1881. | . Entomology. 745 
properties of Pyrethrum will understand. We have during 
an 
diately affects the insects on that plant with which it comes in 
contact, but it will prove perfectly harmless to all insects which 
come on to the plant half an hour (or even less) after the applica- 
ton; 2. The powder acts in the open air—unless, perhaps, applied 
in very large quantities—only upon actual contact with the insect: 
if €. g., it is applied to the upper side of a cotton leaf the worms 
that may be on the underside are not affected by it; 3. It has no 
effect on insect eggs nor on pupae that are in any way protected 
or hardened. 
These disadvantages render Pyrethrum in some respects inferior 
‘to arsenical poisons, but, on the other hand, it has the one over- 
shadowing advantage that it is perfectly’ harmless to plants or to 
higher animals; and if the cultivation of the plant in this country 
should prove a success, and the price of the powder become low 
enough, the above mentioned disadvantages can be overcome, to 
@ certain degree, by copious and repeated applications. 
In a closed room the effect of Pyrethrum on insects is by far 
More powerful than outdoors. Different species of insects are 
differently affected by the powder. Some resist its action most 
effectually, e. g., very hairy caterpillars and especially spiders 
of all kinds; while others, especially all Hymenoptera, suc- 
cumb most readily. In no case are the insects killed instan- 
taneously by Pyrethrum. They are rendered perfectly helpless “~ 
few minutes after application, but do not die till some time after- 
ward, the period varying from several hours to two or even three 
days, according to the species. Many insects that have been 
treated with Pyrethrum show signs of intense pain, while in others 
the outward symptoms are much less marked. Differences in 
temperature and other meteorological changes do not appear to 
have any influence on the effect of Pyrethrum. 
Modes of Application.—Pyrethrum can be applied, I. In dry 
Powder ; As a fume; 3. As an alcoholic extract diluted; 4. 
= Simple solution of the powder in water; 5. Asa tea or decoc- 
ion. . 
