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], 1g). 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 New 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 protection 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).