CONDITIONS FAVORING INCREASE. 97 



tioned, and possibly many others have escaped observation, 

 but those given may serve in a measure to explain the un- 

 usual increase of the moth. It is during such seasons that 

 its destructiveness is most apparent. It is then that the 

 groves and forests are stripped of their leaves, and whole 

 rows of trees in orchards and along highways appear to have 

 been stripped in a single night. 



DISTRIBUTION AS AFFECTED BY FOOD SUPPLY AND OTHER 

 NATURAL CAUSES. 



If the number of gypsy-moth larvae in a given territory 

 is small and their food supply is large, they do not usually 

 spread of their own volition to any appreciable extent. So 

 long as the supply of food is abundant and accessible the 

 caterpillars usually remain on or near it, and will move only 

 when disturbed or dislodged from the trees or other plants 

 on which they feed. In such cases they will reascend the 

 same tree or crawl to near-by vegetation. When a tree is 

 overcrowded with caterpillars, and by reason of their vo- 

 racity food becomes scarce, they will crawl rapidly in all 

 directions in search of it, and thus they spread out from a 

 common centre over a limited area. Wherever the moth is 

 introduced it has the advantage of such species as the canker 

 worms (Paleacrita vernata and Anisopteryx pometaria) and 

 the tent caterpillar ( Clisiocampa americana) , which are con- 

 fined to a few kinds of food plants. Because of the great 

 number of its food plants it is capable of subsisting in almost 

 any locality whereto it may migrate or be transported. Its 

 spreading is, therefore, more general and its distribution less 

 localized than that of the canker worms. But as the female 

 moth does not fly, the species is limited in its powers of 

 migration, and though the larger caterpillars are very rest- 

 less, their movements show little method except when food 

 becomes scarce. It has been seen that isolated colonies of 

 the moth in woodlands not frequented by men do not often 

 spread to a considerable extent until the caterpillars have 

 increased in numbers so as to destroy all the foliage in the 

 originally infested localities. They then migrate in search 

 of food, and when this movement is once begun they some- 



