MINERAL POISONS FOR KILLING THE WORMS. 
137 
the most effective insecticides. Still, considerable progress has been 
made since the investigation began. The improvements and discover- 
ies will not only greatly benefit the cotton-planter, but the agricultur- 
ist in all parts of the country, since many of the discoveries are of gen- 
eral application to most of our leaf eating insects. 
The most important point to be insisted upon is, that the planter in 
the southern portion of the belt should consider the poisoning of the 
worms as inseparably connected with and part of the cultivation of cot- 
ton. So many days' labor in the year, and a certain expenditure of 
money to purchase, prepare, and apply the poisons, and to buy and keep 
in repair the requisite machinery, are inevitable. In a very favorable 
season the poisoning, it is true, may not be necessary, but experience 
very plainly indicates that no year now passes when the worm is not in- 
jurious in some part of the cotton belt. Cases of complete or compara- 
tive immunity occur almost every year even in the hibernating por- 
tion of the cotton belt, but as they depend on locally restricted condi- 
tions, the planters ought never to rely upon them. It ought never to 
be forgotten that labor and money expended in a judicious application 
of poiBOD are always amply repaid by a corresponding increase of crop. 
The various insecticides now be considered are divisible into two 
classes: first, those which act through the stomach — the arsenical poi- 
sons, and some allied ones; secondly, those which kill the worm upon 
actual contact, such as oily substances and py ret brum. A vast num- 
ber of other substances cannot at present be brought under these two 
heads, as they are still in the experimental stage, and many of them 
will, no doubt, Upon further experimentation, be altogether rejected as 
of little or no value. The action of the poisons of the second class is 
immediate, but not lasting, the substances being more or less volatile; 
whereas the poisons of the first class have no immediate effect, but their 
action is lasting. 
This difference in character requires some modifications in the prac- 
tical application. The greater the adhesiveness given to the poisons of 
the first class, without injury to the plant, the better; while in the 
poisons of the second class no such precaution is necessary. The use 
of arsenical poisons has the advantage that only the enemies of the 
plants are killed directly, though the killing of birds and other enemies 
of the worm, by eating it when poisoned, has occasionally been reported. 
Tho e insecticides that aftect by contact, on the other hand, kill friend 
and foe alike. Other relative advantages and disadvantages will be 
pointed out further on. 
It is very evident, from the habits of the worms, as already detailed, 
that the poisons of either class will prove most satisfactory if applied 
to the under side of the leaves ; and, as we shall see in the chapter de- 
voted to it, the recently invented machinery overcomes the obstacles 
which were formerly in the way of such application. The importance 
of an early application of poison has already (p. 12G) been pointed out. 
