Weed web moths

Agronomist, Terry Edis (Elders) has reported weed web moth (Achyra affinitalis) caterpillars causing damage to a canola crop west of Temora, in the South West Slopes district of New South Wales. Plants have been defoliated in about 20% of the paddock, and some characteristic webbing is evident where the caterpillars are present. Terry says the caterpillars, which are about 5-7 mm long, can be seen on every 1-2 canola plants in the affected parts of the paddock. Weed web moths were prevalent in several parts of southern New South Wales and northern Victoria last year. They are often abundant in seasons with early autumn rainfall and warm weather.

Weed web moth caterpillars are known to attack canola, soybeans, lupins, lucerne and a wide range of broad-leafed weeds. They feed under a fine web, skeletonising foliage and webbing leaves together. They may shred the leaves of seedling crops and 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.

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. Moths have a 20 mm wingspan and are buff coloured with darker brown or reddish flecks, mainly on the forewings.

Click here for images of weed web moth caterpillars.