PREVENTIVE MEASURES 
189 
carbolic acid, but pyrethrum powder is certainly ef¬ 
fective when at all pure. 
Many of the so-called pyrethrum or Persian insect 
powders sold in the shops are impure. The powder 
itself is made from the ground flower-heads of two 
species of the genus pyrethrum, which are composite 
plants not unlike the common ox-eye daisy. It is a 
not uncommon practice for makers of these powders 
to grind the stems as well as the flower-heads, thus 
producing a dilution which greatly lessens the effect 
of the powder. The insecticidal element in this pow¬ 
der seems to be an oleo-resin, and therefore a freshly 
ground powder is more effective than an old one. In 
most of the pyrethrum powders to be found in the 
shops the heads have been imported from Europe and 
ground in this country. 
There are, however, powders of a somewhat higher 
price made from pyrethrum flower-heads grown in 
California in the vicinity of Stockton. These appear 
to be the freshest and strongest, but they cost more. 
It has been the experience of the writer that these 
California powders are effective against house flies 
either when puffed into the air or when burned by 
puffing through a gas jet, or by making moistened 
cones put upon earthen dishes and ignited at the top. 
Repellents 
Flies do not seem to be repelled by odors to the same 
extent that mosquitoes are. Some old ideas in this 
direction, however, may be mentioned. It is stated 
