transmitting the virus from one host to another. The species that do not transmit the

virus are referred to as non-vector species. Studies (Kilpatrick et al. 2005; Hamer et al. 2009)

have suggested that Culex pipiens and Culex restuans are not only the primary vectors in

spreading the disease among birds (enzootic vectors), but also the primary vectors in spreading

the disease into the human population (bridging vectors). Effectively monitoring and controlling

these two key vector species is critical for reducing WNV risk in Southern Ontario.


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The results from TRCA’s WNV larval monitoring program (TRCA, 2006a; 2006b; 2007; 2008;

2010; 2011) have shown that SWMPs and healthy functioning wetlands typically do not support

high densities of mosquito populations. Occasionally, sites of concern for WNV (the numbers of

sites per year ranged from one to six between 2006 and 2011) have been identified through the

larval monitoring program. The ability to detect the sites of concern for WNV through larval

monitoring, subsequently to take appropriate control measures highlights the importance of

regular and continuous seasonal monitoring of the SWMPs and wetlands.

The TRCA WNV program focuses on reducing/eliminating WNV vector mosquito larvae and

dealing with the public concerns on TRCA properties through a three-pronged approach:

Monitoring and Surveillance: to identify sites of concern for WNV on TRCA

properties through mosquito population monitoring, and take appropriate control

measures if deemed necessary;

Public Education and Communication: to respond to public inquiries on WNV and

address standing water complaints on TRCA properties, as well as to provide WNV

information to both the public and TRCA staff; and

Collaboration with Regional Health Units: to participate in WNV advisory

committees in Peel Region, York Region, Durham Region and the City of Toronto

and to share WNV related data.

This report provides a summary of the 2011 West Nile Virus Monitoring and Surveillance

program activities.

2. Larval Mosquitoes Monitoring Methods

2.1 Larval Collection and Identification

TRCA larval mosquito monitoring and surveillance activities began on May 30th, 2011. A total of

37 wetlands and 9 SWMPs across TRCA jurisdiction (Figure 1) were routinely sampled during

the 2011 field season. The routine monitoring sites were selected based on the presence of key

vector species or a large population of any mosquito larvae in the past. The same sampling

protocol was followed as in previous years (TRCA 2004; 2006a; 2006b; 2007; 2008; 2010; 2011).

Each site was sampled four times at approximately three week intervals with a standard

mosquito dipper (diameter = 13 cm). Each site was divided into four comparatively equal

quadrants, and one sample was taken within each quadrant. Each sample consisted of 10 dips

of the mosquito dipper. Several dipping techniques including the shallow skim, complete

submersion, partial submersion, simple scoop and scraping (O’Malley, 1995) were employed to

ensure that the all types of potential mosquito habitats were sampled. Since rain drops disturb

the water surface and consequently cause mosquito larvae to disperse, no samples were

collected during a rain event.


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Collected mosquito larvae were transferred to plastic vials using a dropper and enumerated per

dip for each quadrant. Larvae were then pooled by quadrant and transported to the Boyd Field

Centre in a cooler.

In addition to mosquito larvae collection, TRCA staff also collected in-situ water quality data (pH,

temperature, electrical conductivity, total dissolved solids (TDS), and dissolved oxygen (DO))

using a YSI (650 MDS) meter. Field observation data such as water clarity, type of mosquito

predators (dragonflies, damselflies, frogs, and fish) present, and vegetation type and coverage

were also recorded during site visit.

Mature mosquito (fourth instar stage) larvae collected were killed in boiling water and preserved

in 70% ethanol on the day of collection. Immature larvae collected were reared in heated

terrariums until they reached maturity before they were killed and preserved. The preserved

specimens were identified to species using mosquito taxonomic keys (Wood et al., 1979; Darsie

and Ward, 2005). Ten percent (10%) of identified mosquito specimens were randomly selected

and sent to the Peel Region Public Health Unit for identification verification as part of the Quality

Assurance and Quality Control procedure (QA/QC).

The enumerated mosquito data were entered, verified and stored in the WNV Database Version

2.7 (a Microsoft Access database) maintained on the Boyd Field Centre sever.

2.2 WNV Risk Assessment

WNV risk assessment was carried out to identify vector mosquito hotspots. Sites were ranked

based on the average number of vector larvae found per 10 dips (40 dips/4 quadrants/site)

according to the modified Wada’s method of ranking (Wada, 1956):

Sites with no vector larvae were ranked as “Nil”;

Sites with <2 vector larvae per 10 dips were ranked as “Low” density;

Sites with 2 - 30 vector larvae per 10 dips were ranked “Moderate” density; and

Sites with >31 vector larvae per 10 dips were ranked “High” density sites.

Risk ranking was applied to each vector species independently, instead of the cumulative

number of vector larvae found at each site due to species variation in biology, host preference

and the efficiency of each vector species to transmit WNV.

