Mortality Factors and Mortality Process of Colonial Stages of the Mulberry Tiger Moth, <i>Spilarctia imparilis</i> BUTLER (Lepidoptera: Arctiidae)

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  • クワゴマダラヒトリの集合生活期における死亡要因と死亡過程
  • クワゴマダラヒトリ ノ シュウゴウ セイカツキ ニ オケル シボウ ヨウイン

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The larvae of Spilarctia imparilis hatched from an egg mass in fall formed a communal nestweb and persisted in dense aggregation until hibernation. Effects of the aggregation size on the mortality process of S. imparilis were studied at Nanseicho, Mie Prefecture in 1977 and 1978. The predation by ants was found to be the major mortality factor for the eggs and first-instar larvae. Eggs tended to die as a unit of egg mass and there was no consistent relationship between the rate of eggs killed by predation and the egg mass size, possibly due to the ants attacking egg-masses continuously. The egg-masses tended to form clusters in each tree. There was a negative correlation between the number of egg masses per tree and the rate of egg masses killed by predation. Moreover, the larger the number of hatchlings from an egg mass the higher the survival rate of first- and second-instar larvae. Smaller aggregations tended to be destroyed completely during the early larval stages. The protective role of S. imparilis groups against the predation pressure is discussed and it is concluded that concentrated deposition of egg masses on host plants should have a protective role against the strong predation pressure during the early stages of development of S. imparilis in the urban areas of the southern coast of Japan.

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