vidual chemicals, except that the two chemicals were added as fractions 

 of their activity (96-h LC5o's), and in a 1 to 1 ratio of their inde- 

 pendent toxicities. Toxicity of the mixture of chemicals was computed by 

 summing the LC50 ratios of the combined to the independent toxicities of 

 each chemical in the mixture, and calculating an interaction index (see 

 footnote, Table 3). Evaluations of potential additive activity, and 

 activity greater or less than additive activity were made by computing a 

 range of index values for the 95% confidence intervals around LC50 values. 



RESULTS 



Acute Toxicity 



Fish — 



Toxicities of the candidate forest insecticides ranged between 0.5 to 



11 mg/1 (96-h LCso's), with exception of Orthene and the field formula- 

 tion of Matacil (Table 1). Orthene was the least toxic; up to 100 mg/1 

 did not kill fish within 96 h. The toxicities of Sumithion and Orthene 

 (both organophosphate insecticides) were apparently unaffected by changes 

 in temperature (7-17 °C), water hardness (40-320 mg/1, as CaC03), or ph 

 (6.5-9.0). Further, the toxicities of these two field formulations were 

 similar to those for their technical forms. In contrast, the toxicity of 

 technical grade carbaryl , a carbamate insecticide, was influenced by 

 water quality. It was 3 times more toxic to brook trout at 17 °C than at 

 7 °C; and was most toxic in hard, alkaline water, e.g., it was 4 times 

 more toxic at pH 9.0 than at pH 6.5 (Table 1). The field formulation of 

 carbaryl was tested for toxicity. Because it is not soluble in acetone 

 or water, there were no mortalities, and its toxicity could not be rea- 

 sonably established with these techniques. 



Matacil is also a carbamate insecticide, and, like carbaryl, its toxi- 

 city was appreciably influenced by temperature and pH, but not by water 

 hardness (Table 1). It was over 3 times more toxic at 17 °C than at 



12 °C, and over 6 times more toxic at pH 9.0 than at pH 7.5. However, 

 the most significant finding from these tests was the discovery that the 

 field formulation of Matacil (17% active ingredient) was as much as 70 

 times more toxic than its technical form and could pose a serious hazard 

 to trout. 



The toxicity of the organophosphate insecticide Dylox to brook trout 

 was influenced by temperature, water hardness, and pH (Table 1). The 

 toxicity was 18 times greater at 17 °C than at 7 °C; almost 10 times 

 greater at pH 8.5 than at pH 6.5; and about 4 times more toxic in very 

 hard water (320 mg/1) than in soft water (40 mg/1). The toxicity of the 

 field formulation of Dylox (40% active ingredient) was similar to that of 

 the technical grade material. 



Dimilin is a comparatively new growth regulator that inhibits chi tin 

 synthesis during the molting of immature insects. No extensive toxicolog- 

 ical tests have been conducted with this compound, but preliminary data 

 indicate that it is relatively non-toxic to fish. 



14 



