386 BIOLOGICAL CONTROL 



Among field crop pests the accidental establishment of Pieris 

 rapce in 1929 led to the introduction of the Chalcid Pteromalus 

 puparum. This scheme is proving strikingly successful and the 

 parasite is stated to be destroying over 90 per cent, of the pupae 

 of this butterfly. 



A number of other introductions have been made for the purpose 

 of controlling various insect pests : some of these have proved 

 failures, but as regards the larger number of cases the process of 

 colonisation is either too recent or not sufficiently known to 

 warrant definite conclusions being drawn. The next few years, 

 however, should teach much with regard to the outcome of these 

 experiments. 



Canada. The enormous damage entailed by the European 

 larch saw-fly {Lygceonematus erichsoni Htg.) to Canadian forests 

 led C. G. Hewitt to seek to relieve the situation by parasite 

 introduction. In 1912-13 importations of cocoons of the saw-fly, 

 collected in the larch woods of England, were received in Canada. 

 The distribution of these cocoons in a heavily infested larch area 

 in South Manitoba led to the establishment of the Ichneumon 

 })arasite Mesoleius tenthredinis Motl. Collections of saw-fly cocoons 

 made in 1916 revealed that 19 per cent, contained the parasite, 

 and in succeeding years the proportion steadily increased from 40 

 per cent, in 1919, to 66 per cent, in 1920, and again in 1926 ; while 

 in 1927 it averaged at least 75 per cent., rising as high as 88 per 

 cent, in some localities (Criddle, 1928). In the course of these 

 years the parasite has withstood the rigours of the climate and 

 spread across a patch of open prairie not less than six miles in 

 extent to a larch swamp twenty miles across, penetrating to its 

 utmost limits. The parasite has become well established and. 

 according to Swaine, it has been recovered as far as 200 miles from 



France, and is proving an efficient parasite, destroying its host at all seasons 

 of the year. It has also been introduced from New Zealand into parts of 

 Australia, and re-introduced into South Africa ; in New South Wales it has 

 become as completely successful as in New Zealand, but in South Africa it has 

 so far proved less efficient. Introductions have also been made into France, 

 Germany, Holland, Spain, and some other countries. Although established, 

 it has not, up to the present, given evidence of exercising appreciable control 

 in Germany, or Holland, while in France it appears to be rather more promising 

 in the south than elsewhere. 



