594 Invasive Stink Bugs and Related Species (Pentatomoidea)
of adults of the second generation in late August and early September. Natural day length at this time of
the year already is shorter than the critical day length for winter adult diapause induction in both species.
In Riptortus pedestris, adults can measure day length and respond accordingly, even if the nymphs
develop under different photoperiodic conditions. Thus, when adults emerge in late August, they respond
to natural day length as short days and, after a comparatively brief period of prediapause feeding under
still warm conditions, they successfully enter facultative winter adult diapause.
In Podisus maculiventris, for diapause to be induced, the short-day signals must be received by
nymphs beginning with the third instar; it is not enough if the short-day signals are perceived only by
adults. Thus, the adults emerging under short-day conditions in late August already are programmed for
nondiapause development because as nymphs, they developed under long-day conditions. Photoperiodic
sensitivity of the adult stage is not sufficient to override the response formed during the nymphal stage,
especially because the period when it is still warm is short. The process of diapause induction in this
species starts early (from the third instar) and takes longer than in Riptortus pedestris: adults would need
a month to properly form diapause. Temperature in late August and in September is definitely low for
prediapause feeding of adults of P. maculiventris and, thus, adults of the second generation emerging in
late August fail to properly form diapause (Musolin 1997).
Seasonal development of Perillus bioculatus, another predatory pentatomid, was studied in field
experiments in the southwest of Slovakia (about 45°N) during five seasons (Jasič 1975). The experiments
were carried out with bugs originally brought from Canada (Ontario Province, 45°N) and maintained
in culture for many years in various European countries. Results showed that under Central European
conditions, this species could complete two or three generations per year, depending on the temperature,
and form facultative winter adult diapause. Because of sufficient cold hardiness (down to –12°C), the
adults can overwinter in shelters under the snow cover.
However, it was not clear whether the synchronizing role of day length during the diapause induction
period was preserved after introduction of the species into Europe. As discussed above and in Chapter
11 , an ecological factor can act as an external cue or as an inductor of a certain physiological state, both
components being equally important. Some examples are known in which the inductor function works
properly (i.e., diapause as a physiological state is formed correctly) but the signal function is impaired
(i.e., the seasonal timing of diapause is wrong; Tyshchenko 1980).
To answer this question, parameters of the PhPR of diapause induction of this population of Perillus
bioculatus were studied in the laboratory (Volkovich et al. 1991a). The critical photoperiod changed from
15 hours 30 minutes to 14 hours 30 minutes as the temperature rose from 24°C to 27°C (Figure 12.22;
lines 1 and 2). Furthermore, the critical photoperiod value decreased noticeably under the thermo-
rhythm conditions: it was approximately 14 hours 30 minutes under natural thermorhythm with ampli-
tude 13.7°C (scotophase) to 26.4°C (photophase) and the mean temperature 19.1°C (Figure 12.22; line 3).
Proceeding from these data, it can be assumed that the critical photoperiod of the PhPR of diapause
induction (14 hours 30 min, considering the natural temperature dynamics) was reached in the study
region in Slovakia by August 20 when the temperature dropped to 19°C and induction of winter adult
diapause started. However, it is known that complete formation of diapause requires not only a single
short-day signal but also accumulation of a so-called packet of photoperiodic information (Goryshin
and Tyshchenko 1972, Tyshchenko 1977), or the required day number (Saunders 1976), which is a
species-specific number of short days required to trigger diapause (also see Chapter 11).
In most species studied, this packet of photoperiodic information consists of 18–20 short days. The
filling of the packet of photoperiodic information usually is accompanied by a long preparation for
diapause. Therefore, winter diapause induction in Perillus bioculatus has to be finished by the end of
September, which indeed was observed in the southwest of Slovakia (Jasič 1975). Thus, the multivoltine
seasonal cycle realized in the relocated populations remains under the control of the same physiological
responses that regulate the seasonal development of the species in its native range.
Extensive programs of acclimation of Perillus bioculatus also were conducted in the Soviet Union in
the 1960–1970s, but the numerous attempts yielded no practical results. The main reason for the failure
of acclimation of this pentatomid in the new territory was a mismatch between the emergence of the
predator and its prey in spring after overwintering, which already had been observed in the experiments
of Jasič (1975). In spring, the overwintered adults emerged earlier than their prey and, being narrow