ACTIVITY RHYTHMS OF LEPIDOPTEROUS DEFOLIATORS: II. HALISIDOTA ARGENTATA PACK. (ARCTIIDAE), AND NEPYTIA PHANTASMARIA STKR. (GEOMETRIDAE)

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Abstract
The 24-hour activity rhythms of a gregarious arctiid (Halisidota argentata Park.) and a geometrid (Nepytia phantasmaria Stkr.) were recorded automatically at different constant temperatures and in naturally varying light and temperatures. Larval activity on foliage, adult emergence, and adult flight were all associated with sunset; eclosion began in the dark hours, but in Nepytia was particularly associated with sunrise. The rhythms were found to be adapted to the different seasonal environments normally experienced by the insects, and activity was influenced by changes in light and temperature. The rhythm of Halisidota larvae, which are active throughout the British Columbia coastal winter as well as in the autumn and spring, undergoes a temperature-related phase reversal, the larvae becoming diurnal in the coldest periods of the winter, though nocturnal in warmer periods. Larval Nepytia are found only in summer; they are generally more active on warm days and have little resistance to cold. The activity of both species of moths is less influenced by low temperatures near sunset than late in the night. The sunrise peak in Nepytia males, which normally fly during autumn nights, is less influenced by low temperatures than the corresponding peak in Halisidota, which fly on summer nights.

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