Although some parts of diapause development have been clarified up by endocrinologists, knowledge of the underlying processes remains insufficient. The survey of ecophysiological aspects of diapause development has thus to be limited to inputs and outputs from the blackbox. The terms diapause development, diapause intensity, post-diapause quiescence, horotelic processes of diapause, and tachytelic processes of diapause (reactivation) are defined. Andrewartha's term diapause development has been accepted because it shows diapause as a dynamic event.
In about the last 20 years, some views on diapause development have been updated, while others have fossilised. The assumption that chilling is a general prerequisite for completion of diapause development in all insects still survives in part of the scientific community, in spite of much contradictory evidence and often due to inadequate interpretation of experiments (examples given in figures and tables). On the contrary, it has been generally recognised that in temperate climates overwintering diapause is usually already completed in early/mid winter and the dormancy is then temperature quiescence. The conception of multiple pathways of diapause completion postulates that diapause can be completed either by the normal (slow) progress of diapause development (horotelic processes), or by a faster activation (tachytelic processes). There are important differences between the mechanisms regulating activation and the horotelic processes. Thus, e.g., the photoperiodic response is lost during horotelic completion, while after photoperiodic activation it persists. In addition to photoperiodic activation other kinds of activation are being studied, particularly activation by high temperature.
Some conclusions can be made from modern studies on diapause development. In every individual, several (at least two) possible pathways exist that are evidently interlinked and mutually complementary. Thus the time of diapause passed at any condition has to be considered, as well as the exposure to very low temperatures. In temperature studies the experimental range ought to be adequately wide and less affected by a priori assumptions. Stimulation by temperature increase or improvement in food or other conditions has to be considered.
The success of diapause completion should be measured by at least four parameters: (1) incidence (%) of developmental steps; (2) duration of delay; (3) synchronization; (4) vigour of post-diapause insects (shown by long-term survival or fecundity). The last two parameters have usually been neglected.
Fruit of two almond, Prunus amygdalus Linnaeus, cultivars (Retsou and Truoito) containing diapausing larvae of Eurytoma amygdali Enderlein, were collected in early August from coastal areas in northern Greece. Some larvae were removed from the fruit and maintained singly in open plastic vials and others left in the fruit until the end of the low-temperature period. They were kept at a low temperature of 10°C from the beginning, or after 8 weeks at 20°C. The larvae were subsequently maintained at 20°C and whether they completed the two diapause stages was recorded for 60 more weeks. When the larvae in vials, were kept initially for 8 weeks at 20°C, most of those from Retsou and all of those Truoito almonds completed the first stage of diapause. Of the larvae in the fruits, most of those in Truoito but less than 50% of those in Retsou almonds completed the first stage of diapause after 8 weeks at 20°C. Larvae from different orchards and different almond cultivars differed in diapause intensity. When the larvae were kept at a low temperature of 10°C from the beginning for 4, 8 or 16 weeks and then at 20°C they completed the second diapause stage synchronously, but the time of completion was delayed, and depended on the duration of the low temperature treatment. In several cases the time to diapause completion was bimodally distributed and the relative size of peak depended on the duration of the early exposure to low temperature.
While observational studies led to the assumption that water or moisture (W/M) is a prerequisite for diapause development, the experimental research indicates rather the opposite: usually W/M is needed as late as for the post-diapause resumption of morphogenesis. Recent examples for this type of regulation of dormancy are given: Eggs of the tettigoniid Stictophaula armata, eggs of the grasshopper Oedaleus senegalensis, adults of the bruchid Bruchidius atrolineatus, adults of the endomychid Stenotarsus subtilis (= S. rotundus). In the late diapause of the noctuid Busseola fusca and in eggs of the chrysomelid Homichloda barkeri, moisture is assumed to be the diapause terminating cue. Fall in temperature is assumed decisive for termination of pupal diapause in the saturniid Schausiella santarosensis, although the effect of intense rain after a long dry period has not yet been excluded. Effects of intense changes in environmental conditions and of gradual decrease in diapause intensity with time have often been neglected.