14 THE GIPSY MOTH AS A FOREST IXSECT. 



results, but rarely to the point of noticeable defoliation. Upon others 

 it will barely hold its own, and upon a few there will be a decrease in 

 abundance following any considerable degree of accidental infesta- 

 tion. These trees are what is here called resistant, and nearly all 

 of them are practically that, so long as the "wilt" remains as efficient 

 as it is at present. 



In this group are to be placed the pmes; the spruces; in all prob- 

 ability fir; hendock, with scarcely a doubt, though it is notably more 

 favorable than pine as a food plant; the junipers and cedars; doubt- 

 fuhy larch; some, but perhaps not all, of the poplars; chestnut; 

 probably beech; yellow birch, black birch, and probably paper birch; 

 apparently all the species of hickory; butternut; sycamore; Ameri- 

 can elm, and probably the other species of elm; apparently hack- 

 berry; sassafras; catalpa; the A'arious species of ash; black locust 

 and honey locust; black cherry and bird cherry; probably mountain 

 ash; all tlie indigenous and probably the European species of maple, 

 although the Norway maple is more liable to attack than others; 

 boxelder; tupelo; horse-chestnut; adanthus; tulip tree, and undoubt- 

 edly many other of the less commonly planted shade and ornamental 

 trees. 



A few of the more common trees have not yet been definitely 

 placed, notably basswood or linden, ironwood, and hop hornbeam. 



RELATIVE SUSCEPTIBILITY OF MIXED FORESTS. 



It would thus appear that in a territory in which both disease and 

 parasites, or disease alone, is prevalent the gipsy moth becomes 

 peculiarly an enemy of oak, and this is true in so far as pure stands 

 of trees are concerned, or of isolated trees. It is not so true of mixed 

 stands, however, as was pomted out by ^Ir. Kirkland and by Mr. 

 Wortldey in respect to pine muigled with hardwood, and these mixed 

 stands may generalh^ be considered just a little more resistant than 

 would be a pure stand of the least resistant tree and considerably 

 less resistant than a pure stand of the most resistant tree which goes 

 to make up any considerable portion of the mixture. Thus a pine 

 and oak forest is slightly less liable to injury than a pure stand of 

 oak and much more liable than a pure stand of pine. The same 

 might be said of an oak and hickory mixture or one of oak and 

 chestnut. The reason is that the caterpillars, increasmg unmterrupt- 

 edly upon oak, will finally be forced to leave it and will strip oth(>r 

 trees upon which they would not increase to anything like a similar 

 extent if they were forced to feed upon them for generation after 

 generation. They do not always do this, it is true, bv.t they do it 

 very often, particularly when the oak is abundant and scattered 

 evenly tliroughout the forest. 



