331 



Chapman (R. N.). Observations on the Life-History of Agrilus biline- 

 atus. — Jl. Agric. Research, Washington, D.C., iii, no. 4, January 

 1915, pp. 283-294, 2 plates. 



The four common species of oak in the south-eastern section of 

 Minnesota, Quercus alba, L., Q. niacrocarpa, Michx., Q. rubra, L., and 

 Q. coccinea, Wang, are subject to infestation with Agrilus bilineatus, 

 Weber (the two-lined chestnut borer). Members of the black-oak 

 group seem slightly more susceptible to attack than those of the 

 white-oak group, but in localities where infestation is severe, none 

 of the species is exempt. It has often been found that Armillaria 

 tnellea, Vahl (the shoestring fungus) has apparently been the cause of 

 the weakened condition of the trees, and that the chestnut or oak-borers 

 have followed it. Near Lake Elmo a few dead trees were found with 

 the fungus, but with no traces of Agrilus larvae. In other localities 

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

 showed traces of oak-borers. Dead trees also showed characteristic 

 beetle injury, but no traces of A. mellea were found on examination. 

 The economic importance of this latter fact can hardly be over- 

 emphasised, for it means that the beetles, in spite of their supposed 

 preference for unhealthy trees, chose a healthy one where many trees 

 infested by the fungus were available, thus indicating that the inter- 

 relation between Armillaria mellea and Agrilus bilineatus may not be 

 of such primary importance as would appear at first. On the 17th June 

 1914, the first adult beetles were seen and they reached their greatest 

 abundance about 1st July. There was a noticeable decUne after the 

 first week in July and by the 20th the last record of adults had been 

 made. Never found until a couple of hours before noon, the adults 

 were most numerous shortly after that time. They then gradually 

 disappeared until only few were to be seen late in the day. The females 

 oviposited between 19th June and 13th July, generally in a crevice 

 at the bottom of a deep crack between ridges of bark. Since the bark 

 is usually rougher on the trunk and lower limbs, especially near the 

 ground, more favourable places are to be found there. On one tree 

 which was very badly infested practically every branch more than 

 1| inches in diameter had burrows in it. The larvae on hatching broke 

 through the egg membrane on the side towards the bark and imme- 

 diately began to burrow. Burrows made during the first instar are 

 made across or with the grain. The burrows of the second instar are 

 wider and about twice as long. At the beginning of the third instar 

 quite a different course was usually found, especially in green bark 

 on the trunks of trees, where the burrows were almost always transverse 

 to the grain of the wood, while those of the fourth instar often attained 

 a length of 20-24 inches. Where the bark was thick these were 

 generally transverse to the grain. At the close of the fourth instar 

 the larva burrows out into the bark if it is thick enough, and constructs 

 a cell in which it hibernates. Here pupation takes place in the spring. 

 These cells are found in the ridges of the bark on the trunk and larger 

 hmbs and in the wood of small, thin-barked trees and limbs. It is 

 not surprising that trees infested with Agrilus bilineatus appear to die 

 suddenly when larvae are numerous, considering that each individual 

 may consume cambium tissue equal to nearly twice its own bulk every 

 24 hours. Trees with growing tissue offer the best opportunity for 

 making extended burrows with great nutritive value to the larvae and 



