Table 2 Examples from the USA on the use of adult mosquito management in the control of outbreaks of WNV infection

Louisiana

This study calculated an 86% decrease (compared with a 5-year average) in WNV mosquito vector species in 2002 resulting from increasing control efforts (aerial and ground ULV with Naled (1,2-dibromo-2,2-dichloroethyl dimethyl phosphate) over a 4-month period in St. Tammany Parish, Louisiana.

[93]

Florida

This study estimated a seasonal mean 64.1% Culex nigripalpus density reduction following emergency aerial sprays with Naled in 26 Florida counties during 2004, in response to several hurricanes.

[94]

Boston

Poor efficacy of ground ULV treatments with Resmethrin against Cx. pipiens and Cx. restuans in the Boston area showed that the aerosol plume delivered from the road failed to contact the target mosquitoes because it was blocked by the vegetation. Therefore, the application was unable to reduce transmission of WNV.

[95]

Kentucky

The University of Kentucky evaluated the efficacy of professional application of bifenthrin and lambda-cyhalothrin as barrier treatments with backpack mist blower directed to all vegetative surfaces up to the height of 3 m. Residual efficacy in reducing adult mosquito populations was studied at 24 residential properties (eight replications by three treatments). Mosquito populations were measured on each property by using five methods: CO₂-baited CDC light traps (without a light), human landing rates, CDC gravid traps, ovitraps, and sweep nets. Populations were monitored weekly for two weeks before treatment and eight weeks post-treatment. Additionally, to confirm residual efficacy of each insecticide, a randomly treated leaf underwent a no-choice bioassay with laboratory-reared Ae. albopictus. Trap collections indicate Ae. albopictus and Cx. pipiens as the most abundant species in the area. Both insecticides significantly reduced Ae. albopictus density in comparison with the untreated control areas (85.1-89.5% reduction), and Ae. albopictus bioassay showed significant residual efficacy for both insecticides up to six weeks post-treatment. In contrast, Culex spp. were not reduced by either insecticides. The study seems to therefore indicate that barrier sprays applied to low-lying vegetation do not properly target adult daytime resting sites for Culex mosquitoes but that they can reduce Aedes mosquitoes. Perhaps Culex spp. abundance may be reduced by treating upper tree canopies.

[96]

California

In California, in 2004-2005, a program was implemented to control the amplification and dispersal of WNV using sequential ground ULV applications of Pyrenone® 25-5 (Insecticide containing Pyrethrins (5.0%) and Piperonyl Butoxide (25.0%)). Local treatments were started one month after the initial detection. Evaluations indicated that while the treatments were effective in reducing vector abundance, they had little effect on virus transmission, and WNV was dispersing throughout the area.

[97]

Sacramento

Carney et al. determined the efficacy of the aerial treatments with pyrethrins combined with piperonyl butoxide (PBO) on WNV transmission in Sacramento county during the 2005 epidemic. No human cases occurred in the treated area after repeated treatments (number of treatments not specified), compared with 18 cases in the untreated area. Consequently, they considered the emergency aerial spray to be effective in reducing both mosquito populations and WNV cases.

[98]

California

In the Coachella Valley good results were achieved with early season treatment in mid-April immediately following the first detection of WNV.

[99]

California

In Sacramento County, an aerial distribution of Evergreen EC 60-6 (insecticide containing Pyrethrins (6.00%) and Piperonyl butoxide (60.00%)) over approximately 215 km² obtained a significant decrease in the abundance of both Cx. tarsalis and Cx. pipiens, as shown by pre- and post-trapping realized inside and outside the spray zone.

[100]

New York

Controlling mosquito populations at the end of the season, before Culex females enter refuges, appeared to be an effective way to force declines in the virus circulation

[72]

California

This study examined the efficacy of the 2005 emergency aerial spray in Sacramento County, which used pyrethrins to control adult mosquitoes. An unsprayed area within the county was used as the control, showing a total decrease in Cx. pipiens and Cx. tarsalis, of 57.5%, compared with the pre-spray population in the treated area. They also observed a decrease in WNV infection rates in collected mosquitoes from 6.7/1,000 in the untreated areas to 3.9/1,000 in the treated areas.

[101]

Illinois

In the 2005 the city of Chicago used ground ULV treatments of sumithrin (ANVIL 10 + 10 at the dose of 1.36 g/ha), in areas with high WNV infection rates among Culex mosquitoes (>5 infected mosquitoes/1000). Gravid traps at 87 sites were used for monitoring. Two sequential treatments in weeks 31 and 32 decreased mean mosquito density by 54% (from 2.5 to 1.1 mosquitoes per trap-day), whereas mosquito density increased from 1.3 to 3.3 mosquitoes per trap-day at the non-sprayed sites. The difference between these changes in mosquito density was statistically significant. While other two sequential adulticide treatments in weeks 34 and 35 had no effect on mosquito density (probably because it was late in the season and the mosquitoes were presumably entering diapause and less active). Overall, there was significant decrease in mosquito density at the trap sites treated in all 4 weeks (weeks 31, 32, 34, and 35), while no significant effect was observed following single applications. Maximum likelihood estimates (MLE) of infection rate estimates varied independently of adulticide treatments, suggesting that the adulticide treatments had no direct effect on MLE. In general gravid trap counts were very low, which was probably due to large numbers of alternative oviposition sites, especially catch basins competing with the gravid traps.

[92]