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Calling for help

The plant can mount defences against even specialist feeders. Attacking insects initiate the production of plant scents that are released together with the insects own body odours. Specialist predators and parasitic wasps pick up these signals and are drawn to the plant, attacking the herbivores. The signals may take a day or so to reach full intensity, and disappear soon after the damage stops.
Ninety per cent of Cabbage White caterpillars are attacked by the parasitic wasp Cotesia glomerata that is attracted to these unique signals. The signals have the added benefit of inhibiting other Cabbage White adults from laying further eggs on the plant. When the Mealy Cabbage greenfly attacks cabbages, a different signal is emitted, and the specialist greenfly parasitic wasp Diaretiella rapae attacks the greenfly.
Studying calling signals is usually done in the laboratory. An olfactometer gives the predator or parasitic wasp a choice of directional movement, and allows us to introduce odours produced by the plant material into the air stream. This can show whether an odour influences the insects behaviour. Electro-antennography can show signals working at a physiological level. An insects antenna is covered in sensory cells that can react to single odour molecules. When a sensory cell is triggered it emits a nerve impulse back to the brain. If the antenna is connected as part of an electrical circuit, this activity can be measured.
Non-native crop and garden plants have no guardians to respond to their cries for help. Similarly, plant breeders may have unwittingly bred out the natural defences and signals, so making new plant varieties vulnerable to pest attack.