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Pasture webworm

Pasture webworm can be a problem when cereals are sown into infested pastures using minimum tillage practices. The larvae live in silk-lined tunnels. But can they affect canola?

 

Where have they been reported?

About 10-15% of a canola crop near Forbes in the NSW Central West Slopes & Plains has been attacked, with large patches almost completely eaten bare. The damage arose from stem chewing (‘ring barking’) and chewing of the leaves. The culprits appear to be caterpillars about 10 mm in length, which have been found protected in soil-lined cocoons resting in batches on the soil surface. Specimens have been examined and they appear to be a species of the genus Hednota (sometimes referred to as pasture webworms). There are several species of pasture webworm in NSW and they are difficult to distinguish. Pasture webworms are commonly associated with feeding damage to cereals and grasses.

Pasture webworm habits and behaviour

Pasture webworms are relatively insignificant pests of pastures, but can occasionally cause large losses to establishing cereals. The caterpillars attack establishing crops of wheat, barley and rye although oats are not affected. Damage almost invariably results from a rotation of cereals following a pasture phase. Although rare, pasture webworms have previously been reported attacking canola; this occurred a number of years ago, near Temora in the South West Slopes district of NSW. Webworm caterpillars cut cereal and grass plants at ground level and may pull the plant material down into a tunnel or leave it scattered on the soil surface.

Our advice

Monitor crops that have been seeded into areas that were pasture last year or where stubble and grasses were prevalent in autumn. Digging at the base of recently damaged plants may reveal web-lined tunnels and caterpillars inside, or a search at night may reveal feeding activity. Pasture webworms are easily confused with the pasture tunnel moth (Philobota productella).

Control options

When possible, spot spraying of damaged patches is preferable. A buffer zone of 20 m around the infestation should also be sprayed. Once larvae are killed, badly affected cereal crops will recover provided the crowns are not damaged. Earlier in the season, cultivation to form a 3-week fallow usually provides good control. Infestations are usually worse under minimum tillage, in grassy paddocks and during cool moist situations. Pastures that are heavily grazed before March are generally not attractive to moths and are not favourable for the survival of larvae. This grazing management can be used to reduce the incidence of this pest.

Description

Larvae of most webworm species grow to about 15-20 mm long and are smooth bodied with shiny dark brown heads. They are light to dark brown and may have a tinge of the green gut contents showing through the skin. Older larvae have darker raised patches on each body segment. Young larvae live in silken tubes among leaf blades and older larvae in web-lined tunnels beneath the ground. Moths of several species are slender, straw coloured with darker patterns, and about 10-12 mm long with protruding beak-like mouthparts. It is believed that most species have one generation per year. For more information on pasture webworms, click here.


* Source of field reports of pasture webworm

Jim Cronin – Agronomist, Landmark (NSW Central Western Slopes & Plains)

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