56 



of river flow may allow salt water to enter, bringing with it the marine borers. This 

 development is illustrated by the destruction which occurred in the Carquinez Strait 

 territorv in recent \cars. The reduction in ri\er flow, which permits the increase in 

 salinity, may result from many influences, such as unusually low rainfall or reduced 

 rainfall in mountain sources of streams, new irrigation projects, diversion of streams 

 for industrial or municipal supplies, or storage reservoirs. 



Since salinity may bring with it borer attack, as well as other adverse conditions, 

 it is important that means be established to give information respecting salinity con- 

 ditions where timber is used under such fluctuating conditions. In many cases salinity 

 records are kept by industries located in the vicinity. Such records should be carefully 

 observed to ascertain or predict developments. The technique of salinity determina- 

 tion, while exacting, is relati^•el^' simple and not beyond the reach of any one with the 

 most elementarv chemical knowledge. Salinity records should by all means be sup- 

 plemented by systematic use of test pieces for determining occurrence, character, 

 intensity, and progress of borer attack. Where borer attack is not clearly impossible, 

 it is in the highest degree prudent as well as profitable to use every possible means to 

 foresee a dangerous condition in time to adopt protective measures. 



As time goes on, interior development will tend to alter river flow, in the majority 

 of cases reducing it. In some instances the river flow is increased, as for example where 

 flood control channels are being built to carry off flood waters which now inundate 

 agricultural lands: but while this will temporarily increase the run-off reaching the 

 rivers, it is likely to be oft'set by the influences tending toward decrease. In a few cases 

 locks or dams may be built permanently shutting out sea water. In general, however, 

 it is likely that ocean water will steadily advance into the estuaries of rivers accessible 

 to it. 



Infestation 



The pre\ailing condition of borer infestation is usually a well established fact — 

 the borers are known to be either present or absent, and protective measures are either 

 adopted or disregarded. Perhaps the most dangerous situation is the beginning of 

 borer attack in a region previously without it. An infestation is in reality contingent 

 upon two general factors: marine conditions and the presence of timber which can be 

 attacked. The former will control the intensity of infestation, the latter will provide a 

 breeding ground in which it may be established and continued. 



An infestation may be conveyed from one locality to another either by some such 

 agency as driftwood or log-booms, or by water currents which carry the borer larvae. 

 If unprotected timber exists in a locality reached by such agencies it will eventually 

 be attacked and the infestation established: if no such timber exists, the larvae will 

 soon perish and any borers in driftwood will die off as soon as the wood is destroyed. 

 A locality near an infestation may remain comparatively free from borers if conveying 

 agencies are absent. A structure may even be built of unprotected timber, in an isolated 

 location, where conditions are suitable for borer life and within a few miles of an infested 

 region, and may remain unmolested or only lightly attacked for some time. The possi- 

 bilities of a continuance of such immunity are increasingly unlikely, for some chance 

 circumstance is almost certain to bring the borers; and as time goes on other structures 

 are likely to be built in the vicinity, gi^■ing a greater amount of exposed timber and 

 increasing the possibility of establishing infestation. Under such conditions the rapidity 

 with which the borers breed will soon make the attack as severe as elsewhere. 



The apparent absence of borers in a locality relatively near an infested region 

 has often led to the mistaken conclusion that prevailing conditions unfavorable to 



