DAMAGE CAUSED BY ANIMALS. 113 



development of new buds from the sheath owing to the stumps of 

 the spines being left capable of performing assimilative functions. 



59. Influences favourable and unfavourable to their Increase ; 



Enemies. 



Many forest insects attack sickly seedlings, poles, or trees in 

 the first instance, particularly insects feeding on conifers, which 

 tirst of all seek out the suppressed or unhealthy individuals 

 j with a weakly flow of sap. Thus we may note that bark-beetles 

 (Bostrychini) and cambial-beetles (Hylesinini) deposit their eggs 

 on stems broken by wind, or on newly-felled wood, or on stems 

 already damaged by the attacks of caterpillars, that the large 

 Pine-weevil (Hylobius abietis) selects, when available, sickly plan- 

 tations, or overcrowded crops raised by sowing, for its feeding- 

 ! ground, whilst choosing as its breeding-place the stools of recently 

 i felled trees, and that moths not infrequently select for the 

 I deposition of their ova crops which are backward in growth 

 owing, perhaps, to the inferiority of the soil. 



Certain localities, as for example windfall areas, and falls of 

 I mature timber where the stumps have not been grubbed out, offer 

 favourable conditions as breeding-places for such insects, whence 

 ithey re-issue in vastly increased numbers. Crops backward in 

 growth also form favourable breeding-places, as well as central 

 ipoints for their attacks when feeding. Save during great plagues 

 of insects, attacks only exceptionally take place simultaneously 

 I over extensive areas ; they are otherwise always found to proceed 

 land spread from single detached centres. 



All circumstances combining to form such breeding-places or 

 ifeeding-grounds in large numbers, and especially windfall and 

 'breakage from accumulations of snow, favour the rapid increase 

 pf insects injurious to woodlands ; hot, dry summers are also 

 .favourable to them, as long-continued drought always tends to 

 reduce a number of stems to a sickly state, and warm weather at 

 'the time of caterpillars changing skin, or of imagines swarming, is 

 jfor many kinds of insects (e.g., moths, sawflies) decidedly favour- 

 Sable to their numerical increase. And not infrequently, one 

 insect prepares the way for another, as for instance, after the 

 Black Arches, " Nun," or Spruce Moth (Liparis monaclia) has fed 

 on the needles of Spruce and Pine, the sickly stems yield a 



H 



