Cockroaches are one of the most commonly used laboratory insects. However, few attempts have been made to describe their life-cycle strategy. In this study, the environmental effects on nymphal development and adult reproduction were investigated to understand the seasonal life-cycle strategy and its control mechanism of one forest-dwelling population of a nymphal-overwintering cockroach, Blattella germanica, in Zhuzhou, Hunan (27°N).
Newly-hatched nymphs were reared at constant photoperiods LD 16∶8, LD 14∶10 or LD 12∶12 h and 25℃. Nymphal development was the fastest at LD 16∶8 h, followed by LD 14∶10 h, and the slowest at LD 12∶12 h. Similar results were obtained at 30℃ and 20℃. When nymphs were exposed to LD 16∶8 h during the first 30 days and then transferred to LD 12∶12 h at 25℃, adults emerged later than those reared continuously at LD 16∶8 h and LD 12∶12 h. At constant LD 16∶8 h and LD 12∶12 h the mean nymphal durations were (88.7±15.8) (mean±SD) and (195.1±53.6) days respectively, whereas those transferred from LD 16∶8h to LD 12∶12 h adult emergence took 240.3±29.3 days. However, in the reverse transfer, i.e. from LD 12∶12 to LD 16∶8 h, made 30 days after hatching, the duration of nymphal development was dramatically shortened. Nymphs matured almost simultaneously and developed faster than those kept at constant LD 12∶12 h, the mean durations of nymphal development being 86.1±7.5 days. These results indicate that nymphal development was markedly influenced by the photoperiod and the nymph of this species has a winter diapause that is induced and maintained by short days and terminated by long days.
Both photoperiod and temperature influenced the number of nymphal instars. The number of nymphal instars were 7, 8 and 9 at LD 16∶8h, 14∶10h and 12∶12h under 25℃ and were 7, 7 and 9 at 30℃, 25℃ and 20℃ under long days. These results indicate that this cockroach species varies the number of nymphal instars in response to photoperiod and temperature.
To examine how temperature would influence adult reproductive of this cockroach species, adults obtained at 25℃ were exposed to 30℃, 25℃ and 20℃ in long days. The rate of surviving ootheca were 14.2%, 90.1% and 51.9% and the egg numbers per ootheca were (2.7±6.1), (35.8±7.8) and (20.5±16.1), at 30℃, 25℃ and 20℃,respectively.
To determine the overwintering stages and the time of adult emergence in the field, head widths of nymphs and adults collected from March through December were measured. Insects collected from September through next March showed a unimodal pattern in head width distribution and were all nymphs, whereas those collected in May and July contained nymphs and adults, indicating that they overwinter as nymphs and adults emerge in late Spring in Zhuzhou, Hunan.
Based on these observations and field-census data, it was suggested that this cockroach has a univoltine life-cycle overwintering as diapause nymphs, and that the photoperiod and temperature response controlling nymphal development and the number of nymphal instars might have evolved to adjust the timing of adult emergence and reproduction to the favorable season.