Adaptation of alarm pheromone responses of the pea aphid Acyrthosiphon pisum (Harris)

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Abstract
Adult and fourth-instar pea aphids from Vancouver, B.C., responded to alarm pheromone by either dropping, running, or backing up. Younger instars showed almost no response to pheromone but all instars responded to a pheromone–vibratory stimulus, usually by dropping. We suggest that younger instars respond conservatively to alarm pheromone because they are less agile on the ground and are more likely to die there before finding a suitable food plant. Adult and fourth-instar aphids from the hot, dry region of British Columbia do not respond to alarm pheromone by dropping. This is due to the high risk associated with any instar leaving the host plant where ground temperatures and evaporation rates are high.Clones of aphids contained both individuals which do and do not drop in response to pheromone stimulus. This mixture of behaviour types allows part of the group to exploit new resources while others remain on a known resource.