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272 1.0 08 m 0.6 :::. Q; 04 :l: 0 a.. 0.2 C.S. CHARBONNEAU ET At. S 10 1S 20 Range of Means 2S Fi~..I. Power curve of the ANOVA (N= 10, IX = 0.05), or the prob- ability of detecting a statistically significant difference between the ~eans of the treatment levels (control and recommended applica- tion rate) for enclosure test I. duction in Diptera emergence over the period of pesticide ap- plications. Emergence from the RAR and control followed an almost identical pattern throughout the sampling period. Enclosure tests 2, 3, and 4 did not demonstrate toxic ef- fects of Yectobac-G on the benthic fauna at RAR or 5 x RAR during 1990. There was no statistically detectable effect of Vectobac-G on benthic organisms at the RAR or 5 x RAR in enclosure test 2 (Table 3). Invertebrate densities did not differ significantly (ANOVA p> 0.05) between treatments. Because the power of the test was fairly low (power = 0.37), it is difficult to conclude that there was no effect due to pes- ticide application. Nevertheless, verification test 2, conducted with enclosure test 2, confirms that Vectobac-G applied at the two concentrations (RAR and 5 x RAR) affected mos- quito larvae (1000/0 mortality) (Table 3). 60 50 '" c c Treatment tV Q; 40 ----11--- Control a.. '6 '0 30 t'? Ql .D E 20 => c C tV 10 Ql ;:;: o mo~mo~mO~mQM~QN~M -----NNM----__NN_ ~_______wmm-____~ ."."."."."."." CD CD CD CD CD Date Fig. 2. Mean number of adult dipterans (10 samples per treatment) c~lIec!ed on emergence traps during enclosure test I, following ap- pl.'catlons of Veclobac-G@) on May 10. May 25, and June 22, 1989, ~Iltle Bass Pond, Minnesota Valley National Wildlife Refuge, Bloom- Ington, Minnesota. 30 There were no reductions in the densities of Hyalella azteca or Chironomidae in the treatments of enclosure test 3 (Table 4). Mean densities remained less than one per sample for H. azteca, and actually increased slightly in each treatment for the chironomids. At 5 x RAR, Yectobac-G significantly reduced Oligochaeta densities by about 50%. Verification test 3 validated the potency of Yectobac-G on mosquito larvae in enclosure test 3 with 100% mortality at the RAR and 5 x RAR (Table 4). The natural temporal reduction in densities of chirono- mids that was observed the first field season occurred again during the second field season (Tables 3 and 4); therefore, the enclosures were moved to where densities of chironomids were greater. There were no significant statistical differences due to treatment for any of the organisms in enclosure test 4 (Table 5). However, the likelihood of determining if Yectobac-G affected benthic organisms was low (power = 0.06). In verification test 4, conducted for enclosure test 4, there was 97 and 100% mortality of the mosquito larvae in the two treatment levels of RAR and 5 x RAR, respectively (Table 5). The pattern resulting from the four field tests was clear. Applications were sufficient to completely eliminate mosquitoes but had little detectable effect on the other com- ponents of the benthic community. Toxicity tests. Static acute tests were conducted to deter- mine if chironomid larvae found in the field could be affected by Yectobac-G under controlled conditions. Higher mortal- ity of chironomids was evident at RAR for two tests and at the elevated concentration of 5 x RAR in four of the five tests (Table 6). In static test I, there was 100% mortality of the mosquito larvae (Psorosphera spp.). Chironomid (Mi- cropsectra [Tanytarsus) spp.) larvae exhibited low mortal- ity in the control (27%) but high mortality-87% (RAR), 100% (2 x RAR), and 100% (5 x RAR)-in the three treatments. Although mortality at the RAR approached 50% in static test 2, it was not much greater than the control (40%). There was substantially greater mortality at 5 x RAR (93%). No indication of an effect of Vectobac-G on the chi ron- omid larvae tested in static test 3 was evident. Percentage of mortality at RAR was the same as the control (27%) and was lower than the control at 5 x RAR (23%). Static test 4 demonstrated an effect on chironomid larvae with 17% mortality at the RAR and 57% mortality at 5 x RAR relative to control mortality of 0%. Mosquito (Culex spp.) larvae tested in static test 5 exhib- ited high mortality in both treatments of RAR and 5 x RAR (97%). Chironomids were affected (43% mortality) only at 5 x RAR. In summary, chironomids were affected more se- verely in laboratory toxicity tests compared to enclosure tests. The following series of laboratory toxicity tests were used to evaluate potential factors important in mitigating the toxic- ity of Vectobac-G to chironomids in the field. Laboratory toxicity tests Range tests. The EC50 concentrations for Yectobac-G in overlying water to C. riparius fell within the range of 0.08 to 0.51 ppm. The toxicity of Yectobac-G was less toxic to chi-