Evaluation of the potential effects of methoprene and Bti (Bacillus thuringiensis israelensis) on non-target organisms: A summary of before-and-after sites in western Wright County for 1988, 1989, and 1990

Abstract

This report summarizes the results of a multi-year study initiated in 1987 to assess the effects of two mosquito control agents, methoprene and Bacillus thuringiensis israelensis (Bti). Methoprene is a juvenile growth inhibitor which suppresses metamorphosis of mosquito larvae from pupating, while Bti acts as a poison to the mosquito stomach. Each of these control agents is currently in wide use in the seven county metropolitan area. Wetlands in Wright County, MN were selected as the study area. This area is beyond the current control program of the Metropolitan Mosquito Control District (MMCD), but was selected because of the lack of any prior treatment to the study sites. Two of six possible townships in western Wright County were randomly selected for detailed study, Victor and Corinna Townships. · The experimental design established in 1987 specified ·a "before-and-after" study in which baseline information would be collected for several years prior to treatment. After three years of data collection (1988, 1989, and 1990), the SPRP and invesitigators of this study decided to initiate treatment in 1991. This report is a summary of the "before .. phases of the study prior to any treatment with mosquito control agents. The non-taget organisms selected for this study included: Red-winged Blackbird populations, the general breeding bird community, aquatic insect populations, and zooplankton populations. The study is primarily focused as a statistical sample of a randomly selected group of wetlands representative of western Wright County. As such the sampling is designed to provide "snapshots" of the respective communities over time and to detect selected changes to the communities in response to applications of mosquito control agents. Sampling of Red-winged Blackbirds consisted of measuring clutch sizes, growth rates of nestlings, fledging success, foraging behavior by adults, and population levels of adults in the wetland sites. Sampling of breeding bird, aquatic insect, and zooplankton communities consisted of gathering replicated samples from each of the wetland sites during critical growing season periods. Several additional activities also were included to better understand critical aspects of the wetlands and to verify applications of methoprene and Bti to these wetlands prior to any treatment. Detailed maps of each of the wetland sites were developed to characterize the vegetation and location of sampling stations for each of the subdisciplines. In addition, procedures were developed to independently verify methoprene and Bti application and approximate the dosage. Analysis of data from the pre-treatment phase of the study indicates that populations of birds, aquatic insects, and zooplankton are highly variable among years. For instance, a wetland with a relatively high population for birds, aquatic insects, or zooplankton one year did not necessarily have a high population the next year. Moreover, we also observed that water levels fluctuate annually greatly within and among wetlands. A wetland with a relatively high water level one year may not have a high level the next. On a regional scale rainfall may appear to be relatively uniform, but on a local scale the pattern of rainfall is ~ot uniform. Red-winged Blackbird nest failure rates have been high overall, varying from 72 to 77 % per year. High nest failure rates are likely due to high predation from both birds and mammals, flooding of nests, cold weather periods, and storms. As expeded, many measurements of Red-winged Blackbird reprodudion varied significantly among years, but few differences were observed between the two study regions. Of the 28 bird species commonly found using these wetlands, many showed significant annual variation. In particular, populations of Red-winged Blackbirds decreased from 1988 to 1990 as well as those of two other common wetland species, the Swamp Sparrow and Common . Yellowthroat. The drought years of 1987, 1988, and 1989 likely are associated with these population changes. However, populations of Yellow.:.headed Blackbirds and Marsh Wrens have decreased over the study period. A total of· 96 genera of aquatic inseds from 23 families ·and 5 orders have been sampled from the 27 wetland sites .. Populations of aquatic insects were highest. in 1988 compared with 1989 and 1990. Little data are available on aquatic insect communities of wetlands to make any comparisons; however, the drought of 1987 and 1988 is the likely cause of decreased populations of aquatic insects in 1989 and 1990. It is unclear how quickly these populations will recover from these extremely dry conditions. Zooplankton populations were less variable between years despite dear differences in rainfall, water depth, and temperature among years. However, sampling was limited in 1988 because of the extreme drought conditions when all sites could not be sampled. The first year that all sites could be sampled was in 1990. A method was developed to measure methoprene to a limit of detection of 0.4 ug/1 (ppb). In samples gathered from the 28 wetland sites, no detectable methoprene was found. Similarly, a method was developed to detect Bti in the water. However, because of the relatively low concentrations that methoprene is active and the rapid decline of Bti once applied, it is likely impractical to verify the amount of. ambient methoprene or Bti in each wetland following treatment. As an alternative, protocols were developed to capture methoprene and Bti in particle samplers on each site to verify treatment. · Using the 1988 to 1990 data, 23 variables were selected to test for differences between wetland sites assigned to methoprene treatment, Bti treatment, or to be maintained as reference sites. Sites were randomly assigned to one of the three treatment groups and the groups then treated to see whether there were significant differences (p < 0.05) in any of the variables. After 458 randomizations, a suitable combination of the sites was identified which will be used in subsequent applications of methoprene and Bti for the "after" phase of the experiment. A review of the statistical power of the experimental design based on the 1988 to 1990 "before" sampling phase indicated that reasonable differences can be detected between treatment groups and reference sites with the experimental design. If treatment with methoprene or Bti have an effect on non-target species in these wetlands then it will be reasonable to detect differences in Red-winged Blackbird reproduction, foraging, or populations; zooplankton size, egg production, or densities; or aquatic insed densities. Despite widely varying environmental conditions such as drought and the lack of consistent trends among the sites, many differences can be detected if they occur due to treatment.