Use of Centers for Disease Control and Prevention Gravid Trap in Catch Basins: Proof-Of-Concept Trials.

Gravid traps are commonly used by mosquito control agencies to collect local populations of Culex pipiens, which are then tested for the presence of West Nile virus. Culex pipiens adults disperse a relatively short distance (~2.5 km) from their breeding site, so it can be challenging to position a sufficient number of gravid traps to accurately monitor these mosquitoes in large urban areas. As placement of these traps is often limited to locations out of public?view, the potential for placing these traps belowground in commonly found storm-water catch basins was investigated. We compared the numbers of mosquitoes isolated in the Centers for Disease Control and Prevention (CDC) gravid traps placed aboveground with various types of CDC gravid traps placed in nearby catch basins. We found that the gravid traps placed in catch basins collected significantly fewer Culex pipiens females as compared to the aboveground traps. However, the 2 types of catch basin traps continued to function and collect mosquitoes despite heavy rainfall and runoff, demonstrating their utility for sample collection in an urban setting. The potential advantages and disadvantages of using catch basins for the placement of CDC gravid traps are discussed.

Assessment of Reactive Catch Basin Larvicide Treatments Toward Improved Water Quality Using FourStar(®) Briquets and CocoBear™ Larvicide Oil.

Because it is often logistically impossible to monitor all catch basins within an operational area, local mosquito control programs will preemptively treat catch basins with larvicides each season. However, these larvicides can, ostensibly, be considered water quality pollutants. To experimentally reduce the use of larvicides toward improving water quality, 30 basins within a small 0.7-km(2) residential area were monitored weekly for the presence of larvae and pupae for 14 wk in the summer of 2013. Once a basin was found to reach a threshold of 12 mosquitoes per dip sample, it received a FourStar® Briquet (a 180-day briquet formulation of 6% Bacillus sphaericus and 1% B. thuringiensis israelensis). Each week a FourStar-treated basin surpassed this threshold, it was treated with an application of CocoBear™ oil (10% mineral oil). By the end of trials, all but one basin received a briquet and 13 required at least 4 treatments of CocoBear, suggesting that preemptive treatment is appropriate for the study area.

Evaluation of Culex pipiens Populations in a Residential Area with a High Density of Catch Basins in a Suburb of Chicago, Illinois.

The North Shore Mosquito Abatement District applies extended release larvicides including methoprene-based Altosid® XR Extended Residual Briquets to approximately 40,000 catch basins in the southern half of the District's operational area at the beginning of each season. Treatments begin in May and typically again 9 to 10 wk later when larvicide efficacy appears to wane. In 2013 spinosad-based Natular™ XRT tablets were applied to basins, and a subset were monitored for larvae and pupae weekly with a standard dipper. When setting the threshold for retreatment as 12 juveniles per dip sample it was observed that basins required a second application 9 wk after the initial application, a time period similar to Altosid despite utilizing a different active ingredient. Average counts of weekly larval samples appeared to be positively associated with average numbers of Culex pipiens collected the following week in a gravid trap located among catch basins, highlighting the importance of basins as sources of these mosquitoes.

A comparison of FourStar™ Briquets and natular™ XRT tablets in a North Shore suburb of Chicago, Il.

Stormwater catch basins are the primary source of mosquitoes targeted by the North Shore Mosquito Abatement District, in the northern suburbs of Chicago. Over a 17-wk period (June to September 2011), 3 clusters of catch basins located within a 0.7-km2 area were monitored weekly with dipper samples of immature mosquitoes. During the 2nd week of monitoring, Natular XRT tablets (6.25% spinosad) were applied to northernmost cluster and FourStar Briquets (6% Bacillus û, 1% Bacillus thuringiensis israelensis) were applied to the southernmost cluster; the remaining middle cluster was kept untreated. Following 15 wk of monitoring in 20 basins in each cluster, both Natular and FourStar reduced immature numbers in treated catch basins for 8 consecutive weeks. These results suggest, if effectively timed, a single application of either larvicide may last an entire season.

Year-round Observations of Adult Culex Mosquitoes in Storm Sewer Pipes in the Northwestern Chicago Suburbs.

