394 BIOLOGICAL CONTROL 



6,250). It is further noteworthy that among the Hymenoptera there 

 are only two species of saw-flies, and less than 250 species of the parasitic 

 groups. The paucity of the latter is well emphasised by comparison 

 with Great Britain, where there are about 1,500 species of Chalcidoidea 

 alone. 



In Great Britain none of the three groups of conditions 

 previously mentioned appears to favour parasite introduction. A 

 variable, and on the whole, cool humid climate ; a complex and 

 highly adjusted continental insect fauna ; and a diversity of 

 crops and cultural methods all appear to militate against the 

 possibility of any striking economic results being achieved through 

 the application of biological control. An environmental resistance 

 of this character has stood the country in good stead, since it is 

 almost free from introduced insects affecting agriculture. It is 

 evident, therefore, that parasite introductions would naturally 

 have to be framed to cope with one or other of the major indigenous 

 pests. In the future it may be found feasible to explore the 

 possibilities of north temperate countries with this object in view. 



(b) Localised Continental Areas. Biological control in con- 

 tinental areas has proved successful in certain cases where the 

 crops involved occupy more or less circumscribed tracts of country 

 and require climatic conditions of a warm and equable nature. 

 This applies especially to mulberry cultivation in Italy and to the 

 cultivation of citrus fruits in various parts of the world. These 

 same conditions, it would appear, favour parasite introductions, 

 and a partial explanation may be found therein of the success 

 attending the importation of Prospaltella berlesei and of Vedalia 

 cardinalis into many countries. The effective control exercised 

 by Aphelinus mali has so far only been evident under continental 

 conditions in areas of a more or less localised character, areas 

 where the climate is favourable and apple cultivation is prosecuted 

 on a large scale. ^ 



The successes achieved in California are to be ascribed to a 

 combination of special conditions. Bounded partly by the 

 Pacific Ocean and partly by barriers of mountain and desert, the 



^ As mentioned previously, other contributing factors appear to be the 

 rapid multiplication of the parasites in question, and the fact that they attack 

 hosts which are largely sedentary in behaviour. 



