The mutual relation between the coffee-twig beetle, an ambrosia beetle, and the coffee plant was analysed as an example of the relation between insect and host plant. By boring passages in the twigs of the coffee plant, the beetle killed the twigs. Control was hindered by the feeding of the beetle only on a fungus, Ambrosiella xylebori Brader, which was the chief cause of death. Other plants were also infested and acted as a source of infestation for coffee plantations.The relation between beetle and coffee plant was influenced by: a scent stimulus attracting the females to the plant; a scent stimulus which caused them to remain or fly off; a flavouring influenced whether the entrance passage was bored; the completion of the entrance passage depended on the nutrient reserves of the female and on the structure of the plant tissues; the boring of the brood passage depended whether the ambrosia fungus grew and growth of the ambrosia depended on the sugar content of the plant (positive relation); the number of eggs laid depended on the growth conditions for the ambrosia. Selection of less infested varieties of coffee seemed the best means of control.
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