Over the course of three years, 200 ft to 0.75 mi (60 m to 1.2 km) sections of 3 larger (>6 ft [1.8 m] diam) belowground storm sewer conveyance pipes in the northwestern Chicago suburbs were inspected for the presence of adult mosquitoes. Culex mosquitoes were by far the most common (555 of 556 [99.8%] total mosquitoes) collected within pipes during all four meteorological seasons (i.e. during months of October, January, May, August). These observations support prior work elsewhere, suggesting storm sewer pipes are consistent sites of refuge for adult Culex mosquitoes.

Year-round Observations of Adult Culex Mosquitoes in Storm Sewer Pipes in the Northwestern Chicago Suburbs.

Over the course of three years, 200 ft to 0.75 mi (60 m to 1.2 km) sections of 3 larger (>6 ft [1.8 m] diam) belowground storm sewer conveyance pipes in the northwestern Chicago suburbs were inspected for the presence of adult mosquitoes. Culex mosquitoes were by far the most common (555 of 556 [99.8%] total mosquitoes) collected within pipes during all four meteorological seasons (i.e. during months of October, January, May, August). These observations support prior work elsewhere, suggesting storm sewer pipes are consistent sites of refuge for adult Culex mosquitoes.

Experimental use of Natular XRT tablets in a North Shore suburb of Chicago, IL.

In the northern suburbs of Chicago, stormwater catch basins are the primary source of vector mosquitoes targeted by the local mosquito abatement district, and therefore effective catch basin larvicides are needed. From June to September 2011, Natular XRT was applied at label rate to a cluster of 20 catch basins and compared to a cluster of 20 untreated basins all within a small 0.21 km2 area of catch basins known historically to have high levels of mosquitoes. All monitored basins held immature stages of mosquitoes at least once; however, significantly fewer immatures overall were observed from Natular-treated basins than in untreated basins during 14 wk of monitoring. During the study a portion of Natular-treated catch basins was cleaned as part of scheduled maintenance, and this appeared to remove both the larvicide and any control effects. Better timing of catch basin maintenance events is suggested, particularly if, as the results of this study suggest, a single larvicide treatment may be effective for an entire season.

Assessment of Truck-Mounted Area-Wide S-methoprene Applications to Manage West Nile Virus Vector Species in the Suburbs of Chicago, IL, USA.

West Nile virus remains the leading cause of arboviral neuroinvasive disease in the United States, despite extensive efforts to control the mosquito vectors involved in transmission. In this study, we evaluated the effectiveness of Altosid SR-20 (active ingredient, S-methoprene 20%) larvicide applications using truck-mounted ultra-low volume (ULV) dispersal equipment to target Culex pipiens Linnaeus (Diptera: Culicidae) and Cx. restuans (Theobald)larvae. A combination of emergence bioassays, open-field measurements of deposited S-methoprene and spray distribution using gas chromatography-mass spectrometry, and assessments of adult Culex spp. populations in response to applications were conducted over the summer of 2020 within the North Shore Mosquito Abatement District (IL, USA). Open-field applications revealed that dispersed Altosid SR-20 using ULV equipment was effective (75% emergence inhibition in susceptible lab strain Cx. pipiens larvae) up to 53 m. In suburban neighborhood applications, we found that S-methoprene deposition and larval emergence inhibition (EI) in front yards did not differ significantly from backyards. An overall EI of 46% and 28% were observed for laboratory strain Cx. pipiens and wild Cx. restuans larvae respectively, and both had an EI significantly higher than the untreated control group. The EI of exposed wild Cx. pipiens larvae did not differ from the untreated controls, suggesting an increased tolerance to S-methoprene. No difference in abundance of gravid or host-seeking adult Culex spp. post-application was detected between treated and untreated sites. These results document the ability of area-wide application to distribute S-methoprene, but this strategy will need further modifications and evaluation for Culex spp. management.

Ability of newly emerged adult Culex quinquefasciatus (Diptera: Culicidae) mosquitoes to exit belowground stormwater treatment systems via lateral conveyance pipes.

