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Honeydew and sooty mould

The sap fabricated in host plants is sugar-rich but poor in amino acids, essential components for growth. The aphids have to ingest extremely large quantities of sap to satisfy their need in proteins. The substance produced by digestion, very rich in a wide variety of sugars (mono-, di- and trisaccharides), accumulate in the dilated part of the rectum before ejection, droplet by droplet. This is honeydew. It serves as a highly favorable culture medium, where saprophytic fungi soon settle. These induce sooty moulds, which impede respiration and chlorophyll assimilation or taint edible parts (fruit for example) and make them unfit for release onto the market.

Viral transmission

The aphids take a prime role in spreading viral diseases, both by the number of viruses they could transmit and the number of species involved. Nearly 200 aphid species have been recognized as vectors. One of them, Myzus persicae Sulz, is alone capable of passing on more than 120 diseases. That shows the importance of this group and it is no exaggeration to assert that aphids are more dangerous when they carry viruses than when they take sap from their host. In the case of viral diseases, and contrary to that of direct damage, just a few individuals can be enough to bring on irreversible harm.

Sooty moulds on Takecallis arundicolens honeydew on bamboo leaf	Pea Enation Mosaic Virus (PEMV) transmitted by Acyrthosiphon pisum on peas	Barley Yellow Dwarf Virus (BYDV) transmitted by Rhopalosiphum padi on barley	Virus Y (PVY) transmitted by Myzus persicae on potato