ITS RATE OF INCREASE. 95 



of its natural enemies and from the struggle with other 

 adverse influences which encompass it. The normal rate of 

 increase in such isolated colonies as are found to-day in 

 the outer towns of the infested district is seen to be small. 

 The annual increase can be readily ascertained by noting 

 the relative number of egg-clusters laid in successive years, 

 the unhatched or latest clusters being easily distinguished 

 from the hatched or "old" clusters, and the age of these 

 latter, whether one, two, three or more years, being indi- 

 cated by their state of preservation. The ratio of the aver- 

 age annual increase of ten such colonies was found to be 

 6.42, that is, six or seven egg-clusters on an average may 

 be found in the second season to one of the first season. 



Though even at this rate of increase the progeny of a 

 single pair of moths would be numerically enormous within 

 twenty years,* yet for the first few years, under normal con- 

 ditions, the increase of a small and isolated moth colony is 

 not great enough to work any serious or extended injury. 

 The great army of moths does not advance rapidly by the 

 skirmish line, as it were, but only by the main body, for a 

 great increase and rapid spread to a distance can only occur 

 where the moths have become so numerous over a consider- 

 able area as to have nearly reached the limit of their food 

 supply. 



Conditions favoring Rapid Increase, 

 When any colony under average normal conditions has 

 grown to a considerable size and then receives an added 

 impetus from exceptionally favorable conditions, its power 

 of multiplication and its expansive energy are greatly aug- 

 mented and its annual increase rises above all calculations.! 

 Under such influences hundreds of egg-clusters will appear 

 in the fall where few were to be seen in the spring, and 

 thousands are found where scores only were known before. 

 It is probable that the season of 1889 was particularly favor- 



* At this ratio the number of egg-clusters produced in the twentieth year would be 

 14,148,179,507,899,404. 



t The increase of these large colonies seems to be limited only by the supply of food. 

 Whenever food becomes scarce many of the moths are less prolific. The larvae which 

 do not find sufficient food either die or develop early, and the female moths lay fewer 

 eggs than those which transform from well-nourished caterpillars. 



