64 N. S. ENTOMOLOGICAL SOCIETY. 



ous to foliage) by acting upon them chemically, forming sod'um arsenate and a metallic 

 sulphide. 



On the hypothesis that the presence of sodium in some form increases the toxic value 

 of arsenicals by making them more attractive to insects, several facts, otherwise inex- 

 plicable, may be accounted for. We are informed through a firm of manufacturing 

 chemists that a large section of the potato growers of Maine insist on using finely ground 

 sodium arsenate as a poison with Bordeaux mixture, claiming that no other is as effec- 

 tive against the potato beetle. This may be explained on the ground that the sodium 

 hydroxide formed by the mixing of these chemicals, renders the sprayed leaves more ap- 

 petizing to the insects, resulting in more rapid killing. The Bordeaux mixture with 

 sodium arsenate, mentioned in Table No. 11, was made by first slaking the lime with 

 a solution of sodium arsenate and then diluting and adding to the copper sulphate solu- 

 tion. This mixture killed 61.5 per cent of the larvae that fed upon it, and in the same 

 period that the other four poisons averaged only 40.0 per cent., thus tending to corro- 

 borate the assert'on of the potato growers. The same explanation may account for the 

 effectiveness of the famous "Criddle Mixture" used in grasshopper control, originated 

 by Mr. Norman Criddle, Field Officer, Dominion Entomological Branch, to which com- 

 mon salt (sodium chloride) is added. The old-fashioned home-made lead arsenate is 

 prepared from lead acetate and sodium arsenate, and sodium acetate i" formed at the 

 same time. Growers using the homemade article have often referred to itg superiority 

 to the commercial brands, and, if our hypothesis is sound, their assertions may well be 

 correct. Our experiments certainly seem to indicate that sodium compounds hasten 

 the action of arsenical poisons. Whether this is entirely due to the effect of the sod- 

 ium in increasing the palatability of the leaves upon which the mixture is sprayed, caus- 

 ing the insects to eat more freely, or whether it also hastens the action of the arsenic, has 

 not been determined. 



The relative action of the different combinations as determined by our experiments 

 agrees, in a general way, with the results of other workers. Arsenate of lime is inferior 

 to both acid and neutral (triplumbic) lead in efficiency, and barium arsenate still more 

 inferior. The triplumbic arsenate of lead is inferior to acid or hydrogen arsenate of lead 

 in all combinations except with Bordeaux mixture. It would seem that the Bordeaux 

 does not inhibit the action of this form of lead arsenate to as great an extent as it does 

 that of the other poisons. 



Many of the combinations tested in the experiments, were r ound to be quite un- 

 safe to use on apple foliage, these being marked "o" in Table No. 11. Others are safe 

 under certain conditions, and these are marked "x" in the table. Fo example sodium sul- 

 phide and arsenate of lime gave no burning where only one application was made, but 

 when repeated two, three or four times, it frequently caused yellowing and dropping of 

 the leaves. Apparently, strong-growing, vigorous trees are more resistent to the ill ef- 

 fects of the spray than those in less thrifty condition. The lime sulphur-arsenate of 

 lead combination, which has been used for years by our growers, is discussed in full by 

 one of the writers in another article. 