Only the sites with “high” density of vector larvae or vector “hotspots” were addressed. Once a

hotspot was identified through risk assessment, the respective health unit was informed and

larvicide application options were considered if warranted.

West Nile Virus Vector Monitoring and Surveillance – 2011

February 2012

4

Figure 1. Location of wetland and stormwater management pond monitoring sites, 2011.


5

3. Public Education and Communication

3.1 Distribution of WNV Educational Materials

In 2011, TRCA continued to increase WNV awareness, and educate the public about WNV

through:

sharing tips on personal protection against mosquito bites, reminding the public to

perform good housekeeping practices, and making the latest WNV program annual

reports available on TRCA website (http://trca.on.ca/protect/monitoring/west-nile-

virus-monitoring-program.dot);

sending internal emails to remind TRCA staff the importance of personal protection

against WNV, and providing the latest status of TRCA’s WNV monitoring program

and the status of WNV in the region and in the province; and

displaying WNV newsletters and pamphlets in TRCA offices and conservation areas

throughout the summer.

3.2 Standing Water Complaint Procedure

Complaints from the public or staff regarding standing water or mosquito activities were

addressed according to TRCA’s Standing Water Complaint Procedure (Figure 2). Once a

complaint was received, TRCA staff followed these steps:

1. Acquired background information (location, name of the complainant, contact information,

and the nature of the complaint).

2. Evaluated the location for its proximity to a routine WNV sampling station, and the sensitivity

of the area.

3. TRCA’s Finance and Business Services Division and Planning and Development Division

were consulted to review property ownership, management agreements and land regulation

information.

4. For non-TRCA property or property under management agreement, the respective regional

public unit was notified. For TRCA properties, if deemed necessary, the complaint site was

monitored following the methods described in Section 2: mosquito larval collection and

identification and WNV risk assessment.

5. When a potential hotspot was identified through risk assessment, and if larviciding was

deemed appropriate, the following agencies were notified:

respective regional public health unit

Director of the Ecology Division, TRCA – for approval to proceed with the larvicide

treatments


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Receive Complaints/Enquiries:

Determine nature of Complaint

WNV / Mosquito related

enquiries

Enquiries requesting status of sensitive natural areas for

larviciding permit

Refer caller to MNR Aurora District (John

Pisapio)

TRCA Property?

Determine property ownership, verify land regulations with Planning & Development

Division

NO YES

Property under Management Agreement

Property Managed by TRCA

Notify Health Units of Ownership and

Regulations

Refer to Health Unit and land Managers and notify

if regulated1

Review site, Collect samples, risk rank site

Notify Health Units of results

Determine control options and carry out treatment

Verify land regulations

Notify caller

The Ministry of the Environment (MOE)– to obtain the permit for larviciding

The Ministry of Natural Resources (MNR)– to review the sensitivity of the area

Finance and Business Services Division, TRCA – to obtain a private pesticide contractor

6. Notified the complainant the results of the investigation.

Figure 2. TRCA Standing Water Complaint Procedure


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4. Collaboration with Regional Health Units TRCA has been working closely with its regional public health unit partners since the launch of

the WNV monitoring program. The collaboration efforts involved notification of vector hotspots,

and participation in advisory committees. In addition, TRCA also provided mosquito larval

identification training to staff from the Durham Regional Health Unit in May, 2011. The

participants learned to identify WNV vector and non-vector mosquito larvae typically found in

Southern Ontario.

TRCA was also ordered to assist with the implementation of control measures to reduce the

number of mosquito larvae in the Heart Lake Wetland Complex in Brampton. The Order was

issued on April 7, 2011 by the Medical Officer of Peel Regional under the Health Protection and

Promotion Act, R.S.O. 1990, c. H.7.

Based on sampling activities conducted by Peel Public Health Unit, three locations within the

Heart Lake Wetland Complex were successfully treated from July to September, 2011 (Paul

Proctor, Peel Region, Vector-Borne Disease Team Supervisor, pers. comm.). These locations

were treated with Bacillus thuringiensis israelensis, commonly referred to as Bti. Bti is a

bacterium found naturally in soils, and since 1982, it has been used successfully worldwide as a

biological pest control agent to combat mosquitoes and black flies (Health Canada 2011).

5. Results and Discussion

5.1 Mosquito Larval Density

In total, 8233 mosquito larvae were collected from the routine monitoring sites during the 2011

field season, including 6784 mosquito larvae collected from 37 wetlands and 1449 mosquito

larvae collected from 9 SWMPs. A total of 13 mosquito species were collected across all sites in

2011 including 5 non-vector species (Anopheles earlei, Culex territans, Culiseta inornata,

Psorophora ferox, and Uranotaenia sapphirina) and 8 WNV vector species (Aedes vexans,

Anopheles punctipennis, Anopheles quadrimaculatus, Culex pipiens, Culex restuans, Culex

salinarius, Ochlerotatus japonicus, and Ochlerotatus trivittatus). The most abundant species in

terms of distribution was Culex territans which inhabited 39 of the 46 (85%) sampled sites. The

two key vectors, Culex pipiens and Culex restuans, were found in 37% (n=17) and 22% (n=10)

of the sampled sites respectively.

