Toxicity of Bacillus thuringiensis var. Israelensis to Chironomids
in Pond Mesocosms
Karsten Liber1,
Kurt L. Schmude1 and Daniel M. Rau1
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(1)
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Lake
Superior Research Institute, University of Wisconsin-Superior, 1800 Grand Avenue, Superior, WI, 54880, U.S.A
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Abstract A pond mesocosm study was
conducted in a central Minnesota wetland to evaluate the potential
toxicity of the microbially-derived insecticide Bacillus thuringiensis var. israelensis
(B.t.i.) to chironomids. B.t.i. was applied as VectoBac® G to mesocosms on
two occasions (21 d apart) at five rates (0.3X, 1X, 2.5X, 5X, 10X) with three
replicate mesocosms per rate. The 1X rate (9 kg/ha) was that operationally
used by the Minneapolis-St. Paul Metropolitan Mosquito Control District for
early summer mosquito control. Chironomid abundances following B.t.i.
treatment were compared to abundances in untreated control mesocosms. The
abundance of Chironomidae larvae was significantly reduced at the 10X
treatment 4 d after the first B.t.i. application. Chironomid abundance was
also reduced after the second application with 10X, but showed strong signs
of recovery within 32 d. Chironominae, the numerically dominant subfamily
within the Chironomidae, showed a similar response. The abundance of Orthocladiinae
larvae was significantly reduced at both the 10X and 5X treatments, whereas
the Tanypodinae appeared unaffected by all B.t.i. treatments. Of the two
tribes comprising the Chironominae, the Chironomini displayed a response very
similar to that of its parent subfamily, although reductions in abundance
were not statistically significant. The tribe was dominated by Dicrotendipes,
Einfeldia, and Endochironomus, none of which were significantly reduced
following either 10X application. The second tribe, the Tanytarsini, were
slightly more susceptible to B.t.i. than the Chironomini, displaying
significant reductions in abundance after both 10X applications. The Tanytarsini
were dominated by Paratanytarsus, which were reduced by 91% 4 d after both
10X B.t.i. applications. Tanytarsini and Chironomini were also reduced in
abundance (by 83 and 75%, respectively) at the 5X treatment, but reductions
were not statistically significant. Regressions of larval chironomid
abundance versus B.t.i. treatment rate indicated that the B.t.i. rates
required to reduce chironomid abundance by 25, 50, and 75% were 1.5–2.0X,
2.1–3.3X, and 3.5–11.0X, respectively. Emergence of adult Chironomidae was
significantly reduced at the 10X B.t.i. treatment, but not at 5X. The same
trend was observed for the Chironominae, which comprised 82% of the family,
but not for Orthocladiinae and Tanypodinae. Emergence of Ceratopogonidae and Chaoboridae
was unaffected by all B.t.i. treatments.
Bacillus thuringiensis var. israelensis - toxicity - chironomids - wetland
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