218 NATURE STUDY AND LIFE 



for nearly nine months in the year, the oyster-shell bark 

 louse exists in the egg state, and there is but one gener- 

 ation in a season. Hence the scales will never be found 

 on the later growths of the preceding year ; and while 

 trees are frequently killed by this insect and young trees 

 should not be planted until completely cleansed of them, 

 it is a very slow process, and many trees are weakened 

 rather than killed outright. 



Many other scales will doubtless be found : Kermes on 

 the oaks, interesting from their resemblance to gall-like 

 knots on the twigs; Lecaniums, of a variety of shapes, on 

 all kinds of plants. One of the Lecaniums, a large oval 

 scale nearly a quarter of an inch in length, has become 

 quite destructive to plum orchards in recent years. An 

 instructive story attaches to the cottony cushion scale, 

 Icerya purchasi, which some years ago threatened to 

 destroy the orange groves of southern California. It had 

 been introduced from Australia and before the danger was 

 realized, as is too often the case, had become widely scat- 

 tered. It was finally discovered that in its native home 

 this scale is not particularly destructive, and a reason for 

 this was sought and found in the fact that its numbers are 

 there held in check by several predaceous lady beetles. 

 One of these, Vedalia, a small red and black species, was 

 successfully imported and soon relieved the orange groves 

 of their unnatural burden and returned to them their loads 

 of fruit. 



In general, the smaller an insect enemy, the more dan- 

 gerous it is, and its powers of destruction naturally 

 increase with the number of plant species upon which it 

 is able to feed. We might pass the scale insects by were 



