Seasonal Cycles of Pentatomoidea 575
also see Chapter 11). Correspondingly, the physiological state of the adults in the summer generation
and the possibility of diapause formation are determined by the timing of the final molt. If adults emerge
when day length is below the critical photoperiod of diapause induction, they all enter diapause. In field
experiments carried out in the forest-steppe zone, nymphs that started development on different dates
always molted to adults no earlier than mid-July, when the day length was already below the critical
photoperiod of that particular population. Therefore, we could not obtain nondiapause adult bugs in the
same vegetative season because they all formed facultative winter adult diapause. Attempts to prevent
the onset of diapause were successful in only one experimental series, when nymphal development was
advanced (i.e., artificially shifted towards earlier dates) as compared with the natural phenology of the
local population. In that case, the nymphs reared under quasi-natural conditions molted into adults
much earlier than usually, already at the end of June. As a result, 37% of the females were nondiapaus-
ing and soon started oviposition. However, this scenario never is realized in the field. According to
our observations, even in the warmest years, the first adults of A. custos appear in Belgorod Province
no earlier than the end of July, when reproduction is already photoperiodically suppressed. Therefore,
univoltinism of A. custos under the forest-steppe conditions is ensured not by the invariable, genetically
determined onset of diapause but by the specific traits of its PhPR of diapause induction, namely its high
temperature optimum and the position of the day-length sensitive stage.
The ecological reasons for reduction of the number of annual generations of Arma custos can be only
hypothesized. On one hand, this reduction is certainly related to the temperature; the temperature con-
ditions of the forest-steppe zone permit reliable development of only one generation of this species per
year. Although the sum of effective temperatures (SET) in the region greatly exceeds the value needed
for completion of one generation, it is not sufficient for successful development of two generations. On
the other hand, this predatory bug feeds on the larvae of leaf beetles, most of which produce only one
generation, at least in the forest-steppe zone where our research was carried out. It may be that the com-
bined action of these two factors (i.e., temperature and availability of food) ultimately determine a shift
of A. custos to the univoltine cycle in the region studied.
Another species of this small genus, Arma chinensis (Fallou), occurs in southeast Asia, has a mul-
tivoltine seasonal cycle, and produces two generations per year in Harbin (China; 45.5°N) and three
generations per year in more southern regions (Zou et al. 2012).
Among phytophagous pentatomoids with potentially multivoltine cycles, a similar transition from
multi- or bivoltinism to univoltinism in response to food quality and local SET values that are insufficient
for two complete generations can be observed in the well-studied Italian striped bug, Graphosoma
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Photoperiod, hIncidence of diapause,%12FIGURE 12.6 The photoperiodic response of adult diapause induction in the predatory pentatomid Arma custos from
Belgorod Province, Russia (50°N). Nymphs were reared to adults and then maintained at temperature of 29–30°C ( 1 ) and
24 –25°C ( 2 ) under constant photoperiodic conditions (indicated under the horizontal axis). (From T. A. Volkovich and
A. Kh. Saulich, Entomological Review 74: 151–162, 1995, with permission.)