342 PROCEEDINGS OF THE NATIONAL MUSEUM vol.93 



and Thysanoes LeConte, which live most of their life deep in the 

 wood of their hosts. 



In so far as known, all species breed in the bark or wood of decidu- 

 ous trees or shrubs. Ti ees that are decadent, dying, or recently dead 

 and broken or cut limb's and twigs commonly serve as breeding places. 

 Large colonies of brood burrows are seldom found together, as is so 

 often the case with other scolytids, although with Hylocurus lang- 

 stoni (Blackman) such a massed attack is not uncommon. In most 

 cases, however, only single burrows or small groups of burrows are 

 found at one place. This is probably due to the paucity of indi- 

 viduals of most of the species, especially of the lignivorous forms 

 such as various species of Hylocurus, Micracis, and Thysanoes. 



The proper development of the brood of such forms depends upon 

 whether the wood infested is in proper condition for the nourishment 

 of the larvae. This appears to involve the presence of sufficient 

 moisture and the presence of the hyphae of rapidly developing fungi. 

 If the wood dries too rapidly, fungi do not develop properly and the 

 larvae die either from lack of moisture or from the resultant slowing 

 up of the development of decay fungi. Whether it is lack of water, 

 lack of fungal hyphae, or lack of material produced by fungal decay 

 that is responsible for the death of the larvae cannot be definitely 

 stated. Perhaps the presence of all three is essential. It may well 

 be significant that in hickory, which is a favored host for a number 

 of species, decay of the sapwood is extremely rapid if sufficient mois- 

 ture and warmth are present. 



As regards economic importance the various genera and species 

 of Micracinae vary considerably. None of the forms breeding in 

 bark are known to be strongly injurious, as thej' choose as hosts 

 decadent or broken limbs or twigs. Such species, while they may 

 occasionally kill limbs that might otherwise survive, may in general 

 be classed as mildly beneficial in that they aid in the processes of 

 decay and thus hasten the return of such material to the soil. 



Species of the lignivorous genera Hylocurus, Micracis, and Thysa- 

 noes are potentially more injurious and in several cases are known 

 to be actually injurious. For example, Hylocurus langstoni (Black- 

 man), common in the Southeastern States, often attacks living trees 

 through injured or deadened areas of the bark, breeds in the wood 

 beneath such areas, and by feeding on the adjacent living bark 

 extends the injury until the entire tree is killed. Posts and poles 

 set in the soil before being thoroughly seasoned are also often 

 subject to heavy attack, resulting in a type of injury similar to that 

 made by powder-post beetles, by which the entire sapwood is often 

 riddled by larval mines filled with powdery frass. 



