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Weed web moth

Agronomist, Tim Pilkington (Elders), has reported issues with weed web moth caterpillars across two lucerne paddocks north west of Stawell, in the Wimmera district of Victoria. The caterpillars have skeletonized the majority of seedling lucerne plants across one paddock, while in the other paddock - which is an established lucerne stand – a significant area has suffered extensive feeding damage.

Weed web moth (Achyra affinitalis) caterpillars are typically found concealed between leaf surfaces held together by a silken web. They grow to about 15-20 mm in length. In addition to lucerne, weed web moth caterpillars are known to attack canola, soybeans and lupins. They shred the leaves of seedling crops and may cause complete defoliation, which can lead to plant death. Chemical control is occasionally warranted when large numbers build up. Control difficulties have occurred in the past because weed web moth caterpillars require higher rates of insecticides than commonly used against cutworms and pasture webworms.

Caterpillars of weed web moth look very similar to the pasture webworm and diamond back moth, and can be 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 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.

Monitor crops after emergence as seedlings are at most risk of damage. Look for early infestation signs such as terminal damage, webbing and windowing of leaves. Moths can migrate from long distances and larvae can suddenly appear in very large numbers. Warm weather and early autumn rainfall provides ideal conditions for pest occurrence in crops. Inspect broad-leaved weeds for the presence of larvae prior to and during seedling establishment.

Click here to view images of weed web moth.

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