The incidence of larval mortality during the rearing process dropped from 32% (the 5-year

average from 2006 to 2010) to 10% in 2011; this rearing mortality reduction was achieved by

separating mosquito larvae by maturity (size) and genera, and by rearing fewer larvae in each

rearing container.

The overall summary of the results from the 2011 WNV monitoring season is presented in

Appendix C.


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5.1.1 Wetlands

TRCA collected 6784 mosquito larvae among the 37 selected wetlands. Some collected

mosquito larvae (n=761) had died prematurely or had gone missing during the rearing process.

These larvae were unable to be indentified, thus they were not included in the statistical analysis

provide later in this report. The identified larvae (n=6023) consisted of 2775 (46.1%) vector

mosquito species larvae and 3248 (53.9%) non-vector mosquito species larvae. The vector

species to non-vector species ratio fluctuated from year to year (Figure 3) in wetlands,

depending on many complex biotic and environmental factors. At Grenadier Pond in High Park,

a significantly large number of Culex pipiens were collected on August 2, 2011 (n=1336, which

represented nearly half of the vector mosquito larvae collected among all wetlands in the

season). If the site at Grenadier Pond was removed from the data set, 71.5% of the larvae

collected were non-vector species, while the remaining 28.5% were vectors among the other

wetlands.

Figure 3. Vector and non-vector mosquito larvae composition in wetlands from

2006 – 2011.

In total, 13 species (8 vector species and 5 non-vector species) of mosquito larvae were found in

the sampled wetlands. The predominant non-vector species was Culex territans (52.7%),

followed by Uranotaenia sapphirina (1.0%), Anopheles earlei (<0.1%), Culiseta inornata (<0.1%),

and Psorophora ferox (<0.1%) (Figure 4). The predominant vector species was Culex pipiens

(31.2%), followed by Anopheles quadrimaculatus (4.7%), Anopheles punctipennis (4.0%), Aedes

vexans (3.1%), Culex restuans (2.6%), Culex salinarius (0.5%), Ochlerotatus trivittatus (<0.1%)

and Ochlerotatus japonicus (<0.1%) (Figure 4).

61.5% 29.0%26.5%

64.4%

46.1%38.5%

81.9%

71.0%

73.5%

35.6% 53.9%

0

1000

2000

3000

4000

5000

6000

7000

2006 2007 2008 2009 2010 2011

No

. o

f m

osq

uit

o larv

ae

Sampling year

non-vectors vectors


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Figure 4. Mosquito species composition in wetlands, 2011.

The summer of 2011 was dry; the total precipitation over the summer (June to August) was 189

mm, compared to 351 mm in 2010, 309 mm in 2009, and 341 mm in 2008. This unusually dry

weather resulted in a number of sites being either completely or partially dried up by the second

and subsequent sampling events. Refer to Appendix D for the locations of sites that dried up

during the sampling season.

Due to the dry and warm summer conditions in 2011, many of the routine monitoring sites turned

into shallow, warm and stagnant pools of water. Water temperature rises much faster in shallow

pools of water. Studies (Cranston et al. 1987; Bradford 2005) have shown that rising

temperatures promote larval development, and weather conditions directly impacts the

abundance of Culex pipiens, which increases its hatching rate from 3 days at 20 oC to 24 hours

at 30 oC. This might contribute to the increased numbers of Culex pipiens collected in the

sampled wetlands this year (Figure 4).

Aedes vexans, representing 34% (n=1875) of larvae collected from wetlands was the

predominant species in 2010. However, in 2011, much smaller numbers of this species were

found (n=190; 3.1%). The preferred larval habitats for this species are open, shallow grass-filled

depressions in pastures, roadside ditches and temporary woodland pools that have the

tendency to get periodic flooding (Wood et al. 1979). Consequently, the distribution and

abundance of Aedes vexans depends upon rainfall patterns as they depend upon rain to restore

Culex territans53.1%

Uranotaenia sapphirina

1.0%

Culex pipiens 31.4%

Anopheles quadrimaculatus

4.7%

Anopheles punctipennis

4.0%

Aedes vexans3.1%Culex restuans

2.6%

Vector species45.9%

Cx. salinarius, Oc. trivittatus, Oc. japonicus, An. earlei, Cs. inornata, and Ps. ferox accounted for less than 1% of mosquito larvae combined, therefore, these species were not included in the figure.


10

their breeding habitat. The unusually dry summer in 2011 might have contributed to the dramatic

reduction in Aedes vexans population.

