Chemicals emitted from a wild South American potato species repels aphids because it mimics the aphids' alarm pheromone.
"A wild species of South American potato discharges a message-chemical, a pheromone, which resembles that emitted by aphids when they are under attack. So those aphids never land on it." (Attenborough 1995:68)
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Cyndy Parr commented on an older version of Pheromone mimic repels aphids: South American potato:
This text refers to a wild species of South American potato and I have a feeling that is not nightshade. Unless EOL has a less desirable common name at the top.