176 MISC. PUBLICATION 273, U. S. DEPT. OF AGRICULTURE 
Contact insecticides are used against small sucking insects such as 
the aphids, scales, and bugs, and sometimes against small caterpillars 
and leaf miners, and are applied in the form of liquid sprays or dusts 
so as to come in direct contact with the insects. These poisons kill 
the insects by entering the respiratory system, or by penetrating 
through the thin body membranes, or they may destroy them by 
plugging their breathing tubes, paralyzing the nerve centers, or 
actually burning the insects through the caustic properties of the 
chemicals. The young insects are much more easily killed by such 
treatment than the older forms, which often become protected or 
more or less resistant. 
There are a large number of contact sprays and dusts, each with 
certain advantages and special fields of usefulness. Only a few will 
be mentioned here, since the contact sprays, while very useful in the 
control of shade-tree pests, have not as yet been used in the forests 
of the West. 
One of the most useful contact sprays is 40-percent nicotine sul- 
phate diluted with water in the-ratio of 1 to 800 or 1 to 1,000, to 
which is added about 1 ounce of soap to each gallon of spray in order 
to give good spread and adhesion. This spray is particularly useful 
in the control of aphids and other soft-bodied insects. A typical 
formula 1s— 
Nicotine ‘sulphate; 40-percem teas se ee eat ee ee eee 1 pint 
Fish-oil soap_—-—-- Ia pg pe Ry et Yee Reo ane eS he 5 pounds 
Water lo make 2esos 202 eek eee DS Sa TES Toe 2 se ae ee 100 gallons 
Miscible oils, or oils that will mix with water, have come into 
prominence in recent years as effective contact sprays for shade and 
orchard trees, and are useful in nurseries and plantations against 
such forest pests as the spruce bark louse, the green spruce aphid, 
and various scales. These oils are proprietary products, sold under 
a variety of names, and varv somewhat in strength. The winter oils 
are used as dormant sprays and will injure foliage if apphed when 
the trees are in leaf. Lighter or summer oils are also available that 
can be applied to the folage without injury. Instructions as to 
dilution of the oils come with the product. Fish-oil soap is usually 
used as a spreader. 
Soap and kerosene emulsion have been used in the past, but 
results from their use are uncertain, and they are no longer recom- 
mended for shade-tree sprays. 
Lime sulphur is a very effective and inexpensive spray which is 
used in the control of the armored scales. It is applied during the 
winter or early in the spring as a dormant spray. ‘The concentrated 
preparation should have a density of about 30° Baumé and should 
be diluted with eight parts of water. 
Many other insecticides are available, and various combinations 
of ingredients are used for special purposes. Some of these may be 
very harmful to plants and dangerous to use. If special problems 
come up requiring the use of chemical inseeticides, it is best to con- 
sult an entomologist before entering on any extensive control project. 
SPRAYING 
_ Liquid insecticides, either stomach or contact poisons, when used 
in the control of tree-defoliating insects, are applied in the form of a 
