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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., (Copyright © 2021 by The American Mosquito Control Association, Inc.)

During June to September 2017, 7 mosquito control programs in the midwestern United States evaluated a total of 9 catch basin larvicide formulations using similar protocols. Treated basins were monitored among study sites to observe when larvicides failed to control mosquitoes in 25% or more basins within a site. Overall, when monitoring occurred within the maximum label duration of the larvicides, sites treated with a single larvicide tablet or briquet surpassed the 25% fail threshold more often than pellet and granular larvicide formulations. In 438 of the study basins, the depth from sump bottom to catch basin lid was measured. In basins that were deeper than 5 ft (1.5 m), larvicides failed to control mosquitoes significantly more often than those 5 ft or shallower.

Effectiveness in controlling mosquito larvae in stormwater catch basins in the North Shore Mosquito Abatement District (northeastern Cook County, IL) was determined for 2 extended-duration larvicides indicating up to 180 days of control on their labels (Natular™ XRT, FourStar® Briquet) and 3 larvicides indicating up to 30 days of control (Natular™ T30, Natular™ G30, and VectoLex® FG). Over the course of the 26-wk study, catch basins receiving the extended-release products were treated twice (an initial treatment in early April followed by a re-treatment after 16 wk), and catch basins receiving the shorter-duration products were treated every 28 days, with the 1st treatment occurring during the 1st week in April. Control in an individual catch basin was considered to have failed if late-stage larvae or pupae were found in 2-dip samples taken from the catch basin. Control for a treatment was considered to have failed if >25% of catch basins treated with the product failed at a given time period posttreatment. All of the products evaluated in the study demonstrated some degree of control; however, the Natular XRT-, FourStar Briquet-, and Natular T30-treated basins rarely achieved the effectiveness threshold of ≤25% of catch basins failing. By comparison, basins treated with Natular G30 were below that threshold for 3 of 4 wk every treatment round and VectoLex® FG was consistently below that threshold for all 4 wk posttreatment for every treatment round. Compared with untreated catch basins, the total season pupal production was reduced by approximately 48% in the Natular XRT-, FourStar Briquet-, and Natular T30-treated basins, and by 87% and 99% in the Natular G30- and VectoLex FG-treated basins, respectively. Operational quality control observations indicated that effective control (i.e., ≤25% of catch basins failing) ranged from 3 to 12 wk posttreatment for catch basins treated with Natular XRT and from 5 to 9 wk with VectoLex FG, and that there was considerable geographic variation in the duration of effectiveness. The results indicate that 30-day re-treatments with granular formulations in difficult-to-control areas may provide a more cost-effective outcome than using 1 or 2 applications of extended-duration larvicides.