REPORT OF THE STATE ENTOMOLOGIST I912 91 



tain, for the sole purpose of controlling San Jose scale in a satis- 

 factory manner. We are assured by the various parties that the 

 manufacturer's directions were carefully followed. A fruiting 

 apple tree may be conservatively valued at $25 and upward, so that 

 the death of one hundred trees means a loss which can hardly be 

 estimated at less than $2500, a sum most fruit growers would miss. 

 The possibility, we do not say probability, of such extended injury 

 should make the fruit grower careful as to what spray material he 

 applied. 



Contributory causes. Several adverse factors have been men- 

 tioned in the above discussion, and it is only fair to attempt cor- 

 rectly to estimate their value, even though these inimical condi- 

 tions are unavoidable in most cases. A blanket of wet snow or 

 several days of foggy weather following spraying hinder evapor- 

 ation and favor injury on account of the increased penetration 

 likely to result. A long winter with vital activities at a minimum 

 is undoubtedly favorable to penetration, and the same is true of a 

 cold spell in the spring, checking growth and prolonging the period 

 of least resistance on the part of the tree. Some trees have a 

 thicker outer bark than others, and this dead tissue is of much 

 value in preventing oil entering the vital tissues beneath. Vari- 

 ability in the thickness of this outer bark occurs not only in trees 

 of different species but even those of the same kind. Observa- 

 tions go far to show that other things being equal, the thin-barked 

 trees or branches succumb first. 



It may be that excessively low winter temperatures favor pene- 

 tration by oils and thus increase injury. We know that under cer- 

 tain conditions the vitality of trees may be destroyed by extreme 

 cold. This is well known as winter injury and is occasionally a 

 serious factor. It is conceivable that less severe weather might 

 reduce the vigor of a tree to such an extent that penetration by 

 oil and consequent injury would be greatly facilitated. Extreme 

 wiater weather is to be expected and unless there is noticeable 

 winter injury to the same variety of trees in the section, we believe 

 low temperatures can not be held responsible to any material ex- 

 tent for dying or dead trees presenting practically identical condi- 

 tions and which, without exception, had been previously sprayed 

 with an oil preparation or other insecticide known to be dangerous 

 under certain conditions to plant life. 



The fact that serious injury followed the application of oil to 

 sugar maple trees, while spraying at the same time caused little or 



