HOST-SELECTION 



289 



species. One race is specially attached to apple as its food- 

 plant, and the other to hawthorn and blackthorn. The apple- 

 bred moths exhibit a great preponderance of forms with pure 

 white fore-wings, while those reared upon hawthorn seldom 

 exhibit this feature, the wings being most often suffused with 

 grey. The two forms also exhibit differences in cocoon texture 

 and methods of oviposition. The results obtained in these 

 experiments are shown in tabular form below. 



Table XVI 



When the hawthorn race was reared upon apple, the resulting 

 females laid all their eggs upon hawthorn. On the other hand, 

 when the apple race was reared upon hawthorn, 69-2 of their eggs 

 were laid on apple and 30-8 per cent, on hawthorn. Experiments 

 carried out with hawthorn- and blackthorn-reared moths, 

 allowing them free choice of the two food-plants, indicated that 

 the hawthorn-blackthorn is subdivided in the same way. It 

 appears that out of 825 eggs laid by hawthorn-reared moths, 

 18-8 per cent, of them were laid on blackthorn and 81-2 per cent, 

 on hawthorn. Furthermore, out of 4,293 eggs laid by the 

 blackthorn race, 69 per cent, were on blackthorn and 31 per cent, 

 on hawthorn. Thorpe's results, therefore, lend strong support to 

 the host-selection principle and they raise the practical point as 

 to whether it is necessary for the apple grower to deal with 

 infestations on surrounding hawthorn hedges. It would appear 

 from the data already given that such an infestation is unlikely, 

 under normal conditions, and that treatment of the hedges would 

 not be a sound practical proposition. 



R.A. ENTOMOLOGY. 



10 



