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Weed web moth

We have identified weed web moth caterpillars (Achyra affinitalis) for agronomist, Brett McEwan (I.K. Caldwell). Brett reported finding them attacking medic in a paddock near Kerang, in the northern country district of Victoria. Plants had been completely defoliated in some areas, and characteristic webbing was evident around the plants. Brett says they appear to be starting out in sandy areas and spreading out over time. District agronomist, John Fowler (I&I NSW), has also reported small caterpillars, attacking various weeds around Deniliquin, in the Riverina district of New South Wales. These have also been identified as weed web moth, and John says they are very common in the district at the moment. John has found them on weeds such as Bathurst burrs, boxthorns and roly-poly, as well as some native ground creepers.

Weed web moth caterpillars are known to attack canola, soybeans, lupins and lucerne. 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 this pest requires higher rates of insecticides than commonly used against cutworm and 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.

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