Uranotaenia sapphirina has been found from south-eastern Canada to Florida along the eastern

seaboard of the United States. Its range extends into the central states west to North Dakota

and south into Mexico. It has been considered rare in southern Ontario (Wood et al. 1979).

TRCA collected 62 individuals (1% of all larvae collected, Figure 4) in 8 different wetlands in

2011; this finding might suggest that this species could be expanding its distribution

northwards.

5.1.2 Stormwater Management Ponds

A total of 1447 mosquito larvae were collected among the 9 SWMP monitoring sites. Mosquito

larvae (n=70) that had died prematurely or had gone missing during the rearing process could

not be identified and consequently, were not included in the statistical analysis provide later in

this report. The identified larvae (n=1379) consisted of 1151 (83.5%) vector mosquito species

larvae and 228 (16.5%) non-vector mosquito species larvae. The ratio of vector species to non-

vector species observed indicated that the mosquitoes collected from SWMPs were

predominantly vector species; the composition was similar to previous years (Figure 5).

Figure 5. Vector and non-vector mosquito composition in stormwater

management ponds, 2006 – 2011.

96.0%

99.0%

68.2%

86.2% 88.7%

83.5%4.0%

1.0%

31.8%

13.8%11.3%

16.5%

0

500

1000

1500

2000

2500

2006 2007 2008 2009 2010 2011

No

. o

f m

osq

uit

o larv

ae

Sampling year

Non-vector Vector


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During the 2011 sampling period, the following five vector mosquito species were collected in

order of abundance: Culex pipiens (73.7%), Culex restuans (3.8%), Anopheles Punctipennis

(3.8%), Anopheles quadrimaculatus (1.9%), Aedes vexans (0.4%), and Culex salinarius (0.1%)

(Figure 6). Similar to previous years, Culex territans was the only non-vector species collected,

representing 16.5% of the mosquito larvae found in the SWMPs.

Figure 6. Mosquito species composition in stormwater management ponds,

2011.

Compared to the wetlands, the number of species collected and their overall abundance was

lower in the SWMPs. The differences could be attributed to several facts:

Habitat preference of certain species – Species such as Culiseta inornata and

Psorophora ferox prefer temporary, rain-filled pools, particularly in or near wooded areas,

in overflow pools along streams and occasionally in potholes in stream beds after

summer rains (Wood et al. 1979). These species were absent from the sampled SWMPs.

High exposure to wind and frequent runoffs into SWMPs might cause recurrent and

continuous disturbance to habitats that mosquitoes need for depositing their eggs and

breathing. The summers of 2007 and 2011 were drier than usual and our data have

shown that there is a correlation between the lack of precipitation (fewer disturbances)

and the higher number of mosquito larvae collected in the SWMPs (Figure 5).

Culex territans16.5% Aedes

vexans0.4%

Culex pipiens 73.7%


1.9%


3.8%

Culex restuans3.8%

Vector Species83.5%

Culex salinarius was represented by only one individual, and was therefore not included in the figure.


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Regional health units continue to monitor SWMPs and apply larvicide periodically.

5.1.3 Interspecies competition between key vector species

The two key WNV vector species Culex pipiens and Culex resturans are ecologically similar

species and prefer similar breeding sites (Reiskind et al., 2008). Culex pipiens, an introduced

species, and Culex resturans, a native species, appear to coexist in the sampled sites. In 2011,

the abundance of Culex pipiens increased, while the abundance of Culex restuans decreased

slightly. These results might suggest that the introduced Culex pipiens is out-competing the

native Culex restuans in both wetland habitats and SWMPs. More data are necessary to

understand the complex interspecies relationship between Culex pipiens and Culex resturans.

All other interspecies associations were statistically non-significant.

5.2 Risk Assessment and WNV Vector Hotspot Identification

In total, 167 samples (46 sites x 4 sampling events – sites that dried up completely) were taken

from the monitoring sites in 2011, and only 6 samples were determined to have high densities of

WNV vector species. Mosquito control measures were taken to deal with WNV vector hotpots

after they were determined through risk assessment.

5.2.1 Wetlands

Risk assessment was performed on vector species found at each wetland site to determine WNV

risk. Most routinely monitored wetlands were determined to pose “nil”, “low” or “moderate” risk.

Only three wetland sites were identified as WNV vector hotspots in 2011 (Table 1, Appendix D).

Table 1. List of WNV vector hotspots in wetlands, 2011.

Site Region No. of vector larvae/

10 dips

Vector

species

Sampling

date

High Park – Grenadier

Pond

Toronto 334 Culex pipiens August 2

Altona Forest – Lacey’s

Pond

Durham 41 Culex pipiens August 16

TTP – Goldfish Pond Toronto 56 Culex pipiens August 23

The City of Toronto applied larvicide treatments at Grenadier Pond in High Park upon

notification. The treatments were effective. When the pond was sampled again on August 31, the

number of Culex pipiens larvae reduced from 334 to 7 per 10 dips. Lacey’s Pond in Altona Forest

was treated by TRCA’s pesticide contractor (GreenLeaf Pest Control), after a permit was

obtained from the MOE, and the Altona Forest Stewardship Committee was informed. Goldfish

Pond in Tommy Thompson Park was not treated due to the sensitivity of the area (presence of

species at risk) and the late seasonal timing of the discovery of the hotspot.


