Seasonality and Life Histories

In trapping studies of cockroaches it is usually unknown if the failure to collect a particular species is due to the absence of the taxon in the habitat, the absence of the targeted life stage, or the current inactivity of the targeted life stage. Light traps or windowpane traps, for example, will collect only adult stages of volant cockroaches during the active part of their diurnal and seasonal cycle; taxa absent from these traps may be plentiful as oothecae and juveniles in the leaf litter. It is therefore important to discuss seasonal activity within the framework of a particular taxon's life history strategy (Daan and Tinbergen, 1997). There are complex, multivariate interactions among generation time, the size at maturity, age, lifespan, and growing season length (Fischer and Fiedler, 2002; Clark, 2003). Diapause and quiescence further interact with developmental rates to synchronize lifecycles, determine patterns of voltinism, and regulate seasonal phenology.

In seasonal environments life histories typically balance time constraints, with the synchronization of adult emergence most crucial when nymphal development is extended and adults are relatively short lived (Brown, 1983). Hatching must be timed so that seasonal mortality risks to juveniles are minimized. In P. japonica, for example, first-instar nymphs do not recover following tissue freezing, although mid- to large-size nymphs survive (Tanaka and Tanaka, 1997). The most thoroughly studied lifecycles among temperate cockroaches are those of the genus Ectobius. All three species in Great Britain spend winter in egg stage diapause, and hatch over a limited period in June after 6-7 mon of dormancy (Fig. 3.6). Ectobius panzeri is univoltine, while E. lapponicus and E. pal-lidus have semi-voltine lifecycles. Nymphs and eggs of the latter two species diapause in winter in alternate years, but there is complex intrapopulation variability in both species. At the onset of winter the nymphs move to grass tussocks and assume a characteristic posture: the body is flexed ventrally and the legs and antennae are held close to the body. Nymphs may feed during the winter, but no molting occurs from the end of September until the end of April or beginning of May. Adults are short lived; males die shortly after mating in June, but females live until October (Brown, 1973a, 1973b, 1980,1983). It is also notable that of the three species of Ectobius in Great Britain, the smallest species, E. panzeri (Brown, 1952), is the only one with a univoltine cycle. Ectobius duskei, abundant in the bunch grasses of Asian steppe zones, is also univoltine and endures winters of —30 to — 40°C in the egg stage (Bei-Bienko, 1950, 1969). It is thought that short favorable seasons often lead to compressed life histories such as these, characterized by brief developmental times, high growth rates, and smaller adult sizes (Abrams et al., 1996). A radically different life history, however, is exhibited by temperate cockroaches in the genus Cryptocercus, and by members of the blaberid subfamily Panesthiinae. Nymphs have extended developmental periods and the full length of the growing season is required to complete a reproductive episode in both Cryptocercus and Panes-thia (Rugg and Rose, 1984b). Female C. punctulatus paired with males the previous summer begin exhibiting ovariole and accessory gland activity in April and oviposit in late June and early July. Oothecae hatch in late July and early August, with most neonates reaching the third or fourth instar prior to the onset of winter (Nalepa, 1988a, and pers. obs.). Additional temperate species that have been studied include An. tamerlana in the Turkmenistan desert (3-yr lifecycle in males, 4-6 yr in females) (Kaplin, 1995), and P. japonica, with a 2-yr lifecycle. The first winter is passed as early instar nymphs, the second one as late-instar nymphs (Shindo and Masaki, 1995).

Recently Tanaka and Zhu have been studying the life-cycles of several species of subtropical cockroaches on Hachijo Island in Japan. Margattea satsumana is a uni-voltine species that overwinters as a non-diapause adult. Nymphs undergo a summer diapause, but develop quickly in autumn under short-day photoperiods. The authors suggest that the summer diapause of nymphs is related to a need for timing reproduction during the following spring (Zhu and Tanaka, 2004b). Opisthoplatia orientalis and Symploce japanica on this island are both semi-voltine. The latter has a complex 2-yr lifecycle with three kinds of diapause (Tanaka and Zhu, 2003): a winter diapause in mid-size nymphs, a summer diapause in late-stage nymphs, and a winter diapause in adults. Opistho-platia orientalis is a large (25-40 mm) brachypterous species capable of overwintering successfully in any stage

Fig. 3.6 Lifecycle of three species of Ectobius in Great Britain. After Brown (1973b), with permission from V.K. Brown.

without diapause. The ovoviviparous females spend the winter with several different stages of oocytes and embryos held internally, but the growth of these is suppressed. Most of the eggs and embryos do not survive to partition. As a result female ovarian development is reset in spring; there is a synchronized deposition of nymphs in summer, most of which reach the fifth instar prior to winter (Zhu and Tanaka, 2004a). This somewhat odd strategy may be related to the fact that these cockroaches are at the northern limit of their distribution on Hachijo Island, where they are not endemic.

Was this article helpful?

0 0

Post a comment