THE WAR ON INSECTS 277 



even fine road dust has some insecticide value. The 

 fine dust particles get into the trachea and there set up 

 specific irritation, but they must get there to do any 

 good at all. Coarse particles are no better than so 

 much coarse dust and insects with covered spiracles are 

 not affected at all. It is quite possible, therefore, to 

 have an insecticide which is very effective against one 

 series of a species, but which, against another series, 

 will be entirely useless: any material for which it is 

 claimed that it will kill everything, should for this 

 reason be looked upon with distrust from the start. 



Plant lice are among the most common of all the 

 insects to be dealt with in a practical way, because 

 there is almost no vegetation not more or less infested 

 by them. In a small way on house plants, insect powder, 

 i.e., pyrethrum, is the cleanest and simplest remedy. 

 It can be dusted on through a sieve, put on with a little 

 powder puff or bellows, or it can be made into a decoc- 

 tion or tea, using an ounce to two quarts of hot water. 

 This can be put on with an atomizer and, when fresh, it 

 is very effective against all kinds of plant lice, against 

 most small caterpillars and slugs, and against the 

 larvae of scale insects. It loses strength rapidly when 

 exposed to the air, and in corked bottles the decoction 

 ferments and moulds after a day or two. It is alto- 

 gether too expensive for general use in fields, but in 

 gardens or greenhouses its cleanliness and absolute 

 safety to plants are in its favor. 



Tobacco has a much greater range of usefulness. 

 When very finely powdered it may be dusted on and 

 will kill about the same sorts of insects as the pyre- 

 thrum. It may be worked into the soil of pots and 

 benches to kill root lice, besides discouraging many 

 other underground insects. It is also a fertilizer and 

 stimulates the plants to which it is applied. If a spray 



