The History of the Corps: The Story So Far, Volume 2
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Having become established, the rhododendron stays established because no herbivores will eat it. Apart from the fact that it is a foreign species and its natural enemies do not live in this country, rhododendron has the protection of being poisonous. Toxic chemicals, particularly phenols and diterpenes, occur in significant quantities in the tissues of rhododendron, making the plant unpalatable to herbivores, and phenols are particularly abundant in the young emergent tissues, such as leaves and buds, enabling rhododendron to become established more easily.

Young emergent leaf buds also produce a sticky exudate, containing phenols, which means that smaller, insect herbivores would get stuck, if not put off by the unpleasant flavour. Even honey produced by bees which have visited rhododendron is poisonous, causing what is known as mad honey disease, which causes intestinal and cardiac problems, though is rarely fatal. The plant is also not touched by larger grazing mammals because of the toughness and unpalatability of the leaves, but sheep or cattle which eat rhododendron because of lack of experience or extreme hunger then become poisoned by the chemicals in the leaves.

Rhododendron also has an allelopathic property, rhododendron inhibiting the establishment and germination of other species. Because of this, and because of the severe shading caused by the evergreen nature of the plant, few native species survive once rhododendron has entered an area. Once the native plants are gone, another problem arises in that the animals which rely on the native plants, either directly or indirectly, cannot survive.

Even where trees survive above the rhododendron, species such as woodland butterflies disappear because the caterpillars of woodland butterflies need the wild flowers and grasses which normally exist in woodland rides. Also, if there are no herbivores eating the rhododendron, there are no predators to eat the herbivores, so species such as song birds cease to live in these areas, which also means that there are no sparrowhawks and other such species.

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