INSECTICIDES AND REPELLENTS 11 



ISODRIN AND ENDRIN 



Isodrin and endrin are the endo-endo isomers of aldrin and dieldrin, 

 respectively. Their effectiveness against insects of medical impor- 

 tance has not yet been fully evaluated. 



PYRETHRUM AND ALLETHRIN 



Ground pyrethrum flowers and their extracts have long been used 

 as insecticides. The principal acbive constituents are called pyrethrins 

 I and II and cinerins I and II, which occur in different proportions in 

 different samples. In insecticide formulations the mixture of active 

 substances as obtained in extractions with hydrocarbons is frequently 

 called pyrethrins. The material has a rapid paralyzing action on 

 insects but is almost nontoxic to warm-blooded animals. For these 

 reasons it is still used extensively as an insecticide, especially in house- 

 hold sprays. 



Good commercial grades of pyrethrum flowers contain 0.7 to 1.6 

 percent of pyrethrins (total active ingredients) on a dry-weight basis. 

 The standard household spray formerly was prepared by extraction 

 at the rate of 1 pound of ground flowers per gallon of refined kerosene, 

 giving a concentration of about 0.1 percent of pyrethrins. The most 

 highly concentrated commercial preparations contain 20 percent of 

 pyrethrins. These concentrates are used in the preparation of aerosols 

 and sprays. 



Allethrin is the coined name for the substantially pure insecticidal 

 chemical dZ-2-allyl-4-hydroxy-3-metbyh2-eyclopenten-l-one esterfied 

 with a mixture of cis and trans ^-chyrsanthemumic acids. The 

 chemical has also been called the allyl homolog of cinerin I and syn- 

 thetic pyrethrins. Its successful synthesis by chemists of the former 

 Bureau of Entomology and Plant Quarantine after many years of 

 effort was announced in 1949 and was an important milestone in the 

 development of insecticides. 



Allethrin, like pyrethrins, is practically harmless to man and animals 

 and appears to be more stable in the presence of light than the natural 

 product. It is slightly less rapid in action, and less toxic to some 

 insects but more toxic to others. It is being produced commercially 

 in increasing quantities and will evidently find an important place in 

 the insecticide field. 



PYRETHRUM SYNERGISTS 



Several materials have been found that have little toxicity them- 

 selves but when mixed with pyrethrins their effectiveness is greatly 

 increased. They are called synergists, or activators. The first one 

 found was sesame oil and later several piperonyl compounds that were 

 still more effective were synthesized. Two of the most important ones 

 are piperonyl cyclonene and piperonyl butoxide. Recently some of 

 the piperonyl derivatives of chrysanthemumic acid, synthesized at the 

 Beltsville laboratory, > have shown great promise as synergists. 

 n-Octyl sulfoxide of isosafrole (called sulfoxide), iV-isobutylhendecen- 

 amide (IN-930), and propyl isome are also pyrethrum synergists. Of 

 the commercially available materials, sulfoxide appears to be the most 

 effective against lice and piperonyl butoxide against house flies. These 



