Aphid outbreaks can severely damage Australian grain crops such as cereals, oilseeds and legumes. Feeding aphids damage plants in several ways, through direct feeding, transmission of plant viruses, injection of toxins and secondary fungal growth (sooty mould) on honeydew. Although feeding aphids remove nutrients from plant cells, crops usually only sustain significant damage when aphids are present in large numbers. However, low aphid numbers can cause substantial damage if they transmit plant viral diseases. Some species, such as Russian wheat aphid (Diuraphis noxia), inject saliva containing toxins into plants. Last but not least, the honeydew aphids secrete can lead to sooty mould which inhibits photosynthesis and can cause grain handling problems. Aphids, with their piercing and sucking mouthparts, are specialised to feed from the phloem. Under the right conditions, their numbers build rapidly and, when the plant can no longer support large aphid numbers, the aphids produce winged morphs that disperse with the wind. In agricultural monocultures, most of these individuals will find a suitable host thus letting aphid numbers build unhindered. Asexual reproduction and the telescoping of generations (females giving birth to live young in which embryos of the next generation are already developing) underpin aphids' ability to expand their numbers rapidly. In areas such as southern Victoria and Tasmania where plants grow year round, aphids are present all year. In Mediterranean climates, such as those of Western Australia, South Australia and southern New South Wales, aphid numbers plunge in summer with populations re-establishing after autumn rain. Many aphid species can also survive and reproduce at near freezing temperatures which allows them to thrive in winter cropping areas.
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