1907.] The Cottony Maple Scale in Illinois. 353 



less due in part to the fact that the upper surface of the leaf is more 

 exposed to the spray than the under, and possibly also in part to the 

 inferior vitality of the scales on the more exposed surface of the 

 leaf. That there is really such a difference is shown by the differ- 

 ence in ratios of dead to living on the two leaf surfaces before the 

 trees were sprayed. The general average of all the counts is 3.6 

 per cent, of dead on the upper surface of the leaf and 2 per cent, on 

 the lower. 



In the tree sprayed July 3 the effect on the scales on the upper 

 and the lower parts of the top of the tree was brought into compar- 

 ison by examining the scales on 25 leaves from each. Twenty-two 

 per cent, of the scales on the upper leaves had been killed and 35 per 

 cent, of those on the lower, the ratio of killed being more than half 

 as great again for the lower leaves as for the upper. This is doubt- 

 less due, at least in part, to the greater and more prolonged effect 

 produced on the lower leaves by the drip from the upper part of the 

 top. 



We may infer from these experiments that two sprayings with 

 a 10 per cent, kerosene emulsion, one applied at the middle and the 

 other at the end of the hatching period, separated, that is, by an in- 

 terval of about ten days, will produce the maximum effect on the scale, 

 and that they may be expected to destroy 80 per cent, or more of the 

 insects then alive. It should be added that no harm to the tree was 

 done by any of these treatments except a slight burning at the edges 

 of the leaves of the tree which was twice sprayed. This appearance 

 of injury was perhaps due to humid weather which followed the 

 first treatment, delaying the evaporation of the kerosene. Two trees 

 were treated July 1 1 with a 10 per cent, kerosene emulsion, at a 

 time and place when rain was falling with a result to diminish by 

 half the killing effect of the spray. 



The cost of die 10 per cent, emulsion used in these experiments 

 was 4.3 cents per gallon, and the trees were large enough to require 

 three or four gallons each for a single treatment. 



The whale-oil soap solution applied at the end of the hatching 

 period had about two thirds the effect of the corresponding kerosene 

 treatment, as shown by a comparison of the results of the data 

 already given for the latter with those derived from an examination 

 of 77,000 scales on 75 leaves taken from the tree treated with the 

 former insecticide. 



Winter Insecticide Measures. 



Winter spraying, when the trees are bare, has the advantage 

 that stronger insecticides may be used, and that less than half as 