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5.2.2 Stormwater Management Ponds

A risk assessment was performed on vector species found in each SWMP to determine WNV

risk. Most routinely monitored SWMPs were determined to pose “nil”, “low” or “moderate” risk.

Only two SWMPs were identified as WNV vector hotspots (Table 2, Appendix E).

Table 2. List of WNV vector hotspots in stormwater management ponds, 2011.

Site Region No. of larvae/

10 dips

Vector species Sampling date

SWMP 279.1 Durham 50 Culex pipiens July 22

L’Amoreaux Park North Pond Toronto 105 Culex pipiens July 22

L’Amoreaux Park North Pond Toronto 30 Culex pipiens August 18

Similar to previous years, large numbers of Culex pipiens were found in L’Amoreaux Park North

Pond. This pond is under Management Agreement with the City of Toronto and the monitoring

results were forwarded to the public health staff. This site was subsequently larvicided by the

City of Toronto. The pond was sampled again on August 18, 2011, and it was once again

identified as a hotspot; after communicating with the public health staff, it was understood that

the treatment of this pond was on-going.

SWMP 279.1 was identified as a hotspot in late July, 2011. When the Durham Public Health Unit

was notified, TRCA was informed that all SWMPs in Durham Region were already being

monitored and treated if necessary. After a brief meeting with Durham Public Health staff, it was

established that TRCA will suspend the monitoring of all three SWMPs in Durham in 2012. All of

the monitored SWMPs are under management agreements; to avoid duplication in SWMPs

monitoring, a detailed list of TRCA’s monitoring sites will be distributed to all our regional public

health partners before monitoring season starts in 2012.

5.2.3 Standing Water Complaint Sites

In 2011, TRCA received seven standing water complaints associated with TRCA properties. Two

of these properties (YORK04 and YORK05) had been added onto the routine monitoring list

since 2010. Both of these sites dried up completely after the first sampling event (on June 14th,

2011), therefore very few mosquito larvae were collected at these two sites. One property,

Guildwood Park, is under a management agreement with the City of Toronto; therefore, the

information was provided to City staff. The other four properties were investigated and sampled

on a weekly basis following the sampling methods described in Section 2. Three sites were

located in York Region and one site was located in Durham Region. Site YORK02 was identified

as a moderate risk site in July, however the area dried up completely when visited a week later

and remained dry for the rest of the season. The rest of the complaint sites were monitored until


14

the end of the season. They were identified as low risk sites (Table 3), and no larvicide

treatments were warranted.

Additionally, TRCA also received three complaints associated with either public land or private

property; these non-TRCA property complaints were forwarded to the respective regional public

health units.

Table 3. Results of standing water complaint investigation, 2011.

Site Sampling

Date

Species Total

count

# of vector/

10 dips

Risk ranking

DURHAM01 29-Aug-11 Cx. territans 11 Non-vector

An. punctipennis 2 <1 LOW

An. quadrimaculatus 4 1 LOW

06-Sep-11 Cx. territans 25 Non-vector

An. punctipennis 6 1-2 LOW

An. quadrimaculatus 2 <1 LOW

YORK01 12-Jul-11 An. quadrimaculatus 10 2-3 MODERATE


YORK02 11-Aug-11 Cx. territans 64

Non-vector Ur. sapphirina 1

An. walkeri 1


An. quadrimaculatus 5 1-2 LOW

Cx. pipiens 2 <1 LOW

19-Aug-11 Cx. territans 52 Non-vector

Ur. sapphirina 5


25-Aug-11 Cx. territans 21 Non-vector

An. quadrimaculatus 2 <1 LOW

06-Sep-11 Cx. territans 15 Non-vector

Ur. sapphirina 2


An. quadrimaculatus 5 1-2 LOW

Cx. pipiens 3 <1 LOW

YORK03 25-Aug-11 Cx. territans 29 Non-vector

An. punctipennis 8 2 LOW



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5.3 Mosquito species composition and in-situ water quality

In 2011, TRCA continued to investigate the potential correlation between larval mosquito species

composition and water quality (pH, temperature, electrical conductivity, total dissolved solids,

and dissolved oxygen) and site characteristics. However similar to previous years, the data

collected in 2011 showed no statistically significant correlation between mosquito species

composition and water quality. In-situ water quality data were collected at one of the four

sampled quadrants at each site in previous years. In 2012, all in-situ data will be collected at all

four quadrants of each site. More data points might facilitate the discovery of any potential

correlation between mosquito species composition and site characteristics.

