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Resisting insect attack


Plants protect themselves from insect attack by producing a multitude of repellent chemicals. These substances including bitter, toxic alkaloids and tannins - are contained in the leaves and plant sap as well as forming an integral part of the scent and nectar. Non-specialist insects and other animals that attempt to feed on the plant may be repelled, injured through digestive malfunctions or even killed. The repellent substances are costly for the plant to produce, and may only be manufactured when an attack is under way. They can then be moved to parts of the plant not yet under attack. There is good evidence that airborne scents can trigger resistance mechanisms in nearby plants - a kind of neighbourhood watch!
The cabbage is well known for its pungent mustard flavour that deters most animals from consuming it. Two components are held in specialist cells of the leaf. When the plant is attacked, mustard glucosinolate makes contact with an enzyme (myrosinase), releasing a bitter isothiocyanate.
Specialist cabbage feeders tolerate the bitter compound, and in some cases use it to their own advantage. The Cabbage White (Pieris brassicae) caterpillar tolerates isothiocyanates that are produced as it eats. The substances are incorporated into its tissues, making the caterpillar distasteful to predators.
The Mealy Cabbage greenfly (Brevicoryne brassicae) is another serious pest of cabbages. Work at Imperial College at Wye has shown that the greenfly incorporates the mustard glucosinolate into its body but manufactures its own myrosinase. If a predator crushes the greenfly, it releases the isothiocyanate and the alarm pheromone farnesene that signal other greenfly to leave the area.
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