Jan. IS, 1915 Agrilus Bilineatus 285 



it seem probable that the fungus was the primary factor in causing the 

 death of the trees. 



In the vicinity of Robbinsdale, Minn., and in other locahties, the 

 Armillaria fungus is present, but is not so apparent, and all the dead trees 

 show traces of oak borers. Furthermore, many trees with dead trunks 

 started up from the roots the following year and gave every evidence of 

 suffering from the girdling of the cambium layer rather than from a root 

 infestation with the fungus. 



At Robbinsdale a tree was observed on July 27 with its leaves wither- 

 ing as if it had been scorched, a characteristic of trees infested with 

 Agrilus hilineatus. Three days before (July 24) this tree had appeared 

 perfectly normal, but on the 27th the entire trunk of the tree was being 

 girdled by the beetles. It was grubbed and its roots were examined by 

 Mr. F. J. Piemeisel, of the Division of Plant Pathology, who stated that 

 the fungus Armillaria mellea was not present and could have nothing to 

 do with the death of the tree and that the root system seemed perfectly 

 healthy. Other trees examined since gave similar evidence. 



It has not yet been possible to show any relation between the amount 

 of fungus present on a tree and the severity of the attack by the beetles. 

 On land that was being cleared for cultivation near Lake Elmo 40 appar- 

 ently healthy trees were attacked by Armillaria mellea, and none were 

 found to be free from it. Furthermore, W. H. Long states * that a large 

 percentage of the oaks examined by him in the eastern section of the 

 United States have been found to be infested with the fungus. If this 

 is true also in Minnesota and the presence of the fungus is taken as evi- 

 dence of the low vitahty of the tree, all but a small percentage of the 

 oaks in Minnesota are now susceptible to attack by Agrilus bilincalns, 

 and in all these cases the beetles must be looked upon as of only sec- 

 ondary importance. This, however, would possibly place imdue weight 

 on the mere presence of Armillaria mellea. 



As mentioned above, in a few cases in the vicinity of Lake Elmo the 

 death of the trees may be due to Armillaria mellea alone. In the majority 

 of cases the fungus was present, but the trees were girdled by the two- 

 lined chestnut borer, seemingly a hastening factor, at least, in the death 

 of the trees. There still remains the possibility that the fungus was the 

 primary factor. There are cases, as already mentioned, where the 

 borers alone have attacked and killed apparently nonnal trees when the 

 fungus was not present. The economic importance of this condition can 

 hardly be overemphasized, for it means that the Agrilus beetles, in spite 

 of their supposed preference for unhealthy trees, chose one healthy tree 

 when many trees infested with the fungus were available, indicating tliat 

 the interrelation between the Armillaria -mellea and the Agrilus bilinrahis 

 may not be of such primary importance as would appear at first. 



* Long. W. H. The death of chestnuts and oaks due to Annillaria mellea. V. S. Dept. Art. BuI. 3q. 

 p. 4. 1914. 



