APPENDIX. 



vii 



given up the fight for extermination. The main thing for you to 

 do is to try to stamp it out during the active season and use all 

 your energies to that end. The arsenicals are not expensive, and 

 if an analysis is made of the Paris green, or whatever form of 

 arsenic is used, to insure its purity, I can see no reason why the 

 whole area should not be practically cleared off during a single 

 season. That should be carefully followed up by close observa- 

 tion, with a view of repeating it in the case of some omitted centre 

 or point where they may be found in a subsequent year. In case 

 they have not gone into the woods, it seems to me feasible to ex- 

 terminate them. 



I would make one other suggestion, and that is, that as an 

 auxiliary method it would be well to spend five hundred or six 

 hundred dollars in sending one or two persons abroad next sum- 

 mer with no other object than to go to some section of northern 

 Europe to collect and transmit to authorized persons here a certain 

 number of the primary parasites of this species, which are known 

 to check its ravages over there. The insect was undoubtedly 

 brought over by Trouvelot without any of its natural checks. In 

 my judgment it would be well worth trying to import its parasites 

 from abroad. The advantage would be this : if you failed to 

 exterminate it by spraying, its parasites, seeking for this partic- 

 ular host, would be more apt to find the overlooked or escaped 

 specimens than man would. 



Professor Fernald. Do you think that any of our native par- 

 asites will be liable to attack this insect? 



Professor Riley. Experience justifies the belief that some of 

 them may, in time. 



Professor Fernald. Have we a case on record? 



Professor Riley. Yes, though they are not numerous. Pieris 

 rapce, or the imported cabbage-worm, has some native parasites. 

 There are other cases, but I should have to consult my notes. 



[Note by Professor Riley. Several native species attack Scoly- 

 tus rugulosus : the same is true of the hop aphid, Phorodon humuU, 

 and of several important species of bark lice.] 



Professor Fernald. In reply to the question which was first 

 asked, as to whether it is possible to eradicate the insect at all, 

 let me suppose a case. Suppose we have a tree like the elm I see 

 yonder, and suppose we know it to be the only tree in America 

 that is infested. I think you will all agree with me that for a small 

 sum of money all moths on it could be destroyed. Suppose there 

 were two, — suppose all the trees on the Common were infested. 

 If they could be destroyed on all those trees, it is only a ques- 

 tion of time and money to eradicate them from a much larger 



