The case of the toxic moths

In 2001 scientists from the New South Wales National Parks and Wildlife Service (now Department of Environment and Climate Change), La Trobe University and an environmental consulting firm investigated a mysterious death of grass outside a cave in Australia's Snowy Mountains. Heavy rains had washed dead bogong moths (Agrotis infusa) from the cave and grass touched by this outwash died. Investigations revealed that arsenic concentrated in the dead bogong moths poisoned the grass. Worryingly, arsenic occurred in the bodies or the droppings of three mammal species eating bogong moths, so arsenic contamination was spreading in the food chain.

The arsenic came from the plains of Queensland and western New South Wales, where bogong moths breed in autumn (Figure 27.1). The grubs, called cutworm caterpillars, eat grasses and crops before pupating in the soil to develop into winged adult moths. Arsenic-based insecticides were used intensively in the early 20th century and some are still available, so cutworm caterpillars probably absorb arsenic when eating crops. The adults migrate to the Snowy Mountains, aestivating during summer in caves until cooler weather when they return to the plains (Figure 27.2).

The bogong migration is amazing and inspirational, part of the natural legacy bequeathed by the Australian environment to its human occupants. By contrast, the moths' residual arsenic toxicity is disturbing, showing how human intervention may squander or spoil rich natural assets.