抄録
1.日本列島におけるカバマダラの1930〜2002年の記録を総括すると,1997年以降,九州以北で飛来地域数と個体数が激増している。2.日本における食餌植物として,ガガイモ科の9属11種があり,とくに栽培・逸出種のトウワタ,フウセントウワタ,野生種のガガイモが重要である。3.1997・1998年の激増の経過は次のように考えられる。(a)馬毛島が無人島化して,シカによる植生の荒廃がすすみ,毒草トウワタが増加,カバマダラが大発生した。(b)異常気象か,6〜7月に南西風の連続的吹き込みがあって,これが本種の移動を助けた。(c)到着地ではフウセントウワタの栽培が増加しており,各地で発生した。4.南九州の温暖地では,以前から1〜2月の幼虫や春の成虫が発見されていたが,今回も非休眠の幼虫が越冬に成功し,3〜4月に羽化することが確認された。5.2002年,春の越冬世代成虫が次世代を残し,夏世代につながることが,鹿児島市の屋外飼育とマーキング法で確かめられた。6.鹿児島市における成虫の発生は年に4〜8回程度で,夏季には幼生期が20日前後まで短縮され,早い速度で成長する。7.少数が越冬して定着の様相を見せる地域は鹿児島県本土の海岸付近から日南海岸あたりで,その原因は次のように推定される。(a)豊富な食草で,秋に多数の個体が発生し,いろいろなステージの多くの個体が越冬に入った。(b)温暖な地にある食草で,若干の中齢幼虫が冬の低温に耐えて生き残り春に羽化した。(c)越冬世代成虫は,個体数は少なく生息密度も小さかったが,交尾,産卵する能力を持っていた。(d)これらを支えたのは人為的な食草の増加で,それによりカバマダラの個体数が激増し,幸運な幼虫が越冬し,世代をつないだことになる。8.今後日本列島を北上するか否かは,人による食草の提供次第と思われる。9.越冬については,連続した低温が負の要因になっている可能性がある。10.地球温暖化と前世紀後半の漸増傾向との関わりは,今後の詳細な調査を待ちたい。
1. Factors likely to have affected the marked increase in 1997 and 1998 (1) The air current which helped the northward flight Every year many stray or migrant butterflies fly to Japan from the south thanks to the seasonal southwesterly wind and typhoons. In the years in question in particular, the seasonal southwesterly wind blew continuously for a long duration from the end of June to mid July in 1997, and also to early July in 1998 (Table 4). These meteorological conditions are likely to be unusual cases. (2) Spread of a foodplant When A. chrysippus increased remarkably in 1997 and 1998, many researchers noticed that Gomphocarpus fruticosus, one of its larval foodplants, had increased widely in Honshu, Shikoku, and Kyushu. This observation is quantitatively supported by the increased sales statistics of its seeds. Sakata, a leading dealer of plant seeds in Japan, listed this milkweed in its seed catalog in June 1991. The sales grew notably from 1995-1996; Sakata sold 4 liters in 2000, and 10 liters in 2001 (55,000 seeds per liter). Probably the total amount of seeds sold all over Japan, including those sold by other distributors, would have reached twice the above amount. (3) A highly probable departure site formed Departure sites of A chrysippus arriving in Kyushu and further north are presumed to be wide areas including the Ryukyus, Taiwan, the Philippines, southeastern China, the Marianas, etc. Geographically the Ryukyus and Taiwan are their most probable homeland. However, there is no information that A. chrysippus remarkably increased around 1997-1998 in such areas, except for Magejima Island. Magejima, located 12 km west of Tanegashima in the northern Nansei Islands, is a small island 14.3 km in circumference, 7.4 square km in area, and 71.1 m in maximum height. In 1980 all the inhabitants moved away from the island. Drought in 1984 induced a bushfire in 1985, followed by a locust breakout in 1986. After that, the population of deer, endemic Cervus nippon mageshimae, increased excessively, resulting in heavy destruction of the vegetation, but leaving the unpalatable milkweeds intact. In 1993, some researchers reported abundance of the foodplant A curassavica in the abandoned school ground, and proliferation of the butterfly Anosia chrysippus in great numbers there. From 1997 to 2000, some other visitors confirmed the same situations of the butterfly and its foodplant. The outbreak of A chrysippus there is likely due to the luxuriant growth of its foodplant milkweed, which escaped the deer's grazing, owing to its poisonous substances. Magejima could be an ideal departure site for A chrysippus's northward flight (Fig. 1). This probably explains why the recent collection sites of A chrysippus have been scattered in southern Kyushu and along the east coasts of Kyushu, Shikoku, and Honshu. In short, the notable increase of A chrysippus in 1997 and 1998 in Kyushu and further north may have been induced from the following series of events: 1) Emigration of the inhabitants from Magejima → Deer population growth → Destruction of vegetation → Growth of the foodplant Asclepias curassavica → Outbreak of the butterfly Anosia chrysippus. 2) Suitable location of the island for northward departure → Continuous current of the seasonal southeasterly wind in June and July, facilitating the butterfly's flight. 3) Where they arrived, another foodplant G. fruticosus has become abundant → Remarkable incease of A. chrysippus. 