Duration Of Pupal Diapause in the Tobacco Hornworm is Determined By Number of Short Days Received By the Larva

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Abstract
Daylength monitored by the embryos and larvae of the tobacco horn-worm, Manduca sexta, is used to program both the incidence and duration of pupal diapause. Short daylength throughout embryonic and larval development yields a high diapause incidence, but a diapause of short duration. Hornworms transferred from long- to short-day conditions at later stages of larval development enter diapause at a lower rate, but the resulting diapause is of greater duration. The number of short-day cycles the hornworms receive can be varied by partial starvation and the use of different rearing temperatures. Such manipulations consistently support a model in which diapause duration is inversely related to the number of short-day cycles received. Observations with field-reared hornworms are consistent with the model: September hornworms receive more short-day cycles than August hornworms and thus have a shorter diapause. This mechanism apparently functions in synchronizing the initiation of adult development among individuals that pupate at different times. The diapause duration is programmed by the brain and can readily be transferred to other pupae by brain transplantation.