INSECTICIDES 
decrease in price which would result if 
the codling moth damage did not mate- 
rially reduce the crop every year. 
The census of 1900 placed the number 
of apple trees in the United States at 
200,000,000. On the authority of Mr. Tay- 
lor 165,000,000 of these were in bearing 
condition and the cost of spraying and 
other treatment for these will range be- 
tween 5 cents and 10 cents per tree. As 
an offset to untreated orchards the lower 
estimate of costs may be taken, namely, 
5 cents, which gives a charge for treat- 
ment of $8,250,000. Combining the direct 
shrinkage or loss and the cost of protec- 
tion from still greater loss, gives a total 
tax, chargeable to the codling moth, of 
nearly $20,000,000. The insect losses to 
other deciduous fruits are quite as heavy 
as in the case of the apple, and especially 
when the treatment for the San Jose 
scale and other pests are considered, and 
in the case of citrus fruits the cost of 
treatment is much greater and the actual 
losses are again heavy. We are war- 
ranted, therefore, in placing the loss to 
fruits from insect pests as high as 20 per 
cent annually. 
GRANVILLE LOWTHER 
Insect Enemies. See Beneficial Insects, 
Propagating and Distributing. 
Insecticides 
How Contact Insecticides Kill 
It has long been known that certain 
substances when brought in contact with 
the outer surfaces of insects will pro- 
duce death. Why they kill has not been 
so well understood. In order to answer 
this question Geo. D. Shafer, of the di- 
vision of entomology of the University 
of Michigan, undertook a series of elab- 
orate experiments with results as re- 
corded below. 
The insecticides experimented with 
were gasoline, kerosene, benzol, xylene, 
turpentine, creosote, aniline oil, carbon 
disulphide, chloroform, ether, pyro cresol, 
special kreso-dip, creolin, chloronaphtho- 
lium, crel oil, zenolium, to-bak-ine, py- 
rethrum, hydrocyanic acid gas, carbon 
dioxide, nitrogen and hydrogen. 
By “contact insecticide” is meant any 
1161 
substance which kills by contact rather 
than by being eaten, and the definition 
is made to include gases and powders as 
well as the liquid sprays. 
The first part of the experiment was 
undertaken to ascertain if the various 
miscible oils such as kerosene and gaso- 
line plugged the breathing apparatus of 
the insect and so produced death by suf- 
focation. It was found that the plugging 
actually took place, but that suffocation 
was not the cause of death; for it was 
discovered that grasshoppers, beetles, 
caterpillars and other insects would re- 
gain their activities after having lain in 
the water for several hours apparently 
dead or in a closed vessel containing car- 
bondioxide with all oxygen excluded. But 
these same insects when subjected to 
treatment with an insecticide would lose 
all motion in a few minutes and never 
thereafter recover. It was therefore con- 
cluded that death was the result of some 
other cause than mere suffocation. In 
fact numerous experiments extending 
back as far as 1670 show that insects 
succumb from suffocation with difficulty. 
Besides, an oil was found that as effectu- 
ally plugged the trachea of insects as the 
oils mentioned, but that when removed 
from it they gradually recovered. 
Experiments were then made which 
showed that insects would die under the 
influence of the vapor of gasoline as 
readily as if dipped in the liquid itself, 
showing that death was the result of some 
influence of the gasoline rather than suf- 
focation. 
Tests were next made to ascertain if 
the substances used actually passed into 
the tissues of the insect body. This was 
found to be true, but the absorption took 
place in many cases only after consid- 
erable time had elapsed, in some cases 
long after death had taken place. 
Careful observations were then made 
to detect the effect upon heart action and 
respiration. In many cases, though not 
all, heart action was greatly increased, 
sometimes to the point where it was no 
longer possible to count the beats, then 
they would suddenly fall below normal. 
Respiration was irregular also. 
