74 [ DISEASES OF CITRUS TREES 



Aphis, which is often an implacable foe of young Citrus 

 trees. The insect usually makes its appearance early in 

 spring on the new shoots and also on the blossoms. 

 When young it is of a rusty green colour, becomes 

 dark green in adult life and again assumes a rusty 

 green or dingy colour with age. The biology of the 

 Aphidae and their curious phenomenon of parthenogenesis 

 are well-known and require no repetition here. As in 

 other plants, the Orange- Aphis prefers the under surface 

 of young leaves, the petioles and the twigs where the 

 sap is in active circulation. A particularity worth mention- 

 ing is that the winged and wingless female individuals 

 are always found together on the same twig. The winged 

 individuals are of a darker green, nearly black, and 

 the larvae of this form may be distinguished when quite 

 young from those of the wingless form on account of 

 their darker colour and the longer tubercles on the 

 upper part of the body, from which tubercles eventually 

 the wings bud out. The length of the adult insect varies 

 from i^ to 2\ m.m. This insect checks the growth of 

 the new shoots ; the blossons fall without blooming, 

 the very young leaves are destroyed, and the others 

 are stunted in their growth, become curled and deformed, 

 and the young tree may fall into a chlorotic condition 

 which may end in death. Were it not for its numerous 

 enemies the Orange-Aphis would prove very disastrous 

 to the Citrus grove. Its principal enemy is a small gnat- 

 like insect belonging to the Chalcideae, whose name is 

 Trionyx testaceipes, Cresson. The larva of this insect, 

 a small white grub, lives within the body of the Aphis, 

 feeds upon it and at last kills it. In dying the Aphis 

 becomes distended, assumes a globular shape and a 

 brown colour and clings firmly to the undersurface of the 

 leaf upon which it fed. The Tryonyx completes its 

 metamorphosis within its host, and finally makes a small 

 puncture on the back of the Aphis reduced to a brown 

 shell, and issues forth a winged insect ready to continue 

 a successful war for the benefit of the Citrus grove. 



