Oct. 22, 1921 Hopkins Host-Selection Principle 197 



size. An examination in July showed that the oak and chestnut were 

 equally well infested, the hickory contained one larva, and the maple 

 and ash none. 



RoBiNiA, Experiment I ^. — In June, 19 1 7, eight adults from oak were 

 isolated on black locust cut in February, 191 7. The adults laid eggs, but 

 all died later. 



In June, 19 18, the experiment was repeated with 15 adults and wood 

 cut April 15. Many young larvae entered the bark, but by August 15 

 nearly all had died, and none lived to transform the next spring. The 

 experiment was not repeated in 19 19. 



Acer rubrum. Experiment I*. — In June, 191 7, eight adults from 

 oak were isolated on a piece of red maple cut February i, 19 17. A few 

 larvae lived and three adults (two males and one female) emerged in 19 18. 

 They were isolated in a cage containing oak, chestnut, hickory, ash, and 

 maple, all cut April 15, but no infestation occurred in any wood. During 

 August, 19 18, twenty larvae were transferred from oak to maple to con- 

 tinue the species in this host. A few adults emerged in 1919, and were 

 recaged on maple to develop a larger colony, which will be continued 

 several years before testing the selection again. 



Fraxinus. Experiment I ^. — In June, 191 7, eight adults from oak 

 were isolated on a piece of ash cut January i, 19 17. A fair infestation 

 occurred, but the larvae developed slowly, and in May and June, 1918, 

 only seven adults (both sexes represented) emerged, while many larvae 

 were still in the wood. These adults were transferred to a cage contain- 

 ing oak, chestnut, hickory, ash, and maple, all cut April 15, but no infesta- 

 tion occurred in any wood. Larvae were again transferred to ash, and a 

 few adults emerged in 19 19. These were recaged on ash, and several 

 adults emerged in June, 1920, but failed to develop any larvae in the new 

 wood. 



I, I\ I^, P. — During June, 1920, adults emerged from the oak, hickory, 

 chestnut, and maple; strains and adults from all were recaged on the 

 same wood and produced new colonies. No selection tests were made, 

 and these strains will be continued for several years in the same wood 

 before similar experiments are again attempted. 



CONCLUSIONS 



The original oak strain of Xylotrechus colonus shows a decided prefer- 

 ence for a few woods, notably oak, chestnut, and hickory. Two years' 

 trial failed to produce larvae capable of completing their development in 

 locust, while the ash and maple colonies were maintained with difficulty. 

 In nature these woods (ash, maple, and locust) have been found contain- 

 ing thrifty colonies of this species. 



Originally the oak strain showed little preference as between oak, hick- 

 ory, and chestnut; yet, after several years, strains were developed in each 

 wood that showed a growing preference for the given wood. 



