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274 C.S. CHARBONNEAU ET AL. Tahle 6. Percentages of mortality and SDS (in parentheses) of mosquilo and chironomid larvae used in the static acute toxicity tests 1110 Mortality al various trealment levels Dale Species Pond Control RAR" 2 x RARb 5 x RAR< 7/26/89 Micropseclro and Tonylorsus Mpd 27 (6) 87 (6) 100 (0) 100 (0) Psorosphero 0(0) 100 (0) 100 (0) 100 (0) 5/01/90 Chironomus, Dicrolendipes, and Porolonylorsus LBP< 40 (10) 50 (17) 93 (12) 5/08/90 Dicrolendipes LBP 27 (15) 27 (12) 23 (6) 5/19/90 Parolendipes MP 0(0) 17 (21) 57 (21) 6/08/90 Micropseclro. Tonylorsus, and Porolendipes MP 7 (6) 3 (6) 43 (15) Culex 0(0) 87 (15) 100 (0) TeSI no. 2 3 4 5 "Recommended application rate. bTwo times the recommended application rate. <Five times the recommended application rate. dMinnow Pond, Minnesota Valley National Wildlife Refuge. <Little Bass Pond, Minnesota Valley National Wildlife Refuge. rTreatment level not used in the test. pendent of the two water sources (Fig. 6). Toxicity of Vectobac-G was similar in water from a wetland (presumably with more organic and inorganic matter) and in well water. Addition of food did not change the efficacy of Vectobac- G to C. riparius at the tested concentration (Fig. 7). The effect of Vectobac-G at 0.1 ppm or chironomid lar- vae was independent of the percentage of organic matter con- tent in the tested sediment (Fig. 8). Fourth-instar larvae were less sensitive than second- and third-instar C. riparius at 0.0005 and 0.001 ppm (Fig. 9). DISCUSSION Range tests conducted in the laboratory indicated chiron- omid larvae were susceptible to Vectobac-G at levels substan- tially lower than those recommended for field application. However, all four field experiments indicated that Vectobac- G did not reduce abundances of the benthic community un- der conditions found at the Refuge, where it was applied at the operational level, or even at five times the operational level. The high power of enclosure test'l suggests high prob- ability of detecting error; therefore, if an effect of the pesti- cide occurred, it should have been detected. The emergence data from enclosure test I also indicated no effect of the in- vertebrate community after application of Vectobac-G. Al- though enclosure tests 2, 3, and 4 had lower power, none of the post-treatment levels of benthic invertebrates were notice- ably lower than post-treatment controls. Complete elimina- tion of mosquito larvae from the verification tests confirmed that applications of Vectobac-G were sufficient to control mosquito larvae as the product label described. Other formulations of Bti have impacted chironomid pop- ulations in natural habitats. In a review, the World Health Organization [58] reported species of chironomids were af- fected by locally produced cultures of Bti in flooded ditches in West Germany and that there was a 76070 reduction of C. fulvipilus in California after an aerial application of 0.05 kg/L. A wettable powder formulation of Bti was tested throughout the Kern Mosquito Abatement District, Bakers- field, California, resulting in reductions of chironomid lar- vae significantly higher than all other nontarget groups but lower than those of mosquito larvae (22). When a wettable powder formulation of Btiwas applied to experimental ponds, Table 7. Results of range-finding tests I to 4 (10 organisms per ,treatment) conducted with sediment OJo Mortality at various Vectobac-G~ concentrations (ppm) Range EC50" LCLb UCU Statistical test Sediment 10 0.1 0.01 0.001 Control (ppm) (ppm) (ppm) test I Control 100 100 90 60 50 0 0.002 0.00002 0.008 Probit 2 Control 100 100 70 20 0 10 0.04 0.02 0.11 Probit 3 LBpd 100 90 30 0 10 0 0.20 0.08 0.51 Probit 4 BBP< 100 100 40 0 0 0 -0.13 0.01 1.0 Interpolation and binomial .Concentration to which 50'10 of the chironomid larvae showed an effect after 48 h. bLower 95070 confidence limit. <Upper 95070 confidence limit. dLittle Bass Pond, Minnesota Valley National Wildlife Refuge. <Big Bass Pond, Minnesota Valley National Wildlife Refuge.