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Plant-aphid interactions

Plants mount local and systemic defenses against aphid attack. Young leaves in some plants contain chemicals that discourage attack while the older leaves have lost this resistance, while in other plant species, resistance is acquired by older tissues and the young shoots are vulnerable. Volatile products from interplanted onions have been shown to prevent aphid attack on adjacent potato plants by encouraging the production of terpenoids, a benefit exploited in the traditional practice of companion planting, while plants neighboring infested plants showed increased root growth at the expense of the extension of aerial parts. The wild potato, Solanum berthaultii, produces an aphid alarm pheromone, (E)-β- farnesene, as an allomone, a pheromone to ward off attack; it effectively repels the aphid Myzus persicae at a range of up to 3 millimetres. S. berthaultii and other wild potato species have a further anti-aphid defence in the form of glandular hairs which, when broken by aphids, discharge a sticky liquid that can immobilise some 30% of the aphids infesting a plant.

Plants exhibiting aphid damage can have a variety of symptoms, such as decreased growth rates, mottled leaves, yellowing, stunted growth, curled leaves, browning, wilting, low yields, and death. The removal of sap creates a lack of vigor in the plant, and aphid saliva is toxic to plants. Aphids frequently transmit plant viruses to their hosts, such as to potatoes, cereals, sugarbeets, and citrus plants. There are two types of virus transmission between plant-aphid interactions: non-circulative transmission and circulative transmission. In non-circulative transmission, the virus attaches itself to the aphids mouthparts and is released when the aphids feed on a different plant. These non-circulatory transmitted viruses promotes rapid dispersion of the vector, or aphids. In circulative transmission, the virus is ingested and passes through the gut lining to enter the hemolymph, where it is then circulated throughout the entire body. After reaching the salivary glands, the virus is then released into the saliva upon transmission sites in plants. Circulatory transmitted viruses allows for long-term feeding by the aphids and increases the chances of being infected with the virus. The green peach aphid, Myzus persicae, is a vector for more than 110 plant viruses. Cotton aphids (Aphis gossypii) often infect sugarcane, papaya and peanuts with viruses. In plants which produce the phytoestrogen coumestrol, such as alfalfa, damage by aphids is linked with higher concentrations of coumestrol.

The coating of plants with honeydew can contribute to the spread of fungi which can damage plants. Honeydew produced by aphids has been observed to reduce the effectiveness of fungicides as well.

A hypothesis that insect feeding may improve plant fitness was floated in the mid-1970s by Owen and Wiegert. It was felt that the excess honeydew would nourish soil micro-organisms, including nitrogen fixers. In a nitrogen-poor environment, this could provide an advantage to an infested plant over an uninfested plant. However, this does not appear to be supported by observational evidence.