appeared to be no interaction of the organophosphate insecticides Dylox 

 and Sumithion with Aroclor 1254 residues in this species. 



Mixtures of Dylox and Guthion were essentially as synergistic to 

 Atlantic salmon as they were to brook trout, but young salmon containing 

 total body residues of 1.5 yg/g DDT (and its analogs) were no more suscep- 

 tible to the mixture than were those without residues (Table 4). Aroclor 

 1254 residues of 2.3 yg/g in brook trout did not appreciably alter the re- 

 lative synergism of the Dylox with Guthion, or the additive toxicity of 

 any of the other forest insecticide mixtures (Table 3). 



DISCUSSION 



Sumithion, carbaryl, Dylox and Matacil are all relatively much more 

 toxic to young brook trout and Atlantic salmon than are Orthene and 

 Dimilin. However, barring accidental spills or excessive or overlapping 

 applications, the toxic concentrations determined (with one exception) are 

 well above those measured in streams after experimental aerial applica- 

 tions. The exception, the liquid field formulation of Matacil, was 10 to 

 70 times more toxic to brook trout than its technical grade form, and con- 

 centrations exceeding the 96-h LC50 of 0.13 mg/1 could be expected in 

 streams after aerial applications. The hazard connected with use of this 

 formulation would be increased in alkaline waters. Alkaline pH and high 

 water temperatures increased the toxicity of several of the candidate 

 forest insecticides to fish, but for most of these chemicals, even this 

 elevated toxicity does not appear sufficient to pose a significant toxicity 

 hazard. 



Although some of the candidate forest insecticides appear to be syner- 

 gized by other insecticides, this reaction is not likely to be an imminent 

 hazard to brook trout and Atlantic salmon, unless streams receive far 

 greater doses of the chemicals than have thus far been measured in experi- 

 mental aerial applications. A similar relationship exists in fish contain- 

 ing Aroclor 1254 residues, but the relatively toxic field formulation of 

 Matacil could be even more toxic to fish containing significant Aroclor re- 

 sidues. Chemical interactions in invertebrates were not tested. However, 

 considering the high toxicity of all of the compounds except Orthene to 

 these organisms, synergistic interactions could have a dramatic effect. 



The high susceptibility of scuds and stonefly naiads to Sumithion, 

 carbaryl, Dylox, Matacil, and Dimilin suggests that aquatic invertebrates 

 are much more sensitive to these compounds than are fish. In addition, 

 the LC50 values appear to be well within the concentrations measured in 

 streams after experimental aerial applications. Insecticide concentra- 

 tions of 0.24 mg/1 (Haugen, 1976) to 0.013 mg/1 (Marancik, 1976) have been 

 measured at 20 minutes and 24 hours, respectively, after application. 

 However, Orthene should not pose a significant toxicity hazard to fish or 

 invertebrates. 



22 



