The Phasmatid control operation in the Central Highlands of Victoria II. Malathion as a chemical for control, and alternative insecticides.  F. G. Neumann and J. E. Harris.  January 1972.  26 pp. (unpubl.)

SUMMARY

This report reviews the insecticide malathion. In particular, its properties and toxicology, insect resistance, its persistence and residues are discussed. The legislation pertaining to its use in Victoria is also given.

Malathion (Maldison*) is an organophosphorus contact insecticide, soluble in most organic solvents. It is unstable in alkaline media and hence solutions of soaps and detergents are effective detoxifying agents. Many fungicides e.g. Cu-compounds are incompatible with malathion. Potentiation** or antagonism** are not possible in our current spray program against phasmatids as malathion only is used during a 5 week period per year. Malathion exerts lethal action via three interdependent pathways viz.

1. transport through the insect cuticle to the nervous system during which oxidation to toxic malaoxon occurs;
2. primary toxic action on the nervous system involving inhibition of cholinesterase at synapses; and
3. secondary toxic action resulting in the release of neuro-endocrine secretion causing ataxia, paralysis and death.

The toxicity of malathion appears dependent on a number of variables including type of insect, growth stage and sex, formulation and temperature. Also, both dose and the concentration of application appear to determine the percentage and rate of mortality. Thus, at doses ≥ minimum LD100***, increased dilution raises mortality and speed of toxication. Several insect species have developed resistance to malathion, but in our phasmatid campaign, this is not anticipated at present.

Persistence of malathion appears to be very variable and requires detailed investigation in mountain ash forests. Dangerous residues in food stuffs are unlikely to occur as a result of spraying malathion in the Central Highlands as

1. market gardens, orchards and grazing areas are beyond the range of accidental drift; and
2. current daily dietary intake by humans in the U.S.A. and Europe, where malathion has been extensively used close to populated areas, is far below the internationally sanctioned tolerances, reflecting large scale attenuation and degradation during commercial and domestic food conversion processes.

Malathion appears to fulfil its function as an effective and safe phasmaticide and need not be replaced by other pesticides at present.

*Registered Trade Name.
**Potentiation occurs when two jointly administered pesticides produce a much greater effect than that of the sum of two used independently. Antagonism is the exact opposite.
***Lethal dose in mg/kg required to kill all test insects.

Also published:

Neuman, F.G., Harris, J.A. and Wood, C.H.. (1977)  The phasmatid problem in mountain ash forests of the Central Highlands of Victoria.  For. Comm. Vic., Bull. 25, 43 pp.