6. Conclusions and Recommendations

The number of mosquito larvae collected (n=8233) during the 2011 WNV season was the

second highest since the start of the TRCA WNv surveillance program (the highest being the

year 2010 with 9389 larvae) despite a number of sites drying up either partially or completely.

The improved rearing techniques reduced the rearing mortality rate from 32% (the 5-year

average from 2006 to 2010) to 10%.

The data from 2011 supported the findings from the previous TRCA studies. The studies showed

that wetlands do not generally support high densities of WNV vector larvae, and larval

monitoring in the SWMPs yielded lower numbers of mosquito larvae overall, but the majority of

the larvae were WNV vectors. The data also suggested that wetland habitats support a more

diverse mosquito population with greater abundance, and Culex pipiens and Culex resturans

have a preference for SWMPs over wetlands. The unusually dry and hot summer seemingly

created an ideal condition for Culex Pipiens to breed; this may have contributed to the increased

reported human cases in Ontario. The detection of isolated hotspots like SWMP 279.1,

L’Amoreaux Park North Pond, Grenadier Ponds, Lacey’s Pond, and Goldfish Pond signified the

importance of continuous larval mosquito monitoring across TRCA jurisdiction.

TRCA received and addressed seven standing water complaints associated with TRCA

properties as per TRCA’s Standing Water Complaints Procedure. No complaint sites were

deemed hotspots in 2011, however two complaint sites (YORK01 and DURHAM01) could

potentially be sites of concern in the future and additional monitoring effort should be invested in

monitoring and investigating these sites in 2012.

Collaboration with MNR, MOE, local stewardship committees, and TRCA’s management team is

crucial in terms of managing WNV vector hotpots in a timely manner on TRCA properties. A

TRCA WNV Monitoring and Surveillance Program work plan for the 2012 WNV season will be

developed before the field season starts. Once the locations of the routine monitoring sites have

been finalized, the management biologist at MNR will be informed of all the sampling sites, a full-

season permit from the MOE to larvicide high WNV risk sites will be requested, and the local

stewardship committee and the manager responsible for each of the monitoring sites will also be

informed of the WNV activities in the coming year.


16

A predictive statistical model has been developed by the Laboratory of Mathematical Parallel

Systems (LAMPS) at York University; the model uses weather data to predict WNV vector

abundance with reasonable accuracy. The researchers at York University are optimistic

regarding the development of weather-generated forecasting for WNV risk that could be used in

decision support systems for interventions such as mosquito control in Ontario.

According to Dr. Robbin Lindsay, Chief, Field Studies, Public Health Agency of Canada, (Ministry

of Health and Long Term Care 2009) the long-term strategies for WNV management will be that:

―Localized ―hotspots‖ for WNV virus amplification and risk of human disease will likely be

maintained within some/many jurisdictions. The disease burden will vary within jurisdictions from

year to year, driven by abiotic and biotic factors….., managing WNV means to maintain regional

appropriate surveillance and capacity to intervene—at least in areas of greatest risk, …., to

commit to two planks of WNV prevention (personal protective measures (PPM) and Integrated

Mosquito Management (IMM) and vital to all this is the sustainability of programs‖.


17

7. References

Bradford C.M., 2005. Effects of weather on mosquito biology, behavior, and potential for West

Nile Virus transmission on the southern high plains of Texas, a dissertation in

experimental toxicology. Doctoral dissertation Texas Tech University, 2005. 24 pages.

Cranston, P.S., Ramsdale, C.D., Snow, K.R. & White, G.B., 1987. Adults, larvae and pupae of

British mosquitoes (Culicidae). Freshwater Biological Association, Ambleside, Cumbria.

Scientific Publication, no. 48, pp. 1–152

Darsie, R.F and R.A. Ward, 2005. Identification and Geographical Distribution of the Mosquitoes

of North America, North of Mexico. University Press of Florida, Gainesville, Florida, 383

Pages.

Gingrich, J.B., D. Robert, G.M. Williams, L. O’Connor and K. Harkins, 2006. SWMPs,

constructed wetlands, and other best management practices as potential breeding sites

for West Nile Virus vectors in Delaware during 2004. J. Amer. Mos. Con. Asso., Vol. 22

(2): 282 -291.

Hamer GL, Kitron UD, Goldberg TL, Brawn JD, Loss SR (2009) Host selection by Culex pipiens

mosquitoes and West Nile Virus amplification. Am J Trop Med Hyg 80: 268–278.

Health Canada. 2011. Bti – Bacillus thuringiensis subspecies israelensis.

http://www.hc-sc.gc.ca/cps-spc/alt_formats/pdf/pubs/pest/_fact-fiche/bti-eng.pdf

Kilpatrick, A.M., L.D. Kramer, S. Campbell, E.O. Alleyne, A.P. Dobson, P. Daszak. 2005. West

Nile Virus Risk Assessment and the Bridge Vector Paradigm. Emerging Infectious

Diseases., Vol. 11(3) 425-429.

