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.

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.

Variable Efficacy of Extended-release Mosquito Larvicides Observed in Catch Basins in the Northeast Chicago Metropolitan Area.

Since the mid-1990s, the North Shore Mosquito Abatement District (NSMAD) has applied extended-release formulations of mosquito larvicides to approximately 50,000 catch basins in the suburbs north of Chicago, IL, USA. This is performed as part of NSMAD's efforts to reduce local populations of the West Nile virus vector, Culex pipiens. Analyses from NSMAD's monitoring of larvicide-treated basins throughout the District over the 2014 and 2015 seasons suggest that larvicides intended to provide extended durations of control (30-180 days) failed to provide control for the maximum duration specified on the product label in approximately 25% of the District's basins. For larvicides designed to last up to 180 days (or about 26 weeks), failures were found at 1-15 weeks after treatment with most found at five weeks posttreatment. For larvicides formulated to last up to 30 days, failures were found at one to four weeks after applications with most found at three weeks posttreatment. The highest percentages of failing basins (ie, containing late-stage mosquito larvae or pupae during the specified product effectiveness period) were found in communities on the eastern side of the District, bordering Lake Michigan. As the larvicides appeared to function properly in the majority of monitored basins, it appears that the failures likely resulted from basin-specific physical factors (ie, basin volume, sediment content, and hydrology) that cause either product removal or a reduction in the concentration of the larvicide's active ingredient below the effective levels in these basins.

Small-Scale Trials Suggest Increasing Applications of Natular™ XRT and Natular™ T30 Larvicide Tablets May Not Improve Mosquito Reduction in Some Catch Basins.

Stormwater catch basins are commonly treated with larvicides by mosquito control agencies to reduce local populations of mosquito species capable of transmitting West Nile virus. Recent evidence suggests that extended-release larvicides formulated to last up to 180 days in catch basins may not be effective in some basins due to chronic flushing, rapid dissolution, or burying of treatment in sump debris. To investigate if increasing the number of applications could improve effectiveness, a small study was performed over 13 weeks in 2015 to evaluate two extended-release larvicides (Natular™ XRT 180-day tablets and Natular™ T30 30-day tablets) and a larvicide oil (CocoBear™). Over the course of 13 weeks, three groups of eight basins were monitored for mosquitoes, each group receiving Natular™ XRT, Natular™ T30, or CocoBear™ larvicides. All basins received a single application at the beginning of the study period. Once mosquitoes in a basin surpassed the treatment threshold during weekly monitoring, an additional application of the associated larvicide was given to that basin. The number of applications during the study period ranged from 1 to 10 for CocoBear™ basins, 1 to 7 for T30 basins, and 2 to 8 for XRT basins. Overall, the average number of applications and the cost of larvicide per basin were 4.4 applications at $0.66 per Coco-Bear™ basin, 4.4 applications at $6.26 per T30 basin, and 4 applications at $16.56 per XRT basin. Basins treated with XRT and T30 needed reapplications more often than expected, yet were no more effective than CocoBear™, suggesting that increasing the frequency of application of these larvicide formulations may not provide increased mosquito reduction in some basins.

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.

Observed loss and ineffectiveness of mosquito larvicides applied to catch basins in the northern suburbs of chicago IL, 2014.

In the northeastern part of the greater Chicago metropolitan area, the North Shore Mosquito Abatement District (NSMAD) treats approximately 50,000 catch basins each season with larvicide tablets as part of its effort to reduce local populations of the West Nile virus (WNV) vector Culex pipiens. During the 2014 season, an NSMAD technician monitored a subset of 60-195 basins weekly for 18 weeks among the communities of the District for the presence of mosquitoes. Monitoring found no clear evidence in the reduction of mosquitoes with the use of larvicides, and visual inspections of 211 larvicide-treated basins found that the majority (162, 76.8%) were missing tablets 1-17 weeks after applications. This loss of treatment may be due to the rapid dissolution or flushing of larvicides and would help explain why the larvicide appeared to be ineffective.

Identification of larvicide-resistant catch basins from three years of larvicide trials in a suburb of chicago, IL.

The tens of thousands of catch basins found in many urban areas are a primary target of local vector control agencies for seasonal application of extended-release larvicides. A concern with using larvicides in these structures is that active ingredients can be hampered by high flows, debris, and sediment, all of which are common to these structures. As such a certain proportion of basins may be "resistant" to larvicide treatments due to site specific characteristics that may promote these and other factors that hinder larvicide action and/or promote mosquito infestation. Analyses from three years of larvicide efficacy trials suggest that over a quarter of basins in the study area may not be receiving adequate protection from a single dose of larvicide that is routinely applied. Implications of increasing the dose and/or toxicity of larvicide treatments are discussed further.

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.

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.

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.