398 
Journal of Agricultural Research 
Vol. VII, No. 9 
important, for from our present knowledge the insects are killed by the 
absorption of the materials through the trachea, and in this case it would 
be necessary for the solution to spread over the body of the insect in 
order to gain entrance to the spiracles. A slight loss of wetting power of 
solutions which have an efficiency of 75 per cent or less would probably 
not cause an appreciable change in their effectiveness. This is shown 
in the case of group 3, in which the wetting power becomes less as the 
concentration of the solution is increased, but the efficiency rises in a 
normal curve. If a solution with an efficiency of 85 per cent or more 
upon further concentration loses wetting power, there is an appreciably 
greater corresponding loss of efficiency than would occur by increasing 
the concentrations of a solution whose efficiency is less than 75 per cent. 
The reason for this is obvious. A certain percentage of the insects are 
completely covered by the spray, so that the wetting-power influence 
on the efficiency is negligible; but when the insects which are struck by 
only a small quantity of the spray are considered, it is evident that the 
wetting power, as well as the strength of the solution, is an important 
factor governing its efficiency. 
It is evident from the discussion in the preceding paragraphs that the 
loss of wetting power in the more concentrated mixtures is not due to a 
physical effect, but to chemical reactions caused by the nicotine sulphate 
in a soap solution. Since, if the nicotine, without causing chemical reac¬ 
tions, did exert an influence on the physical properties of the solutions, the 
loss of wetting power would be directly proportional to the amount of soap 
in the solution and also to the wetting powers of the soap solutions which 
contained no nicotine. Likewise, the loss of efficiency in the concentrated 
solutions is not due to a reaction which would cause a portion of the nico¬ 
tine to be liberated as free nicotine, for the nicotine contents of the solu¬ 
tions were determined and the percentages were found to remain constant 
irrespective of the amounts of soap added to the solutions. From these 
facts we are led to assume that either a direct loss of wetting power or a 
reduction of the insecticidal value of the soap or both are contributing 
factors in the loss of efficiency of the more concentrated nicotine-sulphate 
and fish-oil-soap solutions. 
SUMMARY 
(1) When using combination fish-oil-soap and nicotine-sulphate sprays 
for the control of insects affecting truck crops, it was found that certain 
concentrated mixtures did not give as satisfactory results as did some of 
lower concentration. In connection with these results it was noticed 
during the spraying operations that some of the more concentrated 
solutions did not possess as high wetting or spreading powers as other 
mixtures which contained less soap. 
(2) The spraying operations were performed on peas, spinach, and 
strawberries against the pea aphid, spinach aphid, and red spider, 
