GARDEN PESTS IN NEW ZEALAND 



CHAPTER VI. 



Sucking Insects (Concluded). 



Plant Lice, or Aphides. 



THE small, soft-bodied plant-lice, or aphides, usually found forming 

 dense colonies on all sorts of plants, are pests well known to every 

 gardener; they attack plants by inserting into the tissues their delicate 

 piercing mouth-parts, and drain the nutrient sap (Fig. 8, Ig). All 

 parts of a plant may be infested, and the insects, owing to their ability 

 to reproduce abundantly and rapidly, may destroy the plant, or at least 

 injure it by stunting its growth, curling the leaves, or deforming the 

 flowers and fruit. In many cases aphides copiously secrete honey-dew, 

 upon which sooty mould grows, rendering the plant unsightly; on this 

 honey-dew ants feed, and are frequently seen associated with aphides. 

 Apart from their direct injurious effects, aphides are of outstanding 

 importance, in that they transmit some of the most serious plant diseases. 

 Of all the species occurring in Xew Zealand, only one species is supposed 

 to be a native. 



Most aphides live exposed upon the host plant (e.g., Rose Aphis), 

 but some (e.g., Woolly Aphis) secrete a protective covering, while others 

 cause a malformation of the plant tissues which form a partial protec- 

 tion as a semi-gall (e.g., Elm-leaf Aphis), or a complete protection as 

 a true gall (e.g., Leaf-petiole Gall-aphis of Poplar). 



Aphides present certain variations in structure, and, generally 

 speaking, the one species presents four or five types (Fig. 8, 1) : the 

 asexual (parthenogenetic) wingless and winged females that give birth 

 to living young (viviparous) in the absence of males, and the sexual 

 forms, both males and females, the latter producing eggs (oviparous). 



The best character by which the Xew Zealand aphides are to be 

 recognised is to be found in the pair of longer or shorter hoirn-like 

 processes, or "cornicles," projecting from the upper surface of the 

 abdomen; in some species, however, the "cornicles" are reduced and 

 inconspicuous (e.g., Woolly Aphis), or altogether absent (e.g., Grape 

 Phylloxera).- The "cornicles" are frequently called "honey-tubes," since 

 for many years it was thought that they secreted the honey-dew; it has 

 been shown, however, that the honey-dew is secreted from the rectum, 

 and that the function of the "cornicles" is to secrete a waxy protective 

 substance, which may take the form of a powder or woolly threads. Th> 

 wings, when present, are membranous, the front pair being much larger 

 than the hind ones, and when not in use usually close roof -like over the 

 bodv. 