2. Larval foodplants recorded in Japan Larval foodplants recorded in Japan all belong to the Asclepiadaceae. Asclepias curassavica: Commonly planted and escaped wild in warm coastal areas south of central Honshu. A. tuberosa: Rarely planted. Reported from Honshu in 1999. Gomphocarpus fruticosus: Indeciduous shrub native to Africa. Since the 1970s, it is known as a larval foodplant that is preferred as well as A. curassavica in Kyushu. As it is cultivated in gardens and often used as a flowering plant for graves, escapees often grow in cemeteries. Cynanchum amplexicaule: Grows wild on damp ground. A few records as a larval foodplant have been reported from Nagasaki in Kyushu. C. austrokiusianum: Wild vine growing in southern Kyushu. In a stand of this plant mixed with A. curassavica, a few eggs and larvae have been found. Calotropis gigantea: Planted. A few cases of use as a larval foodplant have been reported from Okinawa. Metaplexisjaponica: In north-central Kyushu, Shikoku, and Honshu, this native plant grows in waste land and often used as a larval foodplant during the summer. Oxypetalum caeruleum: Perennial plant native to South America. Two records have been reported in southern Kyushu. Hoya carnosa: In Yakushima an escaped larva was found to feed on young leaves of a potted vine of this species. Most common foodplants used in Japan are the two introduced species of milkweeds A. curassavica and G. fruticosus, and a wild deciduous vine M. japonica. The former two species are used all year round, whereas the latter is used only during the summer. In short, in southern Japan the butterfly largely depends on the exotic milkweeds, whereas in northern Japan both native and exotic plants are used almost equally. 3. Remarkable points in its reproductive life cycle Some adults of the overwintered generation mate and lay eggs in the spring even though their population density becomes very low. Summer generation females perform copulation and oviposition within three days after their emergence. Females can mate several times, and males can mate with different females even three days in a row. Some adults survive for more than one month. The duration of the immature stages gets reduced as the temperature rises: 28 days in April-May, 25 days in June, 21 days in July, and about 17 days in August; in particular, the pupal period changes from 20 days in March-April to 6 days in August. They cycle four to eight generations a year with no diapause in Kagoshima in southern Kyushu. Such a rapid reproductive cycle would have been very effective for their settlement in southern Kyushu. 4. Will A. chrysippus expand its range northward? In nonresidential areas, A. chrysippus usually appears in August or September (Table 3). Although in southern Kyushu, a few adults of the overwintered generation emerge occasionally in the spring from late March to early May, emergence of the successive summer generations have not been reported yet. In 2002 in Kagoshima City, the author confirmed their summer broods, 1st and 2nd generations, by mark-release-and-recapture of adults and breeding of immature stages in outdoor cages. The following events may have been the factors for their success in overwintering and proliferation of subsequent broods. 1) Abundant foodplants produces a great increase of A. chrysippus in the autumn population. 2) Consequently, numerous eggs and larvae of various stages enter the winter with no diapause. 3) The lucky few, mostly middle instar larvae, succeed in passing the coldest season, January and early February. Adults, though small in numbers, emerge in March and April. 4) The adults have very effective reproductive behaviour. Some larvae tolerate the severe low temperature in January and February (sometimes -2℃ to 0℃), although many unlucky ones fail in pupation or emergence, or die because of the coldness. Only the lucky few larvae can moult or pupate successfully, when the temperature rises on the foodplants growing in sunny warm places. They happen to survive the winter. It is unpredictable, however, whether they could continue to expand their range northward with this 'urvival of the lucky ones but not of the fittest' strategy alone. An accidental heavy snow or low temperture in the winter may restrict their northern limit of distribution. Reduction of their foodplants, highly dependent on human activities, may cause a steep decline of their number. The destiny of this butterfly, therefore, relies on the climatic conditions and human activities. Today, various pieces of evidence for the global warming continue to accumulate, and the temperature in Japan is gradually rising. Some species of Japanese butterflies are expanding their ranges northward. However, it is unclear whether their range expansions are cases of predicted biological consequences of the trend of the climatic change or not. The average of the lowest temperature in winter has been gradually rising. However, the increased foodplants and subsequent outbreak of A chrysippus population in its departure sites seem to be more important factors affecting their northward range expansion. Studying how low temperatures affect the butterfly's survival will elucidate the relation between the global warming and A. chrysippus's-northward range expansion in future.
