The Phasmatid control operation in the Central Highlands of Victoria III. The effects of malathion insecticide on the environment.  F.G. Neumann and J.E. Harris.  February 1972.  20 pp. (unpubl.)

SUMMARY

This report discusses the likely effects of the organophosphorus insecticide malathion on the environment. Particular emphasis is given to consequences of aerial application over forest at rates of ca. 0.5 lb. a.i./acre, as such rates are used in the Forests Commission’s anti-phasmatid programme. The main conclusions are:

1. Hazards to domestic water supply

There is little possibility of significant contamination of domestic water supply, as malathion sprayed at 0.5 lb. a.i./ acre over forest in the Bunyip-Tarago river catchments in 1965 gave no residue in water at a sensitivity of 0.002 ppm.

2. Hazards to soil/litter organisms

Soil and litter inhabiting organisms are unlikely to receive lethal doses from phasmatid control operations, as the maximum quantity of malathion expected to hit open ground (interception by vegetation absent) approximates only 0.3-0.5 mg a.i./ft². These amounts are far too small to be hazardous. Thus, soil nematodes are killed by a minimum of 4g malathion/ft². Further, according to recent reports, organophosphorus insecticides tend to increase (although not dangerously so) certain soil arthropods.

3. Air pollution

Malathion has very low volatility (a saturated atmosphere at 30°C contains 0.5 ppm) and together with its short persistence, should contribute little to prolonged air pollution over the target area.

4. Hazards to terrestrial insects

Malathion is highly toxic to non- target insects e.g. honey bees. However, for various biological reasons, it appears from studies in the U.S.A. and elsewhere, that bee populations are not significantly reduced over a prolonged period where malathion is sprayed at rates of 0.5 lb. a.i./acre, particularly when oil solutions are used. Concentrates (as in U.L.V. sprays) and dusts appear to be more dangerous than dilute oil sprays at equivalent doses, and should not be applied over honey producing areas.

The Central Highlands are not important for honey production and “knockdown” samples have not contained bees or substantial numbers of other pollinating insects. Eucalyptus regnans F. Muell. flowers well outside the recommended spray period (Neumann and Harris, 1972) and hence it is unlikely that pollination is adversely affected.

5. Hazards to fishes

Toxicity to fish is high. However, in the Central Highlands, this is not a serious disadvantage as streams in the target area are generally too shallow to support fish which live further downstream. Before the pesticide reaches these habitats, it is rapidly and extensively diluted until eventual breakdown. There are no reports of increased fish mortality following spraying in the Central Highlands. However, the Forests Commission’s safety prescription of “no spraying within two chain of Britannia Ck.” is a worthwhile precaution.

6. Hazards to mammals, birds and plants

The claim by the makers of malathion that it is an effective insecticide of low mammalian, avian and also of low phytotoxicity has been verified over two decades of both experimental and operational use throughout the world. Nevertheless, occupational surveillance to detect possible chronic toxic effects is highly desirable.

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.