Diapause in Pentatomoidea 509
winter adult diapause is most characteristic of the whole Heteroptera (Hertzel 1982; Ruberson et al. 1998;
Saulich and Musolin 2007b, 2012; Esenbekova et al. 2015), although exceptions occur. For example,
most species of plant bugs (Miridae) overwinter in the egg stage (Wheeler 2001).
11.3 Environmental Factors Controlling Induction of Winter Diapause
Ecological factors often have a dual mode of action on living organisms. They determine environmental
conditions under which these organisms live; thus, they have a vital function. At the same time, many
ecological factors might be used as reliable predictors of environmental changes that the ecosystem is
going to face in the future; thus, they have a signal function and act as cues (Tyshchenko 1980). For
instance, a vital mode of action of temperature determines a range within which a particular species can
live, whereas the daily rhythm of temperature (thermorhythm) has a signal function and predicts the
coming seasonal environmental changes.
Several abiotic and biotic factors that have regular rhythmicity in nature can be used by insects as sig-
nals (or cues) for synchronization of their seasonal development with environmental conditions. In many
cases, insects use more than one cue, and, thus, the mode of action of different factors can be complex.
11.3.1 Day Length
Day length has an astronomic preciseness and no environmental factor can affect it. Thus, natural day
length is the most reliable environmental cue available. Many species of insects use day length as a reli-
able cue for structuring their seasonal cycle and synchronizing their seasonal development with local
environmental conditions. Precise seasonal dynamics of day length (and night length) are of critical
importance here, but not the changes in energy or intensity of optical radiation.
In eco-physiological laboratory experiments when day length is set artificially, it is called photope-
riod and determined as a ratio between the duration of the light period (i.e., photophase) and the dark
period (i.e., scotophase). Graphically, it can be shown in the following way: L:D 16:8, meaning 16 hours
of light followed by 8 hours of darkness every day in the laboratory.
Many insect species respond in different ways to day length and such physiological responses are
called photoperiodic responses (PhPR). These responses have been found in many species, including
those in the Pentatomoidea (Table 11.2). In the Northern Hemisphere, for species with winter diapause,
short (or shortening) day length becomes a signal of the approaching autumnal decrease of temperature
and serves as a trigger for a hormonal cascade leading to a pause in active metamorphosis (i.e., diapause).
11.3.1.1 Photoperiodic Response of Diapause Induction
In the Northern Hemisphere, formation of facultative winter diapause is controlled by the PhPR of a
long-day type (i.e., one that allows active development under long-day conditions and induces diapause
under short-day conditions). Under long-day conditions (i.e., simulating early summer), individuals
develop directly (so called active development) and produce the next generation; under short-day condi-
tions (i.e., simulating autum or winter), individuals enter winter diapause.
Critical photoperiod (i.e., critical day length, or photoperiodic threshold), is one of the most
important ecological characteristics of the PhPR. This parameter corresponds to the day length at which
50% of individuals of a particular population enter diapause. Typical PhPR of a long-day type has been
found in many pentatomoid species. Figure 11.2 demonstrates the PhPRs of diapause induction in two
pentatomids, Aelia fieberi and Plautia stali. These insects had been reared and then maintained under
particular constant photoperiodic conditions at 25°C from the day when they hatched from eggs. All
females that experienced long-day conditions (i.e., with photophases 15 or 16 hours) became reproductive,
whereas most females reared and then maintained under short-day conditions (scotophase of 12 hours
or more) entered adult diapause. Thus, at 25°C, the critical photoperiod was approximately 13.5 hours
for P. stali and about 14.5 hours for A. fieberi (Numata and Nakamura 2002). These examples clearly