294 THE PRACTICAL APPLICATION OF ECOLOGY 



subject the reader is referred to the general review by Thorpe 

 (1930). 



The Adoption of New Food-plants. A certain amount of 

 experimental evidence exists with reference to the adoption of 

 new plant-hosts by various insects. One of the earliest experi- 

 ments of this kind appears to be that of Schroeder (1903), with 

 reference to the leaf beetle Phyllodecta (Phratora) vitellince. The 

 larvae of this insect skeletonise the leaves of Salix fragilis so 

 as to leave only the upper epidermis intact. Young larvae 

 transferred to Salix viminalis which, unlike ^S*. fragilis, has its 

 leaves densely pilose beneath, adapted themselves to this new 

 food-plant. By cutting and pushing aside the hairy covering 

 they reached the central parenchyma and one larva excavated a 

 mine in the leaf-tissue. After four generations upon S. viminalis 

 the larvae became adapted to leaf-mining, and in each generation 

 the proportion of adults which selected this food-plant in pre- 

 ference to the original host steadily increased from 9 per cent, to 

 42 per cent. 



Experiments conducted by Marchal (1908) with the scale insect 

 Euclecanium persicce involved the transfer of the eggs of individuals 

 living upon peach to false acacia (Rohinia pseudacacia). Only 

 certain of the young scales resulting from these eggs were able to 

 adapt themselves to the new host, but in the following year their 

 progeny had so completely accepted the unusual food-plant that 

 the transfer of individuals back again to peach resulted in their 

 failing to reach sexual maturity. The individuals reared upon 

 Rohinia, it may be added, closely resembled in their large size, 

 form and colour the race or species E. rohiniarum of that same 

 host. Since the Rohinia species was unknown in France prior to 

 1881, Marchal was of opinion that it was a recent derivation from 

 the species E. persicce. Evidence in support of this contention is 

 not entirely conclusive, since the Rohinia is a tree imported from 

 America, where the scale insect E. rohiniarum is also known to 

 occur. 



Sladden (1934), in experiments dealing with induced food- 

 preference in the stick-insect Carausius, came to the conclusion 

 that individuals reared upon ivy showed a greater ability to feed 



