296 THE PRACTICAL APPLICATION OF ECOLOGY 



such change of habit may be equally rapid under natural conditions. 

 There exists in entomological literature many records of insects 

 adopting food-plants which they were previously unknown to 

 affect, but apart from the actual observation of the facts little 

 else is known with respect to this behaviour. In one or two 

 instances, however, well-attested evidence points clearly to the 

 change having occurred within recent years, and appears to be 

 leading to the evolution of special biological races. A well- 

 known example is familiar to North American entomologists in 

 the case of the Trypetid fly Rhagoletis pomonella. The larvae of 

 this insect have long been known to live in the blueberry, but in 

 the north-eastern states its more recent spread to apple is a 

 matter of direct record. According to Woods (1915) it has 

 developed into two distinct races distinguished by the small size 

 of both larvae and adults derived from blueberry as compared 

 with those from apple. Attempts to transfer the apple race to 

 blueberry and vice versa were unsuccessful, the two forms being 

 seemingly independent. What appears to be a parallel example 

 in England is afforded by the Capsid Plesiocoris rugicollis which 

 has now become a widespread and serious pest of the apple. It is 

 practically certain that it was not an apple pest in England 

 prior to 1900, and since that time a race appears to have developed 

 which has adopted this tree as its host in preference to S alloc, 

 which is its usual wild food-plant. This change seems to have 

 involved some readjustment in the life-cycle of the insect since, 

 when living upon willow, it usually occurs from June to August ; 

 whereas, upon apple, eclosion from the eggs takes place from the 

 middle of April onwards, and adults occur from about the first 

 of June until the end of July. The whole life-cycle, it appears, 

 has been advanced by about one month. There is need for 

 exhaustive experiments with reference to the behaviour of this 

 insect ; under artificial conditions it can be readily transferred 

 from willow to apple, or even to new food-plants such as plum. 

 In a state of Nature, however, it shows a tendency to remain 

 upon the host-plant on which hatching took place. In certain 

 cases willow trees were found with their branches actually 

 touching or interlacing those of apple, yet although the willows 



