The Cabbage Root Maggot. 529 



small scale or with choice plants, and is practiced sometimes by 

 extensive cabbage growers with profit. 



{d). By the Use of Insecticidal Substances. 

 Carbolic Acid. 



Since i88r, when Prof. Cook first tested this substance, it has been used 

 with quite uniform success by other investigators ; but all have considered 

 that its effectiveness was due to its odor, which prevented the flies from ovi- 

 positing on the plants treated with it. Mr. Fletcher says : "It must, how- 

 ever, be remembered that it is a ' preventive ' of attack, not a ' remedy ' for 

 application after the attack has begun. It is for keeping the flies from com- 

 ing to lay their eggs. When used as a remedy to destroy them it would have 

 to be of such a strength as would destroy the plants also." He then quotes 

 from Prof. Hilgard to show that "a solution of carbolic acid, that would be 

 instant death to an insect sprinkled with it, becomes inodorous and harmless 

 when filtered through a few inches of soil. ' ' Our discussion of this substance 

 as a preventive, on page 552, should be read in this connection, for it in- 

 cludes the results of the experiments which have been made with the acid. 



While applying the substance in our experiments there recorded, we found 

 that it quickly soaked into the ground, leaving but little odor after a few 

 hours ; and we thus expected but little result from its use. But the results 

 recorded in the preventive discussion of this substance show that it was quite 

 effective, but was not so effective as the Tarred Paper Cards; and yet the 

 much stronger, but similar, odor of the cards was not what gave the protec- 

 tion, for wherever not properly applied the flies crawled under and accom- 

 plished their object. It is also a significant fact in this connection that, in 

 all of the experiments by Mr. Fletcher and Prof Cook, although the appli- 

 cations were doubtless begun before any eggs were laid, yet the applications 

 were continued either until the plants were large enough to eat or until after 

 the time for the maggots to appear. We are therefore inclined to believe that 

 most of the good results heretofore gained by the use of this substance have 

 been through its killing power, either on the egg or maggot, rather than from 

 its having had any preventive effect on the flies. 



The following laboratory experiments show that it has decided killing 

 powers. May 25, loj small maggots, brought from Long Island, were put at 

 the base of each of two well-established cabbage plants. The next day, 

 after the maggots had begun work on the plants, the earth was removed from 

 . about the base of the plants until some of the maggots were laid bare. Then 

 around one plant was poured 4 oz of an emulsion (i lb. soap, i gal water, 

 and Y2 gal. crude carbolic acid) diluted with 50 parts of water, or at the rate 

 of a pint of acid to about 22 gals, of water. Around the other plant was 

 poured 4 oz. of the same emulsion diluted with 75 parts of water. May 30, 

 this latter cage was superficially examined, with the result that 30 dead and 

 3 living maggots, and 14 puparia were found ; many of the dead maggots 



