Objectives: The purpose of this review is to examine the evidence for the effectiveness of larviciding with methoprene in controlling mosquito populations in Ontario that can carry West Nile virus (WNV). Methoprene is one of two larvicides federally registered and provincially classified for commercial use under permit for WNV in Ontario. The other larvicide is Bacillus thuringiensis var. israelensis (Bti). The primary enzootic vector of WNV in Ontario is the Culex pipiens. The breeding site preferences of Cx. pipiens and other mosquito populations in Ontario that carry WNV include standing water in discarded tires, various containers, ditches, catch basins and storm drains. Methoprene is advertised as a safe, effective, and practical method for controlling mosquito populations in these breeding sites. Bti has limited efficacy in these breeding sites because theycontain a high amount of organic matter. It is for these reasons that public health agencies in Ontario are looking at larviciding with methoprene to control mosquito populations that can carry WNV.
Methods: The search strategy followed the guidelines set out by the Cochrane Collaboration for conducting systematic reviews. The search strategy involved a comprehensive electronic search of nationally and internationally published studies, as well as anattempt to locate unpublished studies. Seven electronic databases were searched from inception to February 2004. Literature searches of prominent authors in the field were conducted and reference lists of all retrieved articles were reviewed. Three journals were hand-searched from first publication to the most current release. To capture unpublished studies, a general search on the Internet was conducted, all provincial and state web sites were investigated, and information was requested from experts via the WESTNILEVIRUS ListServ hosted by Cornell University. A standardized relevance tool was used. The only validity criterion considered was whether or not the outcome measure took into account the control group mortality. Allstudies are included in the review. Only interventions that are directed at controlling mosquito species that have been identified as carrying WNV in Ontario are discussed in the text.
Results: A total of 51 articles met the relevance criteria. Of the 51 relevant articles, 12 articles reported outcomes that took into account control mortality, 28 articles reported control mortality separately, 7 articles did not consider nor report control mortality, and 4 articles reported on larviciding with a combination of methoprene and another larvicide. The benchmark for mosquito control with a larvicide in general or methoprene in particular does not appear to be set. Many interventions do not report a time period to determine control a priori nor do they report the expected control period of the product. The effectiveness of methoprene is influenced by mosquito species, weather conditions, water quality, water quantity, product formulation, and dosage rates. The interventions included in the review took place in 8 countries, on more than 26 species of mosquitoes, with multiple formulations of methoprene, containing different amounts of methoprene as the active ingredient, in a range of dosages, and in many different settings making combining the data not possible. Given the many influences on the effectiveness of methoprene, it would be problematic to generalize to Ontario the results of studies that are not similar to the mosquito species and conditions in Ontario. Only four interventions used methoprene in a formulation available in Ontario on a mosquito species that can carry WNV in Ontario. Expanding the criteria to include methoprene formulations not available in Ontario introduced nine more studies andtwo other arms of the above-mentioned studies. Three of the interventions took place in Minnesota, in similar conditions to Ontario. Methoprene pellets achieved greater than 88% emergence inhibition (EI) for the label field-life of 30-days and greater than 76% EI for up to 75 days beyond the label fieldlife. The fourth intervention took place in irrigated pastures in California. The effectiveness of methoprene lasted 20 days or two flood cycles. In interventions using formulations of methoprene not available in Ontario applied to buckets, catch basins, cattail marsh ponds, and floating marsh plots, the formulations were less effective than the pellets in multi-armed studies and achieved lower EI rates in single-arm studies. The effectiveness of the briquet and liquid formulations were effected by the application of below label dosages, water quantity, water quality, wind, photo period and temperature. In many cases methoprene had to bereapplied before the label field-life.
Conclusions: Methoprene has been shown to inhibit emergence of mosquito populations with varying degrees of success for varying periods of time. Studies using methoprene pellets in conditions similar to Ontario on mosquito species found in Ontario report greater than 86% control of emergence for 30 days or more. In most cases, the failure of methoprene to inhibit the emergence of mosquito populations was caused by incomplete application or changes in water quantity or temperature, or air temperature. Proper application procedures and dosages as well as constant monitoring of environmental conditions, particularly heavy rainfall, and mosquito activity with the possible reapplication of methoprene when EI rates start to fall are crucial. Also crucial is creating a knowledge base for mosquito control by documenting and disseminating procedures, conditions and the effectiveness of control measures so that other groups can benefit and build on these experiences.