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 



