66 
As the newly hatched young are especially susceptible to the 
petroleum insecticides, which act by contact, a definite knowledge of 
the hatching period has an important practical value. In central 
Illinois this period extends approximately from June 15 to July 20. 
In and about Chicago it commonly begins about two weeks later, andi 
continues for a period of three weeks, this retardation being appar- 
ently due to the higher latitude and to the neighborhood of Lake 
Michigan. The period varies, in short, as to its beginning time, with 
the advancement of the season, and once begun, the rapidity of the 
hatching will depend, other things being ecpal, on the warmth of the 
weather. It is also influenced locally by the amount of foliage on 
the trees, the eggs hatching later and more slowly in a dense tree- 
top than in one more open to the sun. 
The only insecticides available against these insects are those 
which kill by contact, and of these the kerosene mixtures have thus 
far been found the most useful. Even these can be applied only to 
the young scales shortly after they hatch from the egg, no insecti- 
cide treatment being available for the destruction of the large and 
conspicuous females upon the twigs in May and June. The common 
kerosene emulsion, made by thoroly and intimately mixing kero- 
sene with one-third of its volume of a strong soapsuds, is a satis- 
factory spray when diluted to contain ten percent of kerosene for 
summer use, and sixteen to eighteen percent is used in winter. As 
a summer spray this emulsion must be used twice in succession, once 
when about half the eggs are hatched and again about ten days there- 
after. A single treatment in winter is about the equivalent in prac- 
tical effect of two such summer sprays. Large trees in a sandy soil, 
and especially those in more or less unthrifty condition, should be 
guarded against possible injury to the roots from the dripping of the 
kerosene spray, or from that part of it which may run down the 
trunk and so reach the earth. For this purpose it would be well to 
cover the ground before spraying with a thin layer of straw, packed 
closely around the base of the trunk, and later to. gather this up and 
carry it away. 
The cost of materials for large trees will average approximately 
fifteen cents a gallon for the summer spray, and about twice as much 
for the winter strength. 
Kerosene emulsion is made as follows : Dissolve one pound 
of common soap, or half a pound of whale-oil soap, in one gallon of 
water by boiling, remove from the fire, and add two gallons of kero- 
sene. Then with a spray pump force the mixture back into itself for 
about five minutes, or until it presents the appearance of a thick 
cream and no longer separates on standing. This is the undiluted 
emulsion. For a mixture containing ten percent of kerosene, add 
seventeen gallons of water to the three gallons thus prepared. For 
