Chemicals emitted from a wild South American potato species repels aphids because it mimics the aphids' alarm pheromone.
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"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)
Book The Private Life of PlantsAugust 21, 1995 David Attenborough