Knight, R. L., W. E. Walton, G. F. O’Meara, W. K. Reisen and R. Wass. 2003. Strategies for

effective mosquito control on constructed treatment wetlands. Ecol. Eng., Vol 21: 211-

232. Ministry of Health and Long-term Care, 2009. West Nile Virus Preparedness and Prevention

Plan - 2009. 53 Pages.

http://www.health.gov.on.ca/english/public/pub/ministry_reports/wnv_plan_2009/wnv_pl

an_full.pdf

O’Malley, C. 1995. Seven ways to a successful dipping career. Wing Beats. 6:23-24.

Public Health Agency of Canada, 2011. West Nile Virus MONITOR. Weekly Up Dates: Human

and Dead Bird Surveillance. http://www.phac-aspc.gc.ca/WNV-vwn/index.html.

http://www.hc-sc.gc.ca/cps-spc/alt_formats/pdf/pubs/pest/_fact-fiche/bti-eng.pdf

http://www.health.gov.on.ca/english/public/pub/ministry_reports/wnv_plan_2009/wnv_plan_full.pdf

http://www.health.gov.on.ca/english/public/pub/ministry_reports/wnv_plan_2009/wnv_plan_full.pdf

http://www.phac-aspc.gc.ca/wnv-vwn/index.html


18

Reiskind, M. H. and M. L. Wilson, 2008. Interspecific competition between larval Culex restuans

Theobald and Culex pipiens L. (Diptera: Culicidae) in Michigan. Journal of Medical

Entomology 45(1): 20-27.

Rey, J. R., G. O’Meara, S. M. O’Connell and M. M. Cutwa-Francis, 2006. Mosquito production

from four constructed treatment wetlands in peninsular Florida. J. Amer. Mos. Con.

Asso., Vol. 22 (2): 198 - 205.

Toronto and Region Conservation Authority, 2004. Report on West Nile Virus in Conservation

Wetlands. March 2004.

Toronto and Region Conservation Authority, 2006a: Report on West Nile Virus in Toronto and

Region Conservation Wetlands and Stormwater management ponds 2004 -2005. 85

Pages.

Toronto and Region Conservation Authority, 2006b. Annual Report: West Nile Virus Vector

Status in Toronto and Region Conservation Wetlands and Stormwater management

ponds– 2006. 12 Pages.

Toronto and Region Conservation Authority, 2007. Annual Report: West Nile Virus Vector Status

in Toronto and Region Conservation Wetlands and Stormwater management ponds–

2007. 24 Pages.

Toronto and Region Conservation Authority, 2008. Annual Report: West Nile Virus Vector

Mosquito Larval Surveillance and Monitoring - 2008. 42 Pages.


Mosquito Larval Surveillance and Monitoring – 2009. 31 Pages.


Mosquito Larval Surveillance and Monitoring – 2010. 26 Pages.

Wada, Y., 1956. Population Studies on Edmonton Mosquitoes. Questions ent. Vol.1: 187- 222.

Wood, D.M., P.T. Dang and R. A. Ellis, 1979. The Insects and Arachnids of Canada. Part 6: The

Mosquitoes of Canada Diptera: Culicidae. Publication 1686, Agriculture Canada, 361

Pages.

Appendices

Appendix A. Total Reported West Nile Virus Clinical Cases in Canada: 2002-2011.

This table was adapted from Public Health Agency of Canada website. <http://www.phac-aspc.gc.ca/wnv-vwn/mon-hmnsurv-archive-eng.php#a2002_07>

Province/Territory 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

Newfoundland and Labrador 0 0 0 0 0 0 0 0 0 0

Prince Edward Island 0 0 0 1 0 0 0 0 0 0

Nova Scotia 0 2 0 1 0 1 0 0 0 0

New Brunswick 0 1 0 1 0 0 0 0 0 0

Quebec 20 17 3 4 1 2 2 1 0 37

Ontario 394 89 13 95 42 12 3 4 1 64

Manitoba 0 142 3 55 50 578 12 2 0 0

Saskatchewan 0 937 5 58 19 1285 17 1 2 0

Alberta 0 272 1 10 39 318 1 2 1 0

British Columbia 0 20 0 0 0 19 1 3 1 0

Yukon 0 1 0 0 0 0 0 0 0 0

Northwest Territories 0 0 0 0 0 0 0 0 0 0

Nunavut 0 0 0 0 0 0 0 0 0 0

Total 414 1481 25 225 151 2215 36 13 5 101

Appendix B. WNV vector species of concern in Ontario (Ministry of Health and

Long Term Care, 2009).

*The highlighted species were found within TRCA jurisdiction in 2011.

