522 MISC. PUBLICATION 657, U. S. DEPT. OF AGRICULTURE 
Figure 142.—Mines of the holly leaf miner (Phytomycza ilicis). 
Gall Makers 
Agromyza schineri Gir., the poplar twig gall fly, causes irregular, 
oval swellings about 3 mm. in length on the smaller twigs of Populus 
spp. The larva is greenish yellow and furnished with strong tri- 
dentate, jet-black mandibles. “The species passes the winter as larvae 
within the galls, the adults emerging early in the spring. The adults 
are black and about 1 inch in length. 
bene yza tiliae Coud., the linden bark gall fly, causes irregular, 
val, subcortical twig swellings about 1 cm. long in linden. The 
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neues overwinters in the puparium, the adults issuing in the spring. 
Cambium Miners 
At least four dipterous species mine the cambium of living trees, 
causing the defect in lumber known as “pith-ray flecks” (fig. 143). 
This injury is sometimes serious enough to make the wood unsuitable 
for fine-grade cabinet work or veneers. In cherry a disintegration 
of woody tissue, due to pith flecks, has been observed. The infested 
trees are difficult to detect unless the bark is removed and the cambium 
exposed. 
Brown (66) has given a good account of the origin of pith flecks and 
the injury attributable to them. He stated that specimens of river 
birch examined in the latter part of April were infested with larvae 
of Agromyza pruinosa Coq. that were boring downward in the 
cambium, having already reached a point about 6 feet from the ground. 
A number of mines were traced back to their sources, which were sup- 
pressed lateral twigs on branches of about 5 years’ growth in the crown 
of the tree. 
Some mines could be traced for about 40 feet down the stem to the 
base of the tree; others extended some distance into the roots. The 
greatest number occurred in the basal portion of the trunk, owing to 
the frequent turnings of the immature larva. 
