382 BIOLOGICAL CONTROL 



uniformly effective during the whole period when these are 

 available as hosts. Apart from Bathyplectes, only one other 

 introduced parasite of the weevil is stated to have become properly 

 established. Control of this pest by biological means does not, at 

 any rate, at present seem to afford more than an accessory 

 measure. Cultural and other methods of control remain the most 

 important weapon in combating it. 



The European corn-borer is proving to be one of the most 

 formidable of all foreign pests that have become introduced into 

 North America. Discovered in Massachusetts in 1917, when it 

 was already infesting a known area of about 100 square miles, it 

 has since extended its range with great rapidity, and is still 

 continuing to do so. In 1924 it had infested territories in Southern 

 Ontario and the United States amounting to 42,773 square miles, 

 and by the end of 1926 this area was increased to about 93,000 

 square miles. In 1934 the total area infested was stated to amount 

 to over 312,000 square miles. Substantial commercial losses, 

 however, only occurred in scattered districts over this wide 

 expanse of territory. In 1919 an investigation of the status and 

 controlling factors of the insect in Europe was commenced, and 

 this has led to the accumulation of a large amount of ecological 

 and other data. It appears that, with the exception of certain 

 parts of central Europe, the corn-borer is controlled by a complex 

 of climatic, cultural and parasitic factors, with the result that 

 it seldom attracts attention. These several factors vary both 

 quantitatively and qualitatively in different ecological zones 

 inhabited by the insect, and also from year to year. In so far 

 as parasitism is concerned, it is clear that it forms but one of the 

 many factors operating against the corn-borer. At least nineteen 

 species of parasites had been reared from this host up to 1928, but 

 their relative frequency varies enormously in different regions. 

 Thus in the vicinity of Paris an average parasitism of over 36 

 per cent, has been noted, but only four species of parasites are 

 present, while in south-western France the parasites number nine 

 species, yet their collective parasitism averages less than 15 per 

 cent. Thompson (1928) has computed the relative importance 

 of the different components of the parasitic factor in the control 



