The instars and their comparative destructiveness of the spur-legged phasmatid Didymuria violescens (Leach) (Phasmatodea : Phasmatidae) from ash-type forests of the Victorian Central Highlands.  F.G. Neumann.  August 1972.  41 pp. (unpubl.)

SUMMARY

The objectives were to determine the number of growth stages (instars) of the phasmatid Didymuria violescens (Leach) from ash-type forest of the central highlands of Victoria, to provide data for their identification and of their occurrence in the field, and to compare their destructiveness.

Didymuria violescens was found to have six instars (5 moults). A key, distinguishing between instars and between the sexes were constructed. Also, a simple method, based on size classification of frass pellets, was devised for the identification of “invisible” stages of the insect high in tree crowns.

The sex-ratio (percent males in total sample) in field populations approximated 50% during the summers 1968/69, 1969/70 and 1970/71, implying that sexual reproduction had occurred rather than parthenogenesis, which produces only females. Sexual reproduction favours increases in phasmatid populations, as significantly more progeny hatch from fertilized eggs than from those unfertilized. During summer 1970/71, at elevations below 2000 ft., instar I was predominant throughout November and December. Older instars became increasingly more abundant during December and early-January so that instars III and IV were most frequent by mid-January. The adults first appeared below 2000 ft. during late January and two weeks later at higher elevations. Similar observations were made during summer 1968/69.

In the series of experiments at constant 20°C day and 15°C night temperature, the consumption of eucalypt leaves by nymphs and adults were studied. Results from preliminary work allowed the adoption of frass rate (dry weight of frass defecated per insect per unit time) as the measure of food intake for the adults. Similarly, frass rate was also assumed appropriate as an index of food intake for the nymphs.

It was shown that frass rate increased with each successive instar, but most significantly beyond third. Prior to the fifth instar, the frass rate did not differ between the sexes. For the fifth instar and adult stages, however, females had significantly higher frass rates than males, indicating increased food consumption likely to be associated with ovulation. The older instars preferred Eucalyptus regnans F. Muell to both Eucalyptus nitens Maiden and Eucalyptus cypellocarpa L. H. S. Johnson.

The destructive potential of second instar nymphs was compared with that of adults by measuring rates of leaf areas of E. regnans ingested or exercised. Ovipositing females were most destructive, causing the loss of more than twice the leaf area than that due to adult males and nearly 20x that of second instar nymphs.

The results show clearly that the adults, and particularly ovipositing females, pose the greatest threat to ash-type forests. Thus, aerial insecticidal control should destroy the phasmatid populations before maturity. Present indications are that this can be achieved by completing control by the first week of February at the latest. Future seasonal variations in the occurrence of feeding stages may necessitate a revision of this deadline.

Also published:

Neuman, F.G. (1974)  Autecology and control of Didymuria violescens (Leach) (Phasmatodea : Phasmatidae) inhabiting the Central Highlands of Victoria.  M.Sc.Thesis, Univ. of N.S.W., 230 pp.