Next Gen: weevil larvae

Throughout May we received numerous reports of adult weevils causing crop damage. Now there is more crop damage, this time by the larvae; here it is, the next generation

Spotted vegetable larvae (left) (Source: Copyright © Western Australian Agriculture Authority, 2015); and vegetable weevil larvae showing feeding damage (right) (Source: cesar)

 

Where have they been reported?

Weevil larvae have been reported damaging a range of crops, including barley, wheat and canola, across northern Victoria (Wimmera and Northern Country districts) and up into the Riverina and South West Slopes districts of New South Wales. Whilst many of these larvae were not identified to species, some were identified as spinetailed weevil (Steriphus caudatus) and vegetable weevil (Listroderes difficilis) larvae.

In a barley crop near Deniliquin in the New South Wales Riverina, weevil larvae were feeding on barley tillers approximately 5-10 cm below the soil surface. Damage was broadly seen across two paddocks, resulting in the death of approximately 2-3% of plants in each crop. Uncovering the soil beneath the dead plants revealed the weevil larvae. South of Echuca, weevil larvae were found damaging an irrigated wheat crop. Weevil larvae were causing severe damage in areas that had not been burnt, however, where the paddock had been burnt and grass residue eliminated there was little damage and very few larvae. In both of these cases, the crops had histories of a dominant ryegrass presence within the previous crop or pasture.

In two similar cases from the South West Slopes of New South Wales and the Victorian Wimmera, grey-banded leaf weevil (Ethemaia sellata) larvae had done an excellent job of defoliating stands of marshmallow weed on the edges of crops, indicating how a crop pest can sometimes play beneficial role in weed management.

About weevils

We are still receiving one or two reports of adult weevils in crops; however, it is the larval damage that has recently been most common. For information about a particular weevil species, go to the new series of PestNotes, which includes six of the main weevil pests in broadacre crops and pastures (spinetailed, spotted vegetable, whitefringed, Mandalotus, Sitona, and vegetable weevils).

Our advice

For the vast majority of species, there are no control options for weevils at this time year, mainly due to the sub-soil feeding habits of larvae. At the larval stage, weevils are commonly misidentified as other beetle larvae, wireworms or hoverfly larvae. However, distinguishing weevil larvae from other insect larvae is relatively straightforward – weevil larvae have no legs. Distinguishing between weevil larvae is much more difficult and assistance is often required. In PestFacts Issue No. 2 (2015) we provided a Table of some of the main morphological characteristics that is a useful guide to distinguish weevil adults and larvae.

 

Sources of field reports of weevil larvae

Tim Condon – Agronomist, Delta Agribusiness (New South Wales South West Slopes)

Ben Cordes – Agronomist, Tylers Hardware & Rural Supplies (Victorian Wimmera)

Chris Dunn – Agronomist, Landmark (Victoria’s Northern Country)

Lincoln Harris – Agronomist, Elders (New South Wales Riverina)

Justin Whittakers – Agronomist, Landmark (New South Wales Riverina)