Weed web moth

Weed web moth (Achyra affinitalis) caterpillars have recently been found attacking canola crops in parts of southern New South Wales. Agronomist, Vanessa Warren (Delta Agribusiness), has reported weed web moth in several canola paddocks ranging from cotyledon to the 6-leaf stage. The crops are located around Coolamon, in the Riverina district. Vanessa says the caterpillars can be seen chewing plants while wrapped in their webbing inside rolled leaves. The caterpillars are skeletonizing the middle section of the leaves and restricting plant growth. In some cases whole paddocks have been affected and control has been required. Agronomist, Terry Edis (Elders), has observed caterpillars in several canola crops near Ariah Park, also in the Riverina district. They have been identified as weed web moth. The caterpillars, which are about 10 mm in length, were typically seen inside the folded leaves and protected by webbing. In the worst affected paddock, 2-6 caterpillars per metre square were found, and the extent of the feeding damage has warranted control. Weed web moth caterpillars have also recently been identified for agronomist, Patrick Connell (PC Agronomy). The caterpillars were collected from a canola paddock near Barellan, in the Riverina district.

In addition to canola, weed web moth caterpillars are known to attack soybeans, lupins, lucerne and a wide range of broad-leafed weeds. They shred the leaves of seedling crops and may cause complete defoliation, which can lead to plant death. Control is occasionally warranted when large numbers build up, typically where there is an abundance of green plant material over spring and summer. However, difficulties have occurred in the past because weed web moth caterpillars require higher rates of insecticides than commonly used against cutworms and the pasture webworm.

Caterpillars of weed web moth look very similar to diamond back moth, particularly when they are small. This means they are easily confused in the field. Weed web moth caterpillars are grey-green and pale brown in colour, with a distinctive black head. They are slender, grow up to 15 mm long and generally have a dark line down the middle of their back with three rows of dark spots on either side. Caterpillars tend to wriggle violently or crawl around rapidly when disturbed.

Click here to view images of weed web moth.