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
1992-07
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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
Published Date
1992-07
Publisher
University of Minnesota Duluth
Type
Technical Report
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.
Description
Funding years: 1 April 1987 - 31 March 1992
Contribution Number 84, Center for Water and the Environment, Natural Resources Research Institute, University of Minnesota Duluth
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Niemi, Gerald J; Axler, Richard P; Barnidge, Phyllis; Hanowski, JoAnn M; Hershey, Anne E; Regal, Ronald R; Shannon, Lyle J. (1992). 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. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/200930.
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