Extensive research in the early 1980s demonstrated that the dock moth is harmless to all crops and can only attack dock plants. Eggs and larvae of the moth have been released onto more than 170 000 dock plants at over 150 sites throughout the south-west of Western Australia. A national program, jointly funded by the (then) Department of Agriculture Western Australia and the Meat Research Corporation led to the establishment of the insect in South Australia, New South Wales and Victoria and increased releases in Western Australia.
The moth is wasp-like in appearance with narrow clear wings and bright colours. The female is about 15mm long with a stout dark brown-black body with distinctive orange, black and white bands across the abdomen, bright orange legs and orange and black antennae.
The smaller male (about 12mm) has a slender yellow-brown body with black and white bands, yellow legs and black antennae. The male also has a fan-like tuft of scales at the end of the abdomen.
When at rest, the wings are held horizontally at about 35 degrees to the body, such that the insect resembles an arrowhead when viewed from above. The moth can be found on mature dock flowering stems during spring and summer.
The larva feeds inside the dock root and can reach 25mm in length when fully grown. It is creamy white in colour, with a glossy skin and a shiny pale brown head capsule. It is never seen above ground.
The clearwing moth has a one year life cycle, most of which is spent as a larva in the root of the dock plant. The adults are active during spring and summer but live for only a few days, when they mate and lay eggs. The females lay about 300 tiny dark brown eggs, which they attach to dock flowering stems. The larvae hatch within a few days and make their way down the stem. They tunnel into the dock root just below the soil surface and begin to feed.
The young larva initially feeds just below the surface of the root, producing narrow tunnels near the crown of the plant. As the larva grows, it tunnels extensively in the centre of the root, eating away the root tissue and replacing it with its droppings. From the outside, the only indication of the larva's presence is some sawdust-like material which may be extruded from the base of the stem. At the end of summer, the fully grown larva spins a thick silken cocoon around itself and rests inside the root until the following spring.
As the temperature rises, the larva chews a circular exit hole in the crown of the plant or the base of the stem and pupates down in the larval tunnel in the root. After about two weeks, the pupa pushes through the exit hole and the moth emerges from the partially exposed pupal case. Moths usually emerge in the morning, leaving behind the empty pupal cases protruding from the crown of the plant.
Effect on dock plants
Dock plants are destroyed during late spring and summer, because the larvae feed inside the root while the plant is dormant. By the time the larvae stop feeding at the end of summer, much of the root tissue has been consumed and the plant is less able to regenerate in autumn.
The feeding of the dock moth larvae can kill infested dock plants outright. However, the insect's long life cycle means that the natural build-up of population numbers in the field and their spread from release sites will be relatively slow.
In the long term, a natural balance between the dock moth and the weed will be achieved. The dock population should decrease enough to no longer be a strong competitor with grazed pastures. Similarly, hay paddocks should become more productive when dock is reduced to insignificant levels. These gains in productivity will benefit all producers currently suffering from dock infestations.