DECAY IN TIMBER WITH SPECIAL REFERENCE TO JARRAH (EUCALYPTUS MARGINATA Sm.)

Populations of the phasmatid Didymuria violescens (Leach) have for 10 years been building up in the mountain eucalypt forests of south-eastern Australia, to such a degree that severe defoliation has been experienced. Fears are held for the survival of stands of certain eucalypt species and for catchment stability in heavily infested areas. In January 1961, the State Electricity Commission of Victoria carried out spraying of 6500 acres of rugged forest country (elevation 2000–4200) ft) in the catchment of its Kiewa Hydro-Electric Scheme in northeastern Victoria, using a Bristol Sycamore helicopter to disperse 5½ of “Malathion” in 3 gallons of light fuel oil per acre. The insecticide droplet size range was 150–250 microns. Spraying was carried out from 50 to 100 ft above tree-tops. Air photographs were used to divide the area into units requiring 7–10 loads (62.5 imperial gallons of spray per load) using easily recognizable topographic or man-made boundaries. A navigator travelled in the aircraft to assist the pilot. For accurate location of each swathe, reliance was placed on the judgment of the air crew. Each flight was observed and plotted by ground observers on specially prepared observation points. The high standard of accuracy of spraying achieved under difficult and rather hazardous conditions is attributed to the skill and application of the air crew.

Spraying ceased when oviposition commenced and the phasmatid egg parasite (Cresmophaga sp., Hymenoptera: Cleptidae) wa due to emerge. Frass drop measurements showed virtually complete eradication of the phasmatid population in an experimental area of 50 acres and observation showed that high mortality occurred over the whole of the treated area. By comparison adjoining unsprayed areas showed extensive defoliation. There was no evidence of mortality amongst wild-life, other than insects.

It is hoped that there will be no need for further spraying in the treated area: if no immigration from surrounding untreated areas occurs the drastically reduced population may be prevented from increasing to plague proportions by the combined action of unfavourable weather and natural enemies.

Improvements in techniques are recommended for future operations; for instance better control of droplet size and reduction in spray losses could considerably reduce the application rate.