The egg stage of Didymuria violescens (Leach) (Phasmatodea : Phasmatidae) inhabiting the Central Highlands of Victoria.  F.G. Neumann.  October 1973.  58 pp. (unpubl.)

SUMMARY

Loss of viability with time, rate of embryogenesis and occurrence of diapause, hatching and parthenogenesis (asexual reproduction), was studied in the laboratory and field in about 19,000 eggs of D. violescens from ash-type forest at Powelltown (Victoria). Losses of eggs from predation were also investigated. Embryogenesis refers to the development of embryos within eggs to the hatchable pharate nymph stage, and diapause implies the temporary arrest of embryogenesis, which can only resume upon the action of specific stimulus such as chilling.

The viability of fertilised eggs declined by a mean 42% and, in those unfertilised, by 66% when incubated in litter of E. regnans regrowth from February, 1971 to April, 1973. High egg mortality was particularly marked in samples laid late during the oviposition period, and occurred principally during the second winter (1972), which was warmer and drier than average. Although site differences within E. regnans regrowth apparently had little effect on loss of viability, the eggs tended to retain slightly higher viability in forest with a more open canopy. This suggested that heavy priority thinning to desiccate the eggs in worst affected stands cannot be recommended.

Embryogenesis to the pharate nymph stage was rapid at near room temperature in most fertilised eggs, implying that first diapause (which arrests growth of the undeveloped embryo) was either short or had not occurred. Very few pharate nymphs actually hatched at room temperature, the others died. Second diapause, arresting growth of advanced embryos, is therefore not generally terminated at room temperature. About 80% of fertilised eggs hatched in spring/early summer within the oviposition year (1971) when incubated at room temperature for 3½ months prior to overwintering in 1939 E. regnans regrowth. Thus, first diapause in these eggs was again either short or absent, and winter chilling had terminated second diapause. The latter, therefore, appears less variable than first diapause, and factors controlling instance and termination of the two diapauses may not be identical.

Field-incubated eggs developed comparatively slowly, and about 88% required at least two winters to hatch. Biennial life cycles predominated in the field, but 1, 3 and longer cycling of generations also occurred. In terms of diapause, this means that growth of most embryos is temporarily arrested during both the early and advanced stages of development.

Whilst first diapause supervened in most field-incubated eggs at Powelltown, its incidence can apparently be reduced considerably, unlike that of second diapause, by warm incubation conditions during the first 3½ months following oviposition. With respect to first diapause, these results contrast with those by Readshaw and Bedford (1971), who regard first diapause as essentially obligatory and genetically determined in eggs from highland populations (including Powelltown) of south-eastern Australia, and that chilling at 7°C (6 months) is required for optimum elimination.

First hatching occurred in low-altitude E. regnans stands (330m) during late October, then peaked in mid-November and subsequently ceased by the end of January. At 730m elevation, hatching commenced about 2 weeks later, peaked during mid-December and was completed by mid-February.

Fertilised eggs remained not only significantly more viable in the field but also gave rise to shorter life cycles. Most virgin eggs appeared to require more than two years to hatch, and hence greater vulnerability to destructive agencies is likely. This, together with evidence of increased fecundity of mixed populations suggested that parthenogenesis is an inefficient means of procreation. Its function is seen primarily as a mechanism of survival for the species in spatially isolated stands.

Losses of densely stocked and unconcealed eggs from predation in 1939 E. regnans regrowth reached 76% within 9¾ months. There was evidence however, that predation was density dependent, and hence unlikely to have much impact at sites where eggs are few.

Also published:

Neumann, F.G. (1978)  Studies on the egg stage of Didymuria violescens (Leach) (Phasmatodea : Phasmatidae) inhabiting mountain ash forest in Victoria.  J. Aust. Entomol. Soc. 17 (3): 217-24.