THE SIGNIFICANCE OF THE LIGHT AND DARK PHASES IN THE PHOTOPERIODIC CONTROL OF DIAPAUSE IN METATETRANYCHUS ULMI KOCH

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Abstract
The daily cycle of illumination is one of several agencies which control the onset of diapause in Metatetranychus ulmi. Both light and dark phases in the cycle are concerned in the determination process.In general, a long light phase tends to suppress and a long dark phase to induce a diapause. In any combination, the path of development is decided by the balance between diapause‐preventing (light phase) and diapause‐inducing (dark phase) stimuli. However, as their effectiveness does not increase linearly with duration, the existing balance changes with the phase duration.The effectiveness of the light phase in suppressing diapause increases most rapidly between 8 and 16 hr.; that of the dark phase rises very sharply between 8 and 12 hr. Longer dark periods of up to several days duration also induce diapause but are no more effective than a 13 hr. phase. The inclusion in the cycle of very long periods of light or darkness may also influence diapause by reducing the number of complementary phases experienced by the mite during the sensitive period of development. M. ulmi is highly insensitive to the interruption of effective light and dark phases by short intervals of darkness or light—a further indication of the slow inception of the light‐and dark‐phase reactions.These findings are discussed in terms of hypothetical mechanism involving cumulative synthesis and removal of some active substance, but the experimental results cannot yet be fully reconciled with a simple hypothesis of this kind.