APPLE INSECTS— BUDS AND FOLIAGE 



113 



is the wild cherry, although it will attack apple, peach, plum, 

 and more rarely, witch-hazel, beech, birch, barberry, oak, 

 willow and poplar. When excessively abundant apple trees 

 are frequently completely defoliated and killed (Fig. 121). 

 Destructive outbreaks usually continue for two or three years 



Tree tlcfoliatcd by apph^-trcc tent-caterpillars. 



only and are then followed l)y a longer j)eriod during which the 

 species is rarely noticed. This periodic fluctuation is thought 

 to be the result of complicated interrelations existing between 

 the caterpillar and its parasites, and is a striking illustration 

 of the ups and downs of insect life. 



