season is not likely to harm a tree and has such an excellent
cleansing effect that the benefits to be derived in this direction
alone are often sufficient to meet the cost of the treatment.
Lime-sulfur wash consists of a mixture, boiled one hour, of 40
pounds of lime and 80 pounds of sulfur, in 50 gallons of water. It
may be had in prepared form and should then be used at the rate of 1
gallon to about 9 gallons of water in winter or early spring before
the buds open. At other times of the year and for the softer-bodied
insects a more diluted mixture, possibly 1 part to 30 or 40 parts of
water, should be used, varying with each case separately.

_Kerosene emulsion_ consists of one-half pound of hard soap, 1 gallon
of boiling water, and 2 gallons of kerosene. It may be obtained in
prepared form and is then to be used at the rate of one part of the
solution to nine parts of water when applied in winter or to the
bark only in summer. Use 2 gallons of the solution to a 40-gallon
barrel of water when applying it to the leaves in the summer.
Kerosene emulsion is useful as a treatment for scale insects.

_Tobacco water_ should be prepared by steeping one-half pound of
tobacco stems or leaves in a gallon of boiling water and later
diluting the product with 5 to 10 gallons of water. It is
particularly useful for plant lice in the summer.

The life history of an insect: In a general way, all insects have four
stages of transformation before a new generation is produced. It is
important to consider the nature of these four stages in order that
the habits of any particular insect and the remedies applicable in
combating it may be understood.

All insects develop from _eggs_, Fig. 99. The eggs then hatch into
caterpillars or grubs, which is the _larva_ stage, in which most
insects do the greatest damage to trees. The caterpillars or grubs
grow and develop rapidly, and hence their feeding is most ravenous.
Following the larva stage comes the third or _pupa_ stage, which is
the dormant stage of the insect. In this stage the insect curls
itself up under the protection of a silken cocoon like the tussock
moth, or of a curled leaf like the brown-tail moth, or it may be
entirely unsheltered like the pupa of the elm leaf beetle. After the
pupa stage comes the _adult insect_, which may be a moth or a