Distribution and Habitat
Drury's specimen was given to him by Captain May of Hammersmith and believed to have come from China. Cramer believed the specimen came from Chandernagore in Bengal, however, giving rise to the French name "page de Chadernagor". It is now known that Chrysiridia rhipheus is endemic to Madagascar. Thousands of adult moths seasonally migrate between geographically isolated populations of their host plant Omphalea spp. They can be found almost everywhere on the island, except in the south-west and the extreme subdesertic south of the Androy where their host plant is absent. They migrate from the three species in the dry deciduous forest in the west (O. ankaranensis, O. occidentalis and O. palmata) to the eastern rainforest species (O. oppositifolia). The western species are largely in protected areas. The eastern species, on the other hand, is mostly unprotected and dispersed in widely scattered populations threatened by deforestation. Being the only evergreen species, O. oppositifolia is probably crucial for the moth's survival. The Jamaican moth Urania sloanus, from the same subfamily (Uraniinae), most likely became extinct after the loss of one of its host plant species.
The moths migrate in response to changes in the host plants. Chrysiridia larvae defoliate the whole plant, and even eat the flowers and fruit, and thus have a considerable negative impact on the reproduction and survival of seedlings. The plants probably react by changing their nutrient and secondary compound levels, becoming toxic to the larvae and causing high mortality. Omphalea populations that are not damaged by moths for long periods of time have lower toxicity. These factors cause mass increases in local population, followed by sudden crashes. The population crashes might result from increased larval mortality, but are more likely caused by the emigration of the adult moths. Through semiochemicals, the plant may recruit hymenopteran parasitoids as a protection, hence playing a role in the population dynamics of the moth.
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