Culex quinquefasciatus Say mosquitoes flourish in belowground stormwater systems in the southern United States. Recent evidence suggests that oviposition-site-seeking females may have difficulties locating, entering, and ovipositing inside permanent water chambers when surface entry through pickholes in manhole covers are sealed. It remains unknown, however, if newly emerged adults are able to detect cues necessary to exit these partly sealed systems via lateral conveyance pipes or if they perish belowground. Fourth instar Cx. quinquefasciatus were placed within proprietary belowground stormwater treatment systems to determine the percentage of newly emerged adults able to escape treatment chambers via a single lateral conveyance pipe. Overall, 56% of deployed mosquitoes were captured in adult exit traps with an 1:1 male:female ratio. The percentage of adults captured varied significantly among chambers, but was not associated with structural site characteristics such as the chamber depth or the length and course of conveyance pipe to the exit trap. Empirical observations suggested that longbodied cellar spiders, Pholcus phalangioides (Fuesslin), ubiquitous in these structures, may have reduced adult trap capture. Findings demonstrate that newly emerged Cx. quinquefasciatus can exit subterranean chambers under potentially difficult structural conditions but suggest that a portion may perish in the absence of surface exit points in manhole shafts.

Reduced Potential for Consecutive Truck-Mounted Ultra-Low Volume Mosquito Adulticide Applications in Chicago Metropolitan Area Due to Weather Conditions.

To increase the effectiveness of adult mosquito control, more frequent ultra-low volume (ULV) applications have been suggested. In the Chicago area, weather may be a limiting factor to more frequent applications. To investigate this, ULV adulticidal application dates from the Northwest Mosquito Abatement District during June 2011 to September 2020 were compared to historical weather data-wind and rain patterns to determine optimum conditions for conducting ULV treatments. During the 85 applications performed during 2011-20, there would have been an opportunity, based on weather conditions, for an additional consecutive night for about half (48.2%) of the time. These opportunities diminished with each additional application. The most intensive response possible would have been a spray of 5 consecutive nights, which occurred once in 3 of the 10 years investigated.

The Impact of Adulticide on Culex Abundance and Infection Rate in North Shore of Cook County, Illinois.

Mosquito surveillance is critical to reduce the risk of West Nile virus (WNV) transmission to humans. In response to surveillance indicators such as elevated mosquito abundance or increased WNV levels, many mosquito control programs will perform truck-mounted ultra-low volume (ULV) adulticide application to reduce the number of mosquitoes and associated virus transmission. Despite the common use of truck-based ULV adulticiding as a public health measure to reduce WNV prevalence, limited evidence exists to support a role in reducing viral transmission to humans. We use a generalized additive and fused ridge regression model to quantify the location-specific impact of truck-mounted ULV adulticide spray efforts from 2010 to 2018 in the North Shore Mosquito Abatement District (NSMAD) in metropolitan Chicago, IL, on commonly assessed risk factors from NSMAD surveillance gravid traps: Culex abundance, infection rate, and vector index. Our model also takes into account environmental variables commonly associated with WNV, including temperature, precipitation, wind speed, location, and week of year. Since it is unlikely ULV adulticide spraying will have the same impact at each trap location, we use a spatially varying spray effect with a fused ridge penalty to determine how the effect varies by trap location. We found that ULV adulticide spraying has an immediate temporary reduction in abundance followed by an increase after 5 days. It is estimated that mosquito abundance increased more in sprayed areas than if left unsprayed in all but 3 trap locations. The impact on infection rate and vector index were inconclusive due to the large error associated with estimating trap-specific infection rates.

Discovery of vector mosquitoes (Diptera: Culicidae) in newly installed above- and belowground stormwater treatment systems in San Diego County, California.

Stormwater treatment systems (STS) intended to mitigate the potentially negative public health and environmental impact caused by urban runoff are highly conducive to mosquito production. Thirty-one STS, 15 aboveground extended detention basins (EDBs) and 16 proprietary belowground systems newly installed along State Route 125 in San Diego County, CA, were inspected monthly between July 2008 and June 2009 for presence of standing water and mosquitoes. During the 12-mo study, standing water was observed in 66% of the 180 total inspections to EDBs and at least once in each of the 15 basins, whereas belowground systems held water year-round in permanent-water sumps. With the exception of one EDB, immature mosquitoes were observed in all STS, during every month of the year in EDBs and all months except December in belowground systems. Cumulatively, mosquitoes were noted in 44% of the 372 total site inspections, with a nearly equal number of positive observations from EDBs and belowground systems. Four species were identified from EDBs, Culex tarsalis Coquillett, Cx. quinquefasciatus Say, Anopheles hermsi Barr and Guptavanij, and Culiseta incidens (Thompson). Cx. quinquefasciatus was the sole species identified from belowground systems. Results derived from this study provide additional evidence for mosquito production in STS because of structural design or persistent inflows, or both, of dry-weather urban runoff. Interagency collaboration is needed to ensure that STS are designed and maintained in a way that minimizes their potential to produce mosquitoes that can negatively affect public health.

