534 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 84 



many years. A Canadian by birth and education, he joined the force 

 of the United States Bureau of Entomology and was engaged in 

 jxirasite work at the Gipsy Moth Laboratory in Massachusetts. He 

 afterwards went to Europe among those who first searched for the 

 parasites of the alfalfa weevil, a European insect that had been intro- 

 duced accidentally into the Utah fields. After the World War, for 

 some years he had charge of the European end of the work carried 

 on l)y the Bureau of Entomology in the study and importation into the 

 United States of the European enemies of Pyrausta vubilalis, the 

 well known European corn borer. Later, as we have elsewhere pointed 

 out, he took charge of the important Parasite Laboratory of the 

 Imperial Bureau of F.ntomology at Farnham Royal. All this time he 

 has been studying these questions of natural control, and this last 

 work of his is authoritative and suggestive to a very great degree. 

 He is one of the early and foremost users of mathematics as applied 

 to biological problems, and in this last paper the whole question is 

 studied more or less from its mathematical end. 



His summary, covered in 1 1 points, seems perfectly sound, although, 

 having a non-mathematical mind, I have reached my conclusions in 

 a somewhat different way. Those especially interested should by all 

 means consult this paper, and it will be unnecessary here to quote even 

 the rather long conclusions. Doctor Thompson's No. ii, however, 

 will serve very well to close this chapter : 



Generally speaking, no one species of parasite or predator is likely to bring 

 the host under control over the whole of the infested area. To produce this 

 result, the introduction of additional species will usually be necessary, while in 

 many cases, their efforts must be aided by the methods of agricultural, chemical, 

 or mechanical control. 



IMPORTATION OF THE INSECT ENEMIES OF WEEDS 



It has happened a number of times that when intentionally or acci- 

 dentally imported plants have become weeds suggestions have been 

 made that their natural enemies be introduced from their original 

 homes. Such suggestions have been made not only in this country 

 but in others. I remember that many years ago when the Russian 

 thistle was spreading rapidly in the northwestern United States the 

 United States Department of Agriculture was called upon for assis- 

 tance in importing insect enemies of the plant from Russia. Knowing, 

 however, that a number of the insect enemies of thistles afifect culti- 

 vated plants, I advised against any such attempt, and none was made. 



The case was quite different, however, when certain cacti of the 

 genus Opuntia escaix'd from cultivation in Australia and overran 



