PITH-RAY FLECKS IX WOOD. 11 



different species of the same genus are affected varies. Wood speci- 

 mens of red maple from Pennsylvania and from Florida were abun- 

 dantly infested, wliile those of the western maples were comparatively 

 free. No western species of Crataegus seem to be immune, but this 

 conclusion can be established only by fmther investigation. The 

 genus Pyrus leads in number of species affected. It does not follow, 

 however, that trees of tliis genus form the favorite hosts. As already 

 mentioned, river birch is most often infested in Maryland and Vir- 

 ginia. There appears to be a peculiar local susceptibility of certain 

 species. White buxh, for example, is quite commonly infested 

 tliroughout the New England States, but near the southern limit of 

 its range the wood is usually entirely free of mines. Its successor m 

 range farther south, river birch, becomes the favorite host. It is 

 probable that this peculiar distribution of the insect's work holds 

 also for the ^\-illows and poplars. 



FACTORS AFFECTING LOCAL DISTRIBTITION OF LARV^ AND 



PASSAGES. 



Topography has little to do with the local distribution of the 

 larvae. Trees growing on uplands were found to be infested fully as 

 often as those at lower elevations. It is not at all probable that bark 

 characteristics have anything to do "^"ith the likelihood of infestation, 

 since the mines can be traced to the young bark, which in tliickness and 

 smootliness is much the same on all trees. Organic infiltrations m 

 the cortex may cause immunity in certain hosts. Tannin, perhaps, 

 accounts for the total absence of the mines in American oaks. Resin 

 is doubtless a preservative. Kraus reported pith flecks in the fu's, 

 spruces, and cedars (Juniperus), but later \\Titers contradict tliis. 

 PenhaUow notes the formation in some coniferous woods of trau- 

 matic resin canals, wliich might be easily mistaken for pith flecks in 

 a cursory examination. 



The distribution of pith flecks in mdividual trees is readily mf erred 

 from the life history of the larvae. Samples of wood taken fi'om 

 near the transition point of root and stem in infested trees show the 

 mines in the greatest abundance. This is due to the frequent trips 

 by the larvae back and forth prior to pupation. The number of 

 mines decreases rapidly after the first 10 feet above the ground. 

 Above that distance, in transverse section, there is usually only one 

 pith-ray fleck per larva. 



The adult evidently deposits its eggs in the yoiuiger branches of 

 the tree's crown. The larva, upon hatching out, burrows straight 

 do^vnward until near the ground, then makes several turns, as pre- 

 viously described. In red maple and river birch mines were traced 

 about 40 feet down the stem with only slight deflections, and without 

 a tmii before reaching the base of the tree. Tlio tangential diameter 



