Effects of food plants on the geographic destribution and food habits of phytophagous ladybird beetles.

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The distribution of thistles (Cirsium spp.) and thistle feeding ladybird beetles (Epilachna pustlosa and E.niponica) was surveyed in Oshima Peninsula, southern Hokkaido. E.pustulosa was collected in the morthern part of the peninsula north of the Kariba mountains, occurring on predominant C.kamtschaticum. On the other hand, E.niponica was confined to Ohno Plain and its adjacent areas in the southern part, mainly occuring on C.alpicola. With only one exception, both beetle species did not utilize C.grayanum which was the commonest thistle species all over the peninsula south of the Kariba mountains. Thus there was a zone around the neck of the peninsula that lacked occurrence of thistle feeding ladybird beetles despite there was plenty thistles. Laboratory studies revealed that both beetle species did not prefer C.grayanumand the larvae performed less on that thistle species. Furthermore, E.niponicafemales laid more than 80% of eggs on C.alpicola when they were allowed tochoose between the above three species of thistles in a green house, and the larvae hatched from eggs occasionally laid on C.grayanum did not feed on laeaves of this thistle and dispersed. From these results it was concluded that the distribution of the thistle feedingladybird beetles in Oshima Peninsula was determined by the nature of locally predominant thistle species.