Weevils, grassgrubs and other aphid pests

The importance of distinguishing pest species is highlighted through an array of other pest reports. Knowing pest identity helps to focus management efforts, but can also be elusive

 

Weevil larvae in cereals

Whitefringed weevil larvae (Naupactus leucoloma) have been reported causing extensive damage to a number of barley and wheat crops near Burraboi in the Southern Riverina of NSW. The feeding damage is obvious on tillers just above the crown, and is evenly spread across the paddocks, although less than 20% of the plants are showing visible damage. In another report, cereal weevils located about 10-20mm below the ground have been damaging plants in a cereal crop near Swan Hill in Victoria’s Mallee. Patches of affected plants across the crop have become apparent.

Weevil adults and larvae can cause damage to all field crops, although adults are more frequently associated with above ground damage. Distinguishing weevil larvae from other creamy-coloured insect larvae found in the soil (e.g. cockchafers and wireworms) is relatively straightforward - weevil larvae have no legs. The larvae generally have a tan or dark brown head capsule. However, distinguishing between the many species of weevil larvae is difficult. A guide to weevils found in crops is available here.

Weevil damage can occur at any time of the season, but feeding during autumn and early winter is typically the most critical. Inspect paddocks and nearby weeds prior to sowing and monitor crops for signs of seedling damage and bare patches within paddocks. In spring, look for signs of chewing damage on plants, characterised by scallop-shaped holes along the leaf margins, as well as a loss of plant vigour.

Weevils can be difficult to control with chemicals due to their subterranean habits where they remain protected from insecticide exposure. Several species are also patchy in their distribution within paddocks.

Aphids

We have received a few reports of aphids other than the green peach aphid. Cereal aphids (see PestFacts Issue No. 3) have been observed across a number of barley and wheat paddocks in the Hopetoun area of the Victorian Mallee. Early sown crops, in particular, have 15-20 aphids per plant on most plants. Some BYDV symptoms are beginning to appear in a few crops. The decision to spray is being held over. In the same area, cowpea aphids (Aphis craccivora) (see PestFacts Issue No. 1) are continuing to be reported on vetch.

Grassgrubs or corbies

Pasture and lucerne stand damage in the Bridgewater region of Victoria’s North Central district appears to be caused by one of several possible soil inhabiting caterpillars known as grassgrubs or corbies (Oxycanus antipoda or Oncopera intricate). The larvae have been reported living in soil tunnels and were found attacking clovers and lucerne. The damage was initially thought to be from cutworm. The larvae of this group grow up to 60mm long and are dark-coloured without stripes. The adults are large moths with wingspans ranging from 45-60 mm.

 

*Sources of field reports

Guy Debnam – Agronomist, Landmark (Victorian Northern Country)

Shaun Krahnert – Senior Agronomist, Elders (Victorian Mallee)

Craig Muir – Agronomy Consultant, AGRIVision (NSW Riverina)

John Robertson – Consultant, AgWise Services (Victorian Mallee)