Operational Basin Larvicide Evaluations in Northern Cook County, Illinois During 2019 and 2020.

Over the course of 2019 and 2020, 7 larvicide formulations for use in catch basins were evaluated using a standardized pass/fail protocol. A comparison between 1- and 2-pouch doses of VectoLex® water-soluble pouches (WSP; 10 g per pouch) and 20 g of VectoLex FG (loose granules) suggests that the WSP formulation may have a shorter duration than the FG formulation. Results also suggest that 20 g doses of each of 2 larvicides, Duplex™-G and Sumilarv® 0.5G, may have a minimum effective duration in basins for approximately 40 days. A 20 g dose of Altosid® XR-G and a single briquet Altosid XR each had an observed minimum effective duration of 20 days. Sustain MBG had an observed duration of only 7 wk posttreatment. These observations provide evidence that, in some locations, 20 g granular applications may have a longer effective duration than a single briquet. This work highlights the utility of applying a standardized protocol for routine quality control assessments of the thousands of catch basin larvicide applications performed seasonally by mosquito abatement districts.

Association between Culex quinquefasciatus (Diptera: Culicidae) oviposition and structural features of belowground stormwater treatment devices.

Belowground stormwater treatment devices referred to as Best Management Practices (BMPs) can be persistent sources of Culex quinquefasciatus Say (Diptera: Culicidae) mosquitoes and in urban environments necessitate routine monitoring and insecticide treatment for control. The design of certain structural features of BMPs may discourage mosquito entry and oviposition, potentially reducing the need for control measures. From August 2007 to November 2008, 29 BMPs were intermittently monitored for the presence of mosquitoes and compared against selected structural features. A bucket ovitrap was developed to estimate entry of oviposition-site-seeking females into BMPs via egg raft counts. The number of egg rafts collected was positively associated with larger diameters and shorter lengths of conveyance pipe from the nearest inlet or outlet and with increasing number of pickholes in BMP manhole covers. In contrast, the number of days pickholes were sealed against adult mosquito entry was negatively associated with eggs rafts. Findings suggest that the design of belowground BMPs and their conveyance systems can influence mosquito oviposition inside these structures. Specific design criteria with the greatest potential to minimize mosquito production and provide safe access for monitoring and control activities are discussed.

The need for collaboration among government agencies to reduce mosquito production in mandated stormwater treatment structures.

Efforts to improve water quality increasingly rely on structural stormwater best management practices (BMPs) to remove pollutants from urban runoff. These structures can unintentionally produce mosquitoes and may play a role in the spread of mosquito-borne diseases. A questionnaire was distributed to over 300 stormwater and mosquito control agencies in the United States to assess the prevalence of BMPs and associated mosquito production, identify current measures to control mosquitoes within BMPs, and elucidate the extent of collaboration between these agencies. Responses suggest that agencies often lack basic information relevant to minimizing mosquitoes in BMPs, such as the number of structures within an agency's jurisdiction and the frequency of their maintenance, and that greater interagency collaboration could improve control efforts. Approximately 40% of agencies reported regular collaboration to minimize mosquito production in BMPs; however, barriers to such collaborative work included confusion over roles and responsibilities and a lack of interest. The rapid increase of BMPs in urban environments resulting from increasingly stringent water-quality regulations provides justification for increased collaboration between stormwater and mosquito control sectors of government to aid and strengthen public health efforts.

Evaluation of manhole inserts as structural barriers to mosquito entry into belowground stormwater systems using a simulated treatment device.

Belowground proprietary stormwater treatment devices can produce mosquitoes, including vectors of West Nile virus. Elimination of vertical entry points such as pick holes in manhole covers may reduce the number of mosquitoes entering and reproducing in these structures. Plastic manhole dish inserts were evaluated as structural barriers against mosquito entry through pick holes in a simulated stormwater treatment device. Inserts were 100% effective at preventing mosquito entry through covers when no other openings existed. In devices configured with an open lateral conveyance pipe, the addition of an insert under the cover reduced mosquito oviposition significantly. Subsequent trials to further elucidate mosquito entry through manhole covers found a significant positive correlation between increasing number of pick holes and mosquito oviposition. Results of the study suggest the potential for manhole dish inserts to decrease the number of mosquitoes entering belowground structures. The different available stormwater treatment systems and site-specific installations may, however, provide a much greater variety of possible alternate entry points for mosquitoes than was addressed in the current study. Further work is needed in field installations to quantify the significance of pick holes to mosquito entry and determine under what conditions, if any, manhole dish inserts would be most effective and appropriate.

