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

Abstract

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.

Standardized Operational Evaluations of Catch Basin Larvicides from Seven Mosquito Control Programs in the Midwestern United States During 2017

“…The reasons for the apparent differences between briquets and tablets compared to granules and pellets were not investigated as part of this study. We speculate that the briquet and tablet formulations, as a point source of active ingredient, are more prone to being buried in organic sediment or completely flushed out of basins, as suggested elsewhere (Harbison et al 2015(Harbison et al , 2016Nasci et al 2017). The granular and pellet formulations may provide a dispersed source of active ingredient in the basin that is less prone to being completely flushed out or buried.…”

“…Although routine quality control evaluations of pesticides are recommended for all mosquito control programs that apply larvicides and adulticides, it is unknown how often these evaluations are performed. Recent studies have found some pesticide formulations commonly used for controlling mosquitoes in catch basins (i.e., tablets and briquets) may not be as effective as previously assumed (Harbison et al 2015(Harbison et al , 2016Nasci et al 2017). This led to a broader interest in determining if a similar trend was occurring in catch basin control programs elsewhere in the region.…”

“…Based on previous studies using the same or similar larvicide products in catch basins Novak 1997, 1999;Harbison et al 2015Harbison et al , 2016Nasci et al 2017) it appears that 100% control effectiveness is an unrealistic expectation for many areas. Therefore, to compare effectiveness across the larvicides evaluated, a threshold of 25% or more of the basins scoring ''fail'' in a given site (cluster of catch basins) was set.…”

Environmental stochasticity and intraspecific competition influence the population dynamics of Culex quinquefasciatus (Diptera: Culicidae)

“…larval dynamics and response to rainfall [ 39 ], are known to maintain high abundance of Cx. quinquefasciatus [ 37 , 46 ], are widely distributed in urban areas of the USA, and are the most commonly targeted structure in urban larval control campaigns [ 9 , 11 , 47 , 48 ]. The model also assumes that the effects of evaporation within the system are negligible, and that oviposition site selection of Cx.…”

“…In addition to application cost, vector management in the Cx. quinquefasciatus system is limited by immature larval habitat characteristics [ 9 , 11 – 13 ], fine scale heterogeneity in virus transmission [ 7 , 14 ], and high levels of uncertainty in the prediction of disease outbreaks.…”