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<br />Environrncmal Toxicology and Cherni\lrY. Vol. IJ. No.2. pp. 267-279.1994
<br />Copyright @ 1994 SETAC
<br />Primed in Ihc USA
<br />0730- 7268/94 S6.00 + .00
<br />
<br />EFFECTS OF BACILLUS THURINGIENSIS VAR. ISRAELENSIS ON
<br />NONTARGET BENTHIC ORGANISMS IN A LENTIC HABITAT AND
<br />FACTORS AFFECTING THE EFFICACY OF THE LARVICIDE
<br />
<br />COLETTE S. CHARBONNEAU, *t RONALD D. DROBNEYt and CHARLES F. RABEN It
<br />tThe School of Natural Resources, University of Missouri, Columbia, Missouri 65211 tU.S. Fish and Wildlife Service,
<br />Missouri Cooperative Fish and Wildlife Research Unit, University of Missouri, Columbia, Missouri 65211
<br />
<br />(Received I December 1992; Accepted 2 June 1993)
<br />
<br />Abstract - Field and laboratory studies were conducted to determine the effects of Bacillus thuringiensis var. israelensis (Veclobac.
<br />G@) on waterfowl macroinvertebrate food resources at the Minnesota Valley National Wildlife Refuge, Bloomington, Minne-
<br />sota. Laboratory toxicity tests were conducted to evaluate the influence of environmental variables (e.g., temperature and depth).
<br />The EC50 was 0.20 ppm for Chironomus riparius. Fic:ld studies evaluated the effects of three applications at 5.6 kg/ha on ben-
<br />thic communities during the spring and summer of 1989, and three experiments with one application of 5.6 kg/ha and 28.1 kg/ha
<br />during the spring and summer of 1990. Benthic invertebrates were sampled before each application of Vectobac-G and at 48 h
<br />after application. There was no reduction of the major taxa found in the ponds that could be accounted for by the treatments.
<br />However, results from laboratory toxicity tests conducted with field-collecled Chironomidae indicated that these organisms were
<br />...Jlffected al <; 6 and 28 I h/ha. Further toxicity tests using Ch;;imomus riparius determined that various environmental vari-
<br />ables influenced the efficacy of VeclObaC-G, including temperature, larval instar, water depth, and water surface area coverage
<br />by macrophytes. This study indicates that chironomids (a major waterfowl food source) are adversely affected by Vectobac-G
<br />under a controlled laboratory situation; however, environmental factors reduce the efficacy of the larvicide in the field.
<br />
<br />Keywords- Vectobac-Gl!
<br />
<br />Toxicity tests
<br />
<br />Chironomidae
<br />
<br />INTRODUCTION
<br />Bacillus thuringiensis var. israelensis (Bti), serotype H-14,
<br />is a potent bacterial larvicide used in a variety of habitats
<br />[1-10). Toxicity is limited to nematocerous dipteran families,
<br />including mosquitoes (Culicidae) [1-10), and blacktlies (Sim-
<br />uliidae) [4, II-IS), in which the efficacy depends on feeding
<br />habits, physiology, and habitat of the insect. Besides these
<br />target organisms, other dipteran families may also be im-
<br />pacted. For example, 16 species of aquatic, benthic-dwelling
<br />Chironomidae are known to be susceptible to the delta-endo-
<br />toxin of Bti [4,16-22). Chironomidae are of special concern
<br />because of their ubiquitous distribution in aquatic systems
<br />and their importance as a major link in food webs contain-
<br />ing fish and waterfowl. The efficacy of the larvicide and po-
<br />tential impact on nontarget organisms appear to be related
<br />to environmental conditions within the treated habitat. Water
<br />temperature (23), water depth [24,25], aquatic vegetation
<br />[26,27], and dissolved and suspended organic matter [28-30]
<br />appear to be especially important, and other factors includ"
<br />ing the size or age of the insect [2,31-32] also influence the
<br />efficacy of Bti.
<br />There is a proposal to apply Bti routinely to areas of the
<br />Minnesota Valley National Wildlife Refuge ("Refuge; in the
<br />metropolitan area of Minneapolis-St. Paul, by the Metro-
<br />politan Mosquito Control District ("District"). Because the
<br />
<br />.To whom correspondence mav be addressed.
<br />The current address of C.S. Cha"rbonneau is U.S. Fish and Wild-
<br />life Service, Columbia Field Office. 608 East Cherry SI., Room 200.
<br />Columbia, MO 65201.
<br />
<br />Mosquitoes
<br />
<br />Efficacy
<br />
<br />Refuge is an important breeding area for three species of
<br />ducks, including mallard (Anas platyrhynchos), blue-winged
<br />teal (A. discors), and wood duck (Aix sponsa), all of which
<br />rely on invertebrate food during part of their life cycles,
<br />the question was raised whether hens and their broods
<br />would have an adequate food supply after applications of
<br />Vectobac-G@ .
<br />The goal of this study was to examine effects of Bti ap-
<br />plications on waterfowl invertebrate food resources at the
<br />Refuge. Specific objectives were to determine the effect of
<br />Vectobac-G on benthic invertebrates in an aquatic wetland
<br />community and to conduct laboratory toxicity tests to eval-
<br />uate the conditions under which environmental factors influ-
<br />enced toxicity.
<br />
<br />MATERIALS AND METHODS
<br />
<br />Field tests
<br />
<br />Study Area. The Refuge currently encompasses about.
<br />3,333 ha. A portion of its boundaries occur within the Min-
<br />neapolis-SI. Paul metropolitan area. Field studies were con.
<br />ducted in the Little Bass Pond (LBP) and Minnow Pond
<br />(MP), both located in the Bass Ponds Environmental Study
<br />Area of the Refuge. LBP is a O.4-ha, type 4 wetland (inland
<br />deep fresh marsh), as classified by Cowardin et al. [33], and
<br />MP is a O.I-ha, type 5 wetland (inland open fresh water). The
<br />LBP contained emergent vegetation, including great bulrush
<br />(Scirpus validus); narrow.leaved cattail (Typha angustifolia);
<br />broadleaf cattail (T. lotifolia); and submersed or floating
<br />aquatic vegetation consisting of coontail (Ceratophyllum
<br />demersum), elodea (Elodea canadensis), stonewort (Chara
<br />
<br />267
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