486 STATJO BOAlll) OF AGRKniLTURE. 



SUGGESTIONS FOR POSSIBLE PRACTICE, 

 (a) SOLID AND LIQUID APPLICATIONS. 



It will be observed Hint when the insecticides gained eutrance into 

 the insect tissues as purely contact agents, they did so, ultimately, in 

 the fluid condition. Powdered solid contact insecticides gave oflf volatile 

 bodies to be absorbed by the insect, or else the powdered agent became 

 moistened and partly dissolved in exudations on the insect integument. 

 Experiments showed that gases or volatile insecticides gained entrance 

 to the insect tissues most rapidly. The "miscible oils" usually combined 

 volatile and purely liquid agents. The liquid or the finely powdered 

 solid often gained entrance through the spiracles to the larger tracheae 

 where any volatile bodies that might be carried by the insecticides could 

 then charge the air of the tracheal system. In the case of the powdered 

 solid contact insecticide, the advantage evideutally lay in having the 

 powder so fine and dry that it would adhere Avell and could sift readily 

 into all crevices— even into the stigmata of the insect. Similarly, a weak 

 surface tension for a liquid insecticide enabled it to flow into all ir- 

 regularities of the area treated, and to penetrate often into the larger 

 tracheae of the insect. Even in the case of lime-sulphur wash — which 

 kills scales, perhaps in most cases, without ever having actually come 

 in contact with the bod}' of the insect itself — it would seem to be 

 advantageous if the wash could be given a lower surface tension. The 

 solution could then flow into crevices of the bark and thoroughly wet 

 the scale-covering of insects that miglit otlierwise escape. As is well 

 known, soap cannot be used for the purpose, since it causes in the lime- 

 suli>hur solution a greasy lime-soap precipitate. A number of tests 

 were carried out with the idea of finding agents which might be added 

 to lime-sulphur, without injury, to lower the surface tension and cause 

 it to spread on the bark of plants more readily. In order to be able to 

 compare the "spread" of difl'erent samples of lime-sulphur dilutions 

 readily in the laboratorj', resort was had to what may be termed the 

 ''drop method." For example, the number of drops in 5 c. c. of water 

 were counted as they came from a pipette. The same pipette was then 

 washed out with lime-sulphur prepared by diluting a concentrate with 

 a water solution of the agent to be tested. The number of drops in 5 c. 

 c. of the latter lime-sulphur dilution were then counted from the pipette, 

 and the ratio of the number of drops of water to the number of drops of 

 lime-sulphur dilution was obtained. As long as the temperature of the 

 liquids was practically the same and the drops were counted from the same 

 cleaned pipette, the ratio between the number of drops in equal vol- 

 umes of water and any lime-sulphur dilutions of approximately equal den- 

 sities afforded a direct means for comparison of spread in the various dilu- 

 tions. Actual trial on a tree covered with scale insects, showed that 

 when this ratio equalled about 0.85*, the "Agent plus L. S. solution" had 

 sufficient spread; that is, the spray readily flowed into crevices of the 

 bark and wet the waxen scale coverings. The method was adapted from 



^ . Number drops in 5 c. c. of w ater =0 85") 



^' Number drops iu 5 c. c. of "Saponin-water-dilution of Cone. L. S." 



