Invertebrates-- 



The 96-h LCso's for scuds and stoneflies were in the range of 0.002 to 

 0.04 mg/1 of the insecticides tested, with the exception of Orthene (Table 

 2). In general, invertebrates appear to be about 100 times more suscep- 

 tible to these chemicals than are brook trout. Although Dimilin was not 

 toxic to brook trout, it was highly toxic to scud. Of all six compounds, 

 Orthene was by far the least toxic to both brook trout and the aquatic in- 

 vertebrate tested. 



Toxicity to Eggs and Sac-Fry 



To date, studies of the effects of insecticides on life stages of 

 brook trout have been completed only for Sumithion. Eyed eggs (2 days be- 

 fore hatch) were exposed to concentrations of Sumithion similar to those 

 detected in Maine streams after experimental aerial applications of the 

 compound (Marancik, 1976). These tests were conducted in a flow-through 

 system described by Mount and Brungs (1967), and test concentrations were 

 reduced by one-half for 4 consecutive days and then stopped. For example, 

 with the highest concentration tested, eggs were exposed to 0.1 mg/1 on 

 day 1, 0.05 mg/1 on day 2, and the sac fry to 0.025 mg/1 on day 3, and 

 0.012 mg/1 on day 4. Treatment was then stopped, but the fry were ob- 

 served for an additional 30 days to monitor anomalies or delayed morta- 

 lity. The Sumithion concentration of 0.1 mg/1, which was several times 

 greater than the highest post-treatment concentration reported by Maranick 

 in streams, did not significantly (p=0.05) affect survival or development 

 of brook trout sac fry. 



Chemical Interactions 



The U.S. Forest Service anticipates that several of the candidate 

 forest insecticides may be applied to forests at about the same time, con- 

 currently for different pests, or that applications of different compounds 

 to infested areas may overlap. In addition, fish such as Atlantic salmon 

 and brook trout containing background residues of DDT and PCB's may also 

 be exposed to Guthion (an organophosphate insecticide), which is used in 

 blueberry culture. Therefore, the likelihood of salmon and trout receiving 

 multiple chemical exposures is high, and potential toxic interactions must 

 be explored. 



Insecticide Interactions-- 



Brook trout and Atlantic salmon were exposed to paired mixtures of 

 candidate forest insecticides, or to a combination of Dylox and Guthion. 

 The toxicities of all insecticide combinations were simply additive, ex- 

 cept for the Dylox and Guthion mixture which was synergistic in brook trout 

 and Atlantic salmon (Tables 3 and 4). This synergistic action of Dylox 

 and Guthion was also observed by Marking and Mauck (19/5) in studies with 

 rainbow trout [Salmo QaJjidneAl) . 



Insecticide, DDT, and PCB Interactions-- 



The carbamate insecticides carbaryl and Matacil were about twice as 

 toxic to brook trout containing Aroclor 1254 (PCB) residues of 2.3 yg/g as 

 compared with trout containing lower residues (Table 5). However, there 



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