SCALE INSECTS OF IMPORTANCE 293 



Injuries. The harm done by scale insects is seldom appreciated till 

 it is too late. The scales may be noticed on the bark in considerable 

 numbers, but so long as the tree shows no marked injury, the majority 

 of people are inclined to believe that but little harm has been done. They 

 appear to overlook the fact that a tree, like a man, may put forth every 

 possible effort to sustain itself and apparently succeed in doing so, only 

 to collapse suddenly at the end. Every living scale insect, after it has 

 become established, is an automatic pump drawing the vital fluids from 

 the host plant through a slender, hair-like beak or proboscis (pi. 3, fig. 10). 

 The amount insects are capable of abstracting in this way from a tree is 

 truly surprising. I have repeatedly seen showers of honeydew falling 

 from elms badly infested with the elm bark louse, the excretion being so 

 copious as to keep the walk beneath wet even on good drying days. 

 This abundant excretion is not seen in the case of the armored scales, 

 like the species to be considered later, but the production of their firm, 

 protective coverings, as well as the nutrition of the thousands of insects, 

 must make an enormous draft on the infested tree. This is proved by 

 the fact that not infrequently trees are unable to withstand the drain and 

 succumb. The injury these species can inflict is in a measure directly 

 proportional to their productivity. A moderately proHfic species pos- 

 sessing the ability to develop several generations in a season is one to be 

 feared, because under favoring conditions a much larger number of in- 

 dividuals might be produced than would be possible for a much more 

 prolific species which was limited by nature to one generation annually. 

 It is the same for one year as the relation existing between arithmetical 

 and geometric progression. This is why the San Jose scale is so danger- 

 ous. It is not only moderately prolific, but it develops a number of gen- 

 erations in a season. It has been estimated that in one year in the lati- 

 tude of Washington (D. C.) a single female might produce, all condi- 

 tions such as food supply, etc. being favorable, the enormous number of 

 3,216,080,400 descendants. 



Means of dispersal. This is an extremely important matter, par- 

 ticularly to the man whose trees are free from these pests. The perio-d 

 when any of the scale insects to be considered below can travel of their 

 own free will is very limited and, excluding the males, which may be dis- 

 regarded in this connection, they are wingless and their crawling powers 

 by no means great. These scale insects depend almost entirely on some 

 external agency to transport them even from tree to tree, unless the limbs . 

 interlock. It has been demonstrated by Prof. W. G. Johnson that the 



