タマナギンウワバ幼虫の体色変異個体のキャベツ畑における発生状況と発生条件についての検討

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  • Occurrence of Colour Variation in Larvae of <i>Plusia nigrisigna</i> WALKER (Lepidoptera, Noctuidae)
  • タマナギンウワバ ヨウチュウ ノ タイショク ヘンイ コタイ ノ キャベツバタケ ニ オケル ハッセイ ジョウキョウ ト ハッセイ ジョウケン ニ ツイテ ノ ケントウ

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Larvae of Plusia nigrisigna WALKER show a distinct colour variation in late stages of their development. Generally their colouring are uniformly pale-green, but some larvae have a pair of conspicuous black spots around the ocelli. Thoracic legs of some of the latter type are entirely blackened, and on such individuals the cephalic spots are over more conspicuous.<br>In field servey, it was unable to find the larvae with black-spots before the fourth instar. But the larvae with black spots seemed to emerge afterward. Proportion of the individuals with cephalic spots in the population seemed to increase with advancement of larval maturity, because the spotted larvae were more abundant in the fifth (last) instar population than in the fourth. Proportion of larvae with cephalic spots was also variable along with growing stages of the host plant and with the seasons. The spotted larvae were generally more numerous on fully-grown cabbages than on younger ones, and more in autumn than in summer. The highest proportion of the spotted type was observed in autumn. It was noticed that the larvae changed their occupation-site on a plant from the upper leaves to lower ones as they grew, and that there was a relation between the occupation-site and the emergence of the spotted larvae.<br>When the non-spotted larvae of various instars were collected from fields and reared consecutively in a laboratory. The spotted larvae emerged in this population as larvae grew. And the proportion was far larger as compared with the proportion of the spotted larvae in the native field population.<br>The present information indicates that emergence of the spotted larvae is due to an accumulated effect of certain environmental factors. Although the factors responsible for colour variation was not determined yet, even light intensity and air temperature seem to be of importance. Yet, the day-length might not be so responsible.

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