264 Bulletin 177. 



reason why infested trees may not be saved, provided they are not 

 seriously weakened before remedial measures are taken. Herein 

 lies the greatest danger, — not because the insect is hard to kill, but 

 because it is hard to see, and it spreads with amazing rapidity. A 

 slight infestation is not at all noticeable, and one must look sharp 

 to see individual scales. The probability is that most farmers would 

 not notice the San Jose scale on their trees till they are infested so 

 badly as to be hardly worth the trouble of spraying. Taken at the 

 right time, however, the pest can be dislodged. A faithful applica- 

 tion of arsenical sprays will hardly save us 80 per cent of wormless 

 apples ; while the same amount of energy directed against the scale 

 may be expected to hold it at bay. 



11. Field Tests of New Insecticides. 



Thehurning of the foliage. — In brder that an insecticide shall 

 have practical value, it must not only check the insect or plant 

 disease but must also work no injury to the plant itself. Many 

 materials of great fungicidal value are worthless for practical pur- 

 poses because they kill or injure the plant as well as the parasite. 

 The standard insecticide is Paris green. As a poison it is very satis- 

 factory ; yet it is expensive, it does not remain long in suspension 

 when applied with water, and it sometimes burns the foliage. These 

 objections have induced several manufacturers to offer substitutes, a 

 few of which this Station has tested in comparison with Paris green. 

 Below is recorded the comparative injury to the foliage by four 

 strengths of each insecticide. It is probable that much if not all of 

 the injury noted could have been prevented by adding lime to the 

 arsenical spray. The object of this test was merely to find out the 

 physiological effect of the insecticides when used alone, and inci- 

 dentally the danger limit of strength. Grades are recorded of 

 injury ranging from doubtful (when the slight imperfections in 

 the foliage may or may not have been produced by the spray), 

 through slight, considerable and severe to very severe (when a large 

 proportion of the leaves had fallen and the young shoots were fre- 

 quently killed). Most of the orchard applications were made June 

 10. The potatoes were sprayed July 10 and a duplicate lot on July 

 17. Final records on tlie fruit trees were taken Julyl; on the 

 potatoes, August 3. The spray barrel held 48 gallons. 



