Pesticides in the forest environment. I. Use of 2,4,5-T for control of woody weeds in Pinus radiata plantations in Victoria.  D. W. Flinn and P. Hopmans.  February 1978.  8 pp. (unpubl.)

SUMMARY

This report is based on a paper published in Forestry Technical Papers No 26 dealing with the use of 2,4,5-T (2,4,5 - trichlorophenoxyacetic acid) for control of woody weeds (mainly Acacia dealbata) in Pinus radiata in Victoria. Emphasis is given to methods of application and the chemistry, mode of action, toxicity and degradation of 2,4,5-T in an attempt to clarify many of the misconceptions associated with the use of this pesticide.

The application method is dictated by the density and size of the weeds. Basal bark spraying and stem injection are used where weed density is low and an aerial application at 1.1 kg/ha 2,4,5-T where weeds are denser.

Laboratory tests have shown that pure 2,4,5-T is only moderately toxic, but a contaminant TCDD (2,3,7,8 – tetrachlorodibenzo - p- dioxin) formed during synthesis of 2,4,5-T is very toxic and has foetus-deforming properties. It is this contaminant that is largely responsible for the controversy surrounding 2,4,5-T. All 2,4,5-T sold or used in Victoria must comply with the range of requirements prescribed in the Pesticides Act 1958 including a limitation on the TCDD content. Studies have demonstrated that 2,4,5-T is degraded in the forest environment, principally by photodecomposition and soil microorganisms, to harmless end products. Less is known about the breakdown of TCDD.

The Forests Commission, Victoria enforces rigid prescriptions to minimise application of 2,4,5-T to non-target areas, and work has shown that 2,4,5-T levels in streams draining sprayed areas are well below the upper limits set by the National Health and Medical Research Council for potable water. Allowing for maximum exposure levels following the use of 2,4,5-T in the forest environment and the conservative safety margins required when extrapolating results from laboratory experiments with animals to safe doses for humans, it was concluded that the use of this pesticide in Victorian forests does not constitute a hazard to either humans or the environment.

Also published:

Flinn, D.W. and Hopmans, P. (1977)  Pesticides in the forest environment. I. Use of 2,4,5-T for control of woody weeds in Pinus radiata plantations in Victoria.  For. Comm. Vic., For. Tech. Pap. 26: 5-10.