RESUMEN
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.
RESUMEN
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.
