Seasonal Cycles of Pentatomoidea 577
series of 1995). By contrast, in the experimental series of 1996, the dates of the final molt were artificially
shifted (in the same way as in the experiments with Arma custos described just above) towards an early
period in summer during which the natural day length exceeded the critical day length of the species. In
this case, up to 70% of the females remained physiologically active and started oviposition. These results
clearly confirm the role of day length in induction of facultative winter diapause in G. lineatum and explain
the univoltine type of its seasonal development in the forest-steppe zone. The immediate cause of the shift
to the univoltine seasonal cycle may be the trophic factor: the developing nymphs and prediapause adults
feed on the seeds of umbellates that ripen in the second half of summer and may not be abundant later in
the season.
Similar results also were obtained in experiments with the phytophagous pentatomid Dybowskyia
reticulata (Dallas) in Japan (Nakamura and Numata 1998). The temperature optimum of the PhPR
of adult diapause induction in this species also was shifted into the high temperature range: adequate
response to day length was observed only at 27.5°C and higher temperatures, whereas a decrease in
temperature even to 25°C induced diapause in all adults under long- and short-day conditions. In Osaka
(Japan, 34.7°N), D. reticulata completes only one generation in cold years and two generations in
warmer ones. The relatively low summer temperature (25°C and lower) “switches off” the physiological
mechanism of response to day length, so that all the adults enter diapause regardless of the dates of their
emergence. The need to limit the number of generations likely is related to the fact that D. reticulata is
a narrow oligophage feeding on the seeds of umbellates that are available briefly in summer and shatter
by the beginning of September.
Interesting results were obtained in experiments with another pentatomid Graphosoma rubrolinea-
tum (Westwood) (Nakamura and Numata 1999), which also mostly feeds on the seeds of umbellates.
Two populations of this species were studied in Japan: the northern (Hokkaido Island, 44.2°N) and the
southern one (Osaka, 34.7°N). Individuals of both populations revealed a common and strong tendency
to enter diapause and, as a result, their PhPRs of diapause induction look similar (Figure 12.8).
The difference between the populations was manifested in the action of high temperatures on the
parameters of the PhPR of diapause induction: in the northern population, the fraction of reproduc-
tive females increased as the temperature grew under the long-day conditions, but no such effect was
observed in the southern population. The strong influence of high temperatures on the percentage of
reproductive individuals may lead to the production of a partial second generation in particularly warm
years in the north of the species’ range (see below), whereas in more southern regions all the first-
generation adults enter diapause. It is believed that such an inversion of voltinism may be related to the
availability of food. According to observations, in the Osaka region, seeds of umbellates ripen by the
middle of summer and shatter by the beginning of autumn; whereas on Hokkaido Island, they remain
available until late autumn so that the second-generation nymphs would not experience shortage of food.
10050Incidence of diapause,%0
12 13 14Males
FemalesMales
Females15 16 17 18
Photoperiod, hA
10050Incidence of diapause,%0
12 13 14 15 16 17 18
Photoperiod, hBFIGURE 12.8 The photoperiodic response of facultative winter adult diapause induction in the pentatomid Graphosoma
rubrolineatum. A, Population from Hokkaido Island, Japan (44.2°N); B, Population from Osaka, Japan (34.7°N). Nymphs
were reared to adults and then maintained at 25°C under constant photoperiodic conditions (indicated under the hori-
zontal axis). (Modified from K. Nakamura and H. Numata, Applied Entomology and Zoology 34: 323–326, 1999, with
permission.)