APPLE INSECTS 



143 



ously injured, the leaves being badly curled and the growth of 

 the tree checked. The aphids secrete a sweet liquid known as 

 honey-dew in which a black fungus de- 

 velops and gives the infested foliage and 

 twigs a sooty appearance. In large, bear- 

 ing apple orchards, these plant-lice some- 

 times appear in incredible numbers and 

 not only curl and kill some of the ter- 

 minal leaves but attack the young fruits, 

 preventing their full development and 

 giving them a knotty, stunted appear- 

 ance (Figs. 157, 158 and 159). The three 

 species of these aphids now common on 

 apple trees throughout the United States 

 are Aphis pomi, Aphis sorbi and Sipho- 

 coryne avence. 



The interesting and somewhat compli- 

 cated life histories of these plant-lice have 

 not been fully worked out. The last two 

 species breed on the apple trees for only a few generations in 

 the spring and early summer, then migrate to other food-plants, 

 but return to the trees in the fall, where the hibernating eggs 

 are laid. The first species remains on the trees throughout 

 the year. The wonderful rapidity with which these aphids 

 multiply parthenogenetically enables even those that work on 

 the trees only a part of the season to develop in sufficient num- 

 bers to do serious injury to the foliage or fruit before they mi- 

 grate to their summer food-plants. 



Natural enemies of the apple aphids. 



Cold, heavy rains, both in early spring and late autumn, re- 

 move and doubtless kill many of the aphids, thus often prevent- 

 ing serious infestation in orchards. Fungous diseases also 

 destroy large numbers under favoraWe weather conditions. 

 Several species of ladybird beetles, aphis-lions and maggots of 



Fig. 155. — Newly 

 hatched aphids clustering 

 on an opening apple bud. 



