The human transport of animals across the Northern Atlantic 223 



of Tachinid flies, all of them imported from N. America, have established them- 

 selves in France. 



Phthorimaea operculella Zell. (Potato Tuber Moth). Probably of American 

 origin, now almost cosmopolitan. Control by means of several parasitic wasps 

 was attempted, i.a. a Habrobracon imported from California to France, but without 

 noticeable success. 



Phylloxera vitifoliae Fitch (vastatrix Planch.) (Vine Aphis or Grape Phylloxera). 

 Indigenous in E N. America, it was introduced into California about 1858, in 

 Europe not earlier than 1863, but now a terrible pest. No biological control has 

 succeeded in this case; the insect is kept down by grafting European vine upon 

 more resistant American roots. 



Eriosoma lanigeriim Hausm. (Woolly Aphis). Introduced from N. America before 

 1800, now generally distributed in Europe (except in the north). Its severe damage 

 in Europe is accentuated by the fact that it has cancelled the yearly migration be- 

 tween the fruit-trees and a second host (Ulmus). The Chalcid wasp Aphelinus mali 

 Hald. was introduced from N. America into France in 1920, has subsequently 

 become well established in most parts of Europe and elsewhere, and proved very 

 useful. 



The danger of noxious insects being introduced with the continuously increasing 

 air traffic seems to have been exaggerated. The appearance of a European Grasshop- 

 per, Roeseliana roeselii Hag., near the airports of Montreal was, however, inter- 

 preted in this way (Urquhart & Beaudry, 1953). 



On the minimum size of a viable population 



Students of animal populations often postulate that there exists a minimum 

 number (a minimum density) of individuals constituting the population, below 

 which the population (if isolated), or even the entire species, is unable to recover. 

 They apparently assume that a sufficiently low number of individuals in itself, 

 regardless of other factors, may be the initial stage of complete extinction. 



The picture of an introduced animal (or plant) species colonizing a new continent 

 is fundamentally opposite (already pointed out by Mayr, 1954, p. 174). There 

 is no doubt that these rapidly increasing populations in many cases are descend- 

 ents of one single female, fertilized before, during or after the intercontinental 

 transport. 



We are entitled to maintain that in such cases no "minimum size" of population 



