367 



Slow action of insecticides on dormant scales. — An examinatiou of tlie 

 records of these experiments, in connection M'itli results with other 

 scale insects, strongly emphasizes the point that in the dormant resting 

 condition scale insects respond to insecticides very slowly and grad- 

 ually, and this has an important bearing on the determination of the 

 usefulness of an application. The scale larvse during the growing sea- 

 son are killed in a few minutes, or a few hours at farthest, just as 

 other soft-bodied insects, but the mature scale does not for some time 

 exhibit the effect of the Avash unless it be so radically strong as to be 

 unnecessarily expensive or as to endanger the life of the plant. It will 

 be seen that little can be predicated of the ultimate result within two 

 weeks, and at the end of a month the estimate can be only a lu'ovis- 

 ional one, while at least two months are necessary to reach approxi- 

 mately final conclusions. 



The slow progressive death of the scales must be largely due to the 

 gradual penetration of the insecticide, and also indirectly to the soften- 

 ing and loosening of the scale itself, enabling the weather conditions of 

 moisture and cold to be more tiital. 



It will be noted, also, that it is the destruction of the last 5 or 6 per 

 cent of the scale insects which necessitates the great strength of the 

 wash and the accom])anying heavy expense. It is comparatively easy 

 to kill 75 to 90 per cent of the scales, and this with comparatively weak 

 and inexpensive washes, but to reach the remaining few, double or 

 treble strength is required.* 



Effect of the washes on the health of the tree and on the fruit. — With 

 some of the stronger washes, particularly those of soap and resin, a 

 marked diminution was shown in the amount of bloom and in the 

 fruit set. This, however, was usually accompanied by an increase in 

 the amount of foliage, and the results in this direction were especially 

 marked in the cases of the two stronger whale-oil soap washes. The 

 two trees so treated were noticeably vigorous in foliage, and presented 

 a liner appearance than any in the orchard, but contained scarcely any 

 fruit. The general effect of the washes on the trees was, however, in 

 large measure vitiated by the damage already done them by the scale; 

 and in most instances where there was a greater or less amount of 

 injury, or the death of the tree resulted, it was due to the scale rather 

 than the wash, as evidenced by the similar condition of adjoining 

 untreated trees. The jmre kerosene emulsion, however, had a disas- 

 trous effect on the trees, and pure kerosene killed outright the trees 

 treated with it. An application of soap made late in spring, or after 

 tlie trees were already blooming, did not seem to affect seriously the 

 setting of the fruit in the case of the ])each, and in the case of the 



* This is illustrated notably in the resin wash experiments against Diaspis lanatiis 

 in the winter of 189-l:-95, where a double summer strength killed 90 per cent, an appli- 

 cation twice as strong only 95 per cent, and one three times as strong, or six times- 

 summer strength, was necessary to eftect comiilete extermination. 



