Abstract
Environmental factors that regulate egg diapause in the walking-stick insect, Ramulus irregulariterdentatus, were studied. Insects collected in fields at Okayama, Japan soon after hatching in spring were reared under constant laboratory conditions. After adult emergence, their eggs were placed outdoors or incubated under constant laboratory conditions. Most eggs kept at 15 or 20°C hatched more than 100 days after oviposition. At 25°C, however, many eggs remained in diapause throughout the experimental period although a small number hatched slightly earlier than those kept at the lower temperature. The maternal photoperiod affected the egg period at 20 and 25°C; eggs from females reared under short day conditions hatched significantly earlier. A low temperature of 10°C shortens diapause development, whereas 5°C did not have a clear effect on time to hatching. When eggs of females reared under long day conditions were placed outdoors from June to early July, most hatched the next spring. On the other hand, eggs of females reared under short day conditions hatched before winter if they were laid before mid-July. The short day eggs laid in August and September hatched successfully the following April, and there was a positive correlation between the date of oviposition and time of hatching. It is concluded that low temperatures in mid-winter do not terminate diapause development. The results revealed that both maternal regulation of diapause intensity and the rate of diapause development play an important role in maintaining a monovoltine life cycle in Ramulus irregulariterdentatus.
Highlights
Diapause is a dynamic process, which consists of various phases, each with different prerequisites
To determine whether the mothers control the intensity of egg diapause, insects were reared under a long day (16L : 8D) or short day (12L : 12D) photoperiod at 25 ± 1°C in a group of about 10 individuals in a plastic container (35 × 30 × 38 cm in height)
In insects in which the mother can determine the intensity of diapause of her offspring it is more intense and occurs at a higher incidence if mothers are exposed to short photoperiods
Summary
Diapause is a dynamic process, which consists of various phases, each with different prerequisites. Diapause intensity is an important factor in determining the duration of diapause. It is variable among different species and within a species (Masaki, 1999, 2002). A positive correlation between diapause intensity and photoperiod is recorded for pupae of Manduca sexta (Bell et al, 1975). The latter type of photoperiodic response may be adaptive as it prevents diapausing insects from resuming post diapause morphogenesis before winter when diapause is induced early (Masaki, 2002)
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