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Redefining the Problem that is Container Mosquitoes

Breakthroughs in Aedes control show potential for WNv programs, but more study is needed.

WALS™ has gained notoriety in the recent past because of its demonstrated success and essential role in the management and prevention of arboviral diseases transmission by Aedes spp. mosquitoes. The use of VectoBac® WDG in WALS applications to control A. aegypti and A. albopictus – often characterized as container mosquito species – has been at the core of response to dengue and Zika virus outbreaks in Florida and beyond.

Key to the utility of WALS in these programs is its ability to deliver bacterial larvicide droplets efficiently to the many hard-to-find and hard-to-treat larval habitats of container Aedes. Yet while control of A. aegypti and A. albopictus using WALS has been well documented, the potential for WALS to control other important mosquito vectors is less well researched.

Potential Culex Program Benefits Observed

In 2016, during follow-up of aerial VectoBac WDG WALS applications for the control of invasive A. aegypti in Indio, CA., an interesting observation was made by surveillance teams of the Coachella Valley Mosquito and Vector Control District. Backyard inspections revealed numerous containers with dead Culex quinquefasciatus larvae in the WALS treated area. This finding generated significant interest in California and elsewhere for the potential for WALS integration into West Nile virus (WNv) vector control programs.

Control of WNv vectors is a complicated endeavor, especially in urban areas where larval sources of Culex spp. vectors are both ubiquitous and diverse. Over the course of the 20 years since West Nile’s appearance in the U.S., vector control professionals across the country have gained valuable experience in effectively managing the visible and treatable Culex sources such as street catch basins, stormwater retention facilities, wetlands, and waste lagoons. Even hidden underground storm drainage sources have been managed with innovative aerosol application technologies that distribute larvicides throughout the underground systems. What difficult larval sources remain tend to be backyard containers beyond the capacity of inspection crews to identify and treat by hand.

Complicating matters, many vectors of WNv have developed resistance to pyrethroid insecticides, which are the front-line defense whenever virus activity is confirmed in mosquito pools in a community. Further evidence suggests that these vectors may be also developing resistance to organophosphates – heavily relied upon for aerial responses to elevated WNv risk.

Taking all of these factors into account, it’s easy to understand why some leading vector control districts in California have begun investigating the potential for WALS applications to support WNv prevention. Districts including the Coachella Valley MVCD, the Sacramento-Yolo MVCD and the Greater Los Angeles VCD have all begun testing efficacy of WALS for control of Culex spp. in container habitats within residential areas. Each of these districts has undertaken projects aimed specifically at understanding the efficacy and utility of WALS in WNv programs.

The first steps were taken in 2018, with both ground and aerial evaluations, to characterize equipment capacities, determine efficacy in single-pass and block sprays, and optimize operational systems. On June 26th, the Coachella Valley MVCD hosted a multi-district characterization event during which equipment from around southern California was characterized. The district also completed subsequent aerial and ground block spray evaluations.

Similarly, the Sacramento-Yolo MVCD completed equipment characterization, single-pass spray evaluations and block spray evaluations. The Greater Los Angeles VCD also completed a WALS block spray evaluation at a large sewerage treatment plant known for Culex production.

Initial Results Promising, but More Work Remains

Generally speaking, mortality of Culex spp. larvae in bioassay of containers placed in both front and back yards throughout spray blocks in these trials has been high. Success has been weather dependent, with the best results seen from sprays applied during the night when an inversion is present. In vehicle-mounted ground sprays, it has been observed that some wind is critical to dispersion of spray droplets across residential blocks. It follows that work to optimize spray system settings for different wind conditions will be important. In aerial applications, it has been observed that applications are highly affected by downwind movement. Targeting strategies will need to be optimized.

The critical questions to answer in 2019 are whether WALS applications can impact adult Culex spp. population densities in WNv affected areas, and if WALS can be integrated into programs to pre-emptively reduce WNv risk. Through the use of GIS technology, several districts have begun to identify “hot spots” where virus activity tends to be concentrated. The collective hope is that early WALS intervention in these areas may delay WNv activity sufficiently to reduce the need to respond with mosquito adulticide applications when virus activity reaches critical levels.

While these details will be the object of further study in the months to come, what’s known is that the potential benefits of WALS as part of IVM programs for WNv vector control are significant. Many questions remain to be answered, but the entire mosquito abatement community deeply appreciates the leadership of the mosquito control professionals doing the hard work to complete these evaluations.

Gravid Culex Females Could be a WALS Target

One question of considerable interest to field research teams is if “non-larvicidal” effects of WALS-applied Bti can contribute further to break the WNv transmission cycle. Research published by Nayer Zahiri of UC Riverside showed that gravid female Culex mosquitoes can be killed when imbibing Bti-treated water during oviposition (a typical behavior). Researchers suggest that if this adulticidal effect occurs to any great degree after WALS applications, there could be a tremendous disease control benefit. Gravid females are the most likely to be infected with WNv, and following oviposition will seek additional blood meals. This is what drives transmission, and if they are killed during oviposition, transmission can be interrupted. .