Standardized Back Checks of Catch Basin Larvicides Across Three Modes of Action in the North Shore Suburbs of Chicago, USA.

During June through September 2018, the North Shore Mosquito Abatement District evaluated a quality control protocol for larvicide-treated catch basins that allowed for comparisons of larvicides with different modes of action. Results suggest that when applied at a rate of 2 tablespoons (approximately 20 g of product) per basin, these larvicides (VectoLex® FG Lysinibacillus sphaericus, Natular™ G30 Spinosad, and Altosid® Pellets Methoprene) could reduce the need for retreatment for up to 4 wk or longer. Using this same protocol, Spheratax® SPH (50G) L. sphaericus was applied to a subset of approximately 900 basins and met the threshold for retreatment 1.1 wk postapplication. Despite these larvicide applications, pupae continued to be observed in non-Altosid® treated basins, suggesting 100% mortality may not be attainable.

Gravid Culex pipiens Exhibit A Reduced Susceptibility to Ultra-Low Volume Adult Control Treatments Under Field Conditions.

In July and August of 2018, a field trial was conducted to examine the effectiveness of the North Shore Mosquito Abatement District's operational ultra-low volume (ULV) adulticide program. Two study sites were selected in Skokie, IL, and treated by truck-based ULV with d-phenothrin and prallethrin synergized with piperonyl butoxide over the course of a month. Natural mosquito populations were sampled via Biogents (BG)-counter baited with CO2 or Alfalfa infusion. The results from this study demonstrate that host-seeking mosquitoes were reduced by 65.3% after ULV treatment while gravid mosquitoes were reduced by only 29.2%. In addition, host-seeking mosquitoes rebounded dramatically (303.1%) 3 days posttreatment while gravid mosquitoes did not (5.7%). Based on the differential effect between gravid and host-seeking mosquitoes, we concluded that the gonotrophic cycle and timing of ULV adulticide operations are important factors affecting the resistance of West Nile virus vectors to pyrethroid exposures.

Effect of conveyance pipe dimension and orientation on mosquito oviposition in a simulated stormwater management device.

Simulated stormwater management devices baited with alfalfa infusion were constructed to test conveyance pipe dimension and orientation as a potential deterrent to mosquito oviposition. Various configurations of pipe diameter, length, and orientation were evaluated based on egg raft counts. Field trials tested pipes of 1.3-, 5-, and 10-cm diam and 0-, 90-, or 270-cm lengths, in both horizontal and vertical orientations. Additional trials of 10-cm-diam horizontal pipe evaluated the effects of a 90 degrees bend, single or dual entry points, and lengths greater than 270 cm. Significantly fewer egg rafts were collected in pipes of smaller diameter and longer length in both horizontal and vertical orientations. A 90 degrees bend or removal of an entry point to pipes of fixed length had no significant effect on oviposition. A maximum tested length of 24.4 m did not preclude oviposition. The results of this study suggest that manipulating diameter and length of conveyance pipe in stormwater management devices may not be an effective strategy to deter oviposition. The need for integrating improved, novel, nonchemical mosquito control measures into designing and operating stormwater management structures is discussed.

Development time of Culex mosquitoes in stormwater management structures in California.

A widely recommended strategy to minimize mosquito production in structural stormwater Best Management Practices (BMPs) is to ensure they hold captured water for no more than 72 h. However, this standard may be overly conservative for many mosquito species found in urban environments and may impede or prevent the capacity of BMPs to fulfill more stringent water quality standards in environmentally sensitive areas. Egg-to-pupa development of Culex tarsalis, Cx. pipiens, and Cx. quinquefasciatus were examined during July, August, and September 2006 in stormwater management basins and in water collected from these basins in 3 climatically distinct regions of California: the Lake Tahoe Basin, Sacramento Valley, and Los Angeles Basin. The observed minimum times to pupal development were 6 days for Cx. tarsalis and Cx. quinquefasciatus and 8 days for Cx. pipiens. Multiple linear regression models were used to estimate minimum predicted development times under optimal conditions for each region. The results suggest that water residence times of up to 96 h will not significantly increase the potential for Culex mosquito production in stormwater BMPs in the 3 regions included in this study.