Webworms & false wireworms

There have been unconfirmed reports of false wireworm (Coleoptera: Tenebrionidae) attacking canola crops in parts of the Wimmera and Western district of Victoria. Feeding damage to pastures resembling pasture webworm (Hednota spp.) larvae has also been observed in some areas of the Western district. However, no positive identification was possible.  

False wireworm is a general description given to a mixture of species of soil dwelling beetle larvae. The species are similar in appearance with larvae generally growing between 10 - 20 mm in length. In western Victoria, the grey false wireworm (Isopteron punctatissimus) appears to be the most important species associated with canola.

In crops, they are mostly found in paddocks with high stubble and crop litter contents. They may affect all winter sown crops. There is anecdotal evidence suggesting that stubble retention and minimum tillage are contributing to the build up of false wireworm populations.

False wireworms chew into the seedling stem and roots, weakening the plant or ring barking the stem. The injury to the seedlings makes them susceptible to dehydration and infection by disease. Feeding damage is often most severe when germination is slowed by continued dry weather.

Pasture webworm larvae are caterpillars of the genus Hednota. There are several species which can be difficult to distinguish. The larvae are light-dark brown in colour, growing to about 15 mm long. They feed at night and hide in web-lined tunnels beneath the ground by day.

Pasture webworms are relatively insignificant pests of pastures, but can cause large losses to establishing cereals. The caterpillars commonly attack establishing crops of wheat, barley and rye. Damage almost invariably results from a rotation of cereals following a pasture phase.

The adults are long-legged moths with the enlarged mouthparts arranged in a beak-like projection. They are about 12 mm long with a wingspan of about 22 mm. Moths are easily detected. They fly up from pastures in the autumn; this serves as a warning of the risk of damage later that year.

Although unconfirmed, the reports serve as a reminder to be on the lookout for these pests. In particular, monitor crops that have been seeded into areas that were pasture last year or where stubble and grasses were prevalent in autumn.

Seek advice from your agronomist if you’re unsure about pest identification. Ute guides are still a valuable source of information. Insect Ute guides are available from the GRDC bookshop - www.grdc.com.au/bookshop, phone 1800 110 044.