自然教育園で大発生したキアシドクガ(鱗翅目,ドクガ科)成虫の小型化について

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Ivela auripes is a well known lymantriid moth, sometimes defoliating Swida controversa and Cornus brachypoda (Cornuceae) by outbreak. In the urban area of Tokyo, this moth was not recorded at the end of the 20th Century (Owada et al., 2000), and was discovered in 2001 at the Institute for Nature Study (INS) (Owada et al., 2001). Occurrences of small colonies of I. auripes were also recorded from such large green tracks as Akaksaka Imperial Gardens and the Imperial Palace, in Tokyo, at the beginning of the 21st Century (Owada et al., 2005, 2006). In the garden of INS, defoliation of Swida controversa by I. auripes was not so serious in 2001-2003 (Yano & Kuwahara, 2006). Since 2004, heavy defoliation by I. auripes had been observed there, and many larvae roamed about the ground, fences and walls of buildings nearby INS. In 2005, we tried to collect adult moths as many as possible, and found that very small moths were flying among them. In 2006, the defoliation was advanced more seriously, and 86 trees (ca. 7% of Swida contorversa in the garden) were dead (Yano & Kuwahara, 2007). Moths began to fly on May 24, and the last moths were collected on June 7, 2006 (Table 1). As in Figure 1, miniaturization of INS I. auripes is obvious in comparison with moths from the Imperial Palace, Tokyo, where the defoliation of S. controversa and C. brachypoda was not observed. Mean forewing length and SD of I. auripes in the Imperial Palace 2005, INS 2005 and INS 2006 are shown in Table 2. Length of forewings of Imperial Palace 2005 was significantly longer than those of INS 2005 both in males and females. Variance of forewing lengths was significantly larger in the population of INS 2005 than in that of Imperial Palace 2005. In comparing INS 2005 with INS 2006, miniaturization was progressed and variance was reduced in males in 2006. On the other hand, such a significant miniaturization was not found in females in INS 2006. In the control population of the Imperial Palace, the sizes of males and females were mostly not overlapped each other, while in the population of INS 2005, they were not separated. On the contrary, the forewing length of males was significantly shorter than that of females in INS 2006. The garden of INS is ca. 20 ha, and all the trees of Swida controversa (10 cm over in diameter at breast height) are recorded. In 2005, 1,269 trees of S. controversa were recognized in the garden. Among them, heavily defoliated trees were 508 (ca. 40%), and the defoliation percentage increased to 60% within 51 cm over in DBH trees (Yano & Kuwahara, 2006). The high variance in the forewing length of moths from INS will be derived from variable larvae, whether they faced starvation or not. Many larvae would be dead before pupation, and some moths would be fully grown without starvation like those in the Imperial Palace, Tokyo. It can be surmised that the minimum size of forewing length of I. auripes is 17 mm in male and 18 mm in female, and that smaller moths would be dead in their larval stages. Mean length of control population of the Imperial Palace is 25.7 mm in male and 29.8 mm in female. The miniaturization progressed to 66.1% in male and 60.4% in female in INS population, and in converting the percentages into cubic contents, they are calculated 28.8% and 22.1%. Weight of female must be markedly changed before and after oviposition. Nevertheless, mean of the maximum female specimens (n=5; forewing length: 30-31 mm, weight: 78-145 mg) is 112.0 mg, and mean of the minimum female specimens (n=4; forewing length: 18-19 mm, weight: 14-17 mg) is 15.3 mg, 13.7% of the mean maximum weight. The minimum female (14 mg) is 9.7% of the maximum one (145 mg). It is not clear whether such miniature females are able to copulate and lay eggs normally or not. At least, however, small males fly as active as larger males, and sometimes we observed copulations of smaller males. Larger females are not so active for flying, but smaller females are used to be collected in flight with males. It can be surmised that flight activity of smaller females increases in accordance with decrease of weight, though they may lay fewer eggs. This phenomenon is also considered adaptation to the heavy defoliation of the food plants by I. auripes. There are two ways for the survival strategies in I. auripes. Larvae can survive against shortage of food, making very small pupae, and the small adults can leave their offspring. And, in addition to this, the ability of dispersion increases in smaller females, which can migrate from their critical habitat to a new habitat. The Institute for Nature Study is completely surrounded by urban buildings and roads. Therefore, females, which are going to fly outside INS, may return to INS, and have to lay eggs to trees in INS. During three years, 2002-2005, 100 trees (7.3%) within 1,369 individuals of W. controversa were dead, and even in one year, 2005-2006, 86 trees (ca. 7%) were dead. In INS, parasitoids and predators may not be able to control the population size of I. auripes. Trees of W. controversa might continue to be dead until this extraordinary outbreak of I. auripes comes to an end.
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自然教育園で大発生したキアシドクガ(鱗翅目,ドクガ科)成虫の小型化について

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