Species Name

Culex pipiens

Culex restuans

Culex salinarius

Culex tarsalis

Aedes vexans



Anopheles walkeri

Ochlerotatus canadensis

Ochlerotatus japonicus

Ochlerotatus stimulans

Ochlerotatus triseriatus

Ochlerotatus trivittatus

Appendix C. Overall summary the 2011 WNV monitoring season results.

Species Wetlands SWMPs

Vector Species Density Abundance (# of sites)

Density Abundance (# of sites)

Culex pipiens 1878 10 1015 7

Culex restuans 157 4 52 6

Culex salinarius 27 6 1 1

Aedes vexans 185 7 5 2


241 21 52 5


284 23 26 5

Ochlerotatus japonicus

1 1 - -

Ochlerotatus trivittatus

3 2 - -

Non-Vector Species



Culex territans 3175 32 228 7

Anopheles earlei 5 4 - -

Culiseta inornata 4 3 - -

Psorophora ferox 1 1 - -

Uranotaenia sapphirina

62 8 - -

Missing/rearing mortality

761 70

Total 6784 1449

Appendix D. Risk ranking for WNV of wetland sites for 2011.

Sites were ranked based on the average number of vector larvae found per 10 dips (40 dips/4 replication)

according to the modified Wada’s method of ranking (Wada, 1956): with no vector larvae = “Nil”(_), with <2 vector larvae per 10 dips = “Low” ( L ), with 2 L 30 vector larvae per 10 dips = “Moderate” (M); and with

>31 vector larvae per 10 dips = “High” (H). (x) indicates the site was completely dry during a sampling event

Region

Site

West Nile Virus Vector Species

Cx. pipiens Cx. restuans An.

quadrimaculatus An. punctipennis Ae. vaxans

Sampling Interval 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4

Durham Region

Altona Forest Conservation Area

Altona Forest H L L M M M

Claremont Conservation Area

Wetland 1 L L M M L L

Wetland 2 L L M M L L

Carruther’s Creek

Swamp Complex x x x x x x x x x x

Frenchman’s Bay

Frenchman’s Bay Promenade

L L L L

GreenWood Conservation Area

Greenwood Marsh L L L

Greenwood Pond x M x x L x L x L

Peel Region

Albion Hills Conservation Area

Pond 1

Pond 2 x x x x x x x x x x x x x x x

Pond 4 L L L M M

Clairville Conservation Area

Wetland 1 x x x x x x L x x x x

Wetland 2 x x x x L L x x L L x x x x

Heart Lake Conservation Area

Heart Lake New Site

L L

Glen Haffy Conservation Area

Trout Pond 1 L M M M M M M

Trout Pond 2 L L L M M

Marie Curtis Park

Marie Curtis Wetland 1

L L

Appendix D. (Continued)

Region Site



quadrimaculatus An.

punctipennis Ae. vaxans

Sampling Interval

1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4

Toronto Col. Samuel Smith Park

Mini Pond

Main Pond L L

Eglington Flats

Topham Pond L M M

Woodland Pond M L M L L

High Park

Grenadier Pond L M H M M M M L L L

Humber Bay Boat Park

Mimico Amphibian Pond

Milne Hollow

Milne Pond

Tommy Thompson Park

Triangle Pond

Goldfish Pond H M

York Region

Bruce’s Mill Conservation Area

Bruce’s Mill L M M L L

Boyd Field Centre

YORK04 x x x M x x x x x x x x x M x x x

Granger Wetland (South Pond)

L M M L L

Boyd Wetland M M L L L L L M

Cold Creek Conservation Area

Wetland L L

Pond L L L L L

North Maple

YORK05 x x x x x x x x x x x x x x x

German Mills Creek

Elgin Mills Rd. x M x x L L x L x M M

Keffer Marsh

Keffer Marsh L L L L L L

Kortright Conservation Area

Kortright Marsh L L L L L L

Stouffville Conservation Area

Stouffville Reservoir

L

Toogood Pond Park

Toogood Pond

Appendix E. Risk ranking of stormwater management pond sites for 2011.

Sites were ranked based on the average number of vector larvae found per 10 dips (40 dips/4 replication) according to the modified Wada’s method of ranking (Wada, 1956): with no vector larvae = “Nil”(_), with <2 vector larvae per 10 dips = “Low” ( L ), with 2 L 30 vector larvae per 10 dips = “Moderate” (M); and with >31 vector larvae per 10 dips = “High” (H). (x) indicates the site was completely dry during a sampling event.

Region

SWMPs



quadrimaculatus An.

punctipennis Ae. vaxans

Sampling Interval 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4

Durham Region

253 L L L L L L L

262 M M M L

279.1 M H L L L L L L M M

Peel Region

174 L M L

Toronto L’Amoreaux Park North M H H L L M L L

L’Amoreaux Park South L L L L L

York Region

88.2 M M L L L L L L

139 L L

Killian Lamar L

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