Oct. 22, 1931 Hopkins Host-Selection Principle 205 



In 19 1 9 this was again repeated, but the results showed that pine was 

 infested while the spruce contained no larvae. Both woods were of No- 

 vember cut, but it is not likely that this would have such a decided in- 

 fluence. However, six adults were isolated on spruce to continue the 

 colony, and this wood was very lightly infested. No explanation can be 

 offered as to the reason for this discrepancy from former results unless the 

 spruce wood was in an unfavorable condition. 



During the same season another cage was prepared of pine cut during 

 November, 1918, and green spruce cut in April, 1919. The pine was 

 heavily infested ; the spruce contained no larvse. 



Experiment IV^. To test effects of condition of wood on selection. — 

 April 23, 1920, one pair of adults from spruce was caged on a favorable 

 cut of pine (November) and freshly cut spruce (April) . When the woods 

 were examined on July 5, 1920, the pine contained many larvae while the 

 spruce contained none. At the same time a pair was caged on November 

 pine and November spruce, both optimum cuts. Several days later 

 the female was found dead in the cage and had laid no eggs. 



Acer. Experiment IV^. — ^As maple has been reported as a host of this 

 species, attempts were made in 19 16 to start a colony in this wood. On 

 July 6, six half- grown larvae were transferred to a seasoned piece of 

 wood, but by July 21 all but one had died and it was smaller than when 

 transferred. This one died soon after. None of the larvae fed on the 

 maple. 



Green pine. Experiment IV^. — In 1916 some of the wood used to 

 carry this colony along was cut March i, and consequently little seasoned. 

 It was very unfavorable for the ovipositing of the adults, but some eggs 

 were laid on the pieces. The larvse developed slowly and at the time 

 of pupation were below normal size. The adults secured in 1 9 1 7 averaged 

 about one-half normal size. They were caged again on wood cut in 

 March, 191 7. Adults were secured in 1918 and again caged on the same 

 condition of wood, and a good infestation was secured. While the author 

 was absent for a month from the field station in the summer of 19 18 

 these larvae were all killed by a fungus. The sticks were on the ground 

 and so were caught in a period of rainy weather and were water soaked. 



Experiment IV, IV ^. — In 1920 the pine and spruce strains were 

 continued in the same wood. 



CONCI^USIONS 



The pine form shows a decided preference for that host. 



It can live in spruce and then shows a decided preference for that 

 host. 



It will not live in juniper or maple. 



In producing a new host strain a high mortality occurs in the young 

 larvse. 



