Crop monitoring in late winter? Keep an eye out for these biosecurity threats

The threat posed by exotic grain pests entering Australia is a serious and ongoing risk to the industry. Remaining vigilant to their presence during routine monitoring is a sensible precaution.

Plant Health Australia’s Biosecurity Manual for Grain Producers and Exotic Pest Factsheets contain descriptive information and images of the highest priority exotic threats to the Australian grains industry, including a number of insect pests.

If any of these pests enter Australia, many would become most obvious from and after crop flowering.

However, if there was an incursion a few would start to appear from late winter and early spring.

Importantly, all of these species discussed in this Pest Facts are reported as absent from Australia.

Here, we provide a brief description of three of these exotic pests, and where you might find them if they were present during routine winter crop monitoring. 

Note: each of these pests attack the plant stem from and/or after stem elongation. 

This is an unusual form of pest damage in winter cereals and canola; common native pests of southern Australia are unlikely to cause this.

Winter cereals: Hessian fly (Mayetiola destructor) and barley stem gall midge (Mayetiola hordei

What to look for

It is the larvae of these species that cause damage to wheat and barley plants (leaves, stems and eventually cereal heads).

Damage can result in leaf discolouration (leaves become darker green to blue-green, in seedlings new growth is yellow) and stunted growth.

Larvae of both species would be found at the base of the plant under leaves. Separate the leaf sheath at the base of the stem or node on plants with symptoms.

Closer to harvest, look for hessian fly pupa and puparium at the base of the plant.

Hessian fly pupae are dark brown in colour, slightly tapered in shape and look like flaxseeds. The adults are midges and look a little like mosquitos.

What do the larvae look like?

In the case of the Hessian fly, you would find the maggot-like larva feeding on the stem surface.

For barley stem gall midge, look carefully for a pea-shaped gall formation. If you were to carefully split the gall you would possibly find the larvae inside.

Larvae of both species are initially white in colour and then turn brown and are cylindrical, legless and grow up to 3-4 mm long.

Like all fly larvae, they have no visible head capsule. Larvae could be confused with other legless maggots but there are no major fly larvae pests above ground on winter cereals.

See the fact sheet

Winter cereals: European wheat stem sawfly (Cephus pygmeus) and the closely related wheat stem sawfly (Cephus cinctus

What to look for

Larvae would feed within the stems of cereals and other grasses.

Infected stems would be filled with frass (waste debris produced by the larvae).

The stem may be cleanly cut at the feeding site causing lodging. Distinctively cut, lodged stems should be inspected for signs of frass.

Inspection of the stem below the break would reveal the presence of larvae.

What do the larvae and adults look like?

Larvae are legless (although European wheat stem sawfly has three pairs of reduced thoracic legs) with a head capsule and prominent mouthparts.

They are distinctly different to Hessian fly larvae that have no head capsule.

They have a horn-like projection at the rear end of the abdomen (tubercule).

See the fact sheet

Canola: Cabbage stem weevil (Ceutorhynchus pallidactylus) & rape stem weevil (Ceutorhynchus napi

What to look for

Adult beetles are small and grey coloured, and begin laying eggs during the stem elongation stage of the crop.

Larvae live inside the stems of the host plant.

Overseas, larvae feeding within the stems cause the plant to show a loss of vigour, stem distortion, leaf loss, delayed flowering and lodging.

Close inspection of the stems would reveal very small exit holes caused by the larvae leaving the host plant to pupate underground.

It is important to investigate the cause of lost vigour, split or distorted stems, leaf loss, delayed flowering or lodging.

What do the larvae and adult weevil look like?

Any larvae found within lodged stems and suspected of being exotic should be closely inspected.

Larvae are white coloured with yellowish heads; they grow up to 8 mm long and 0.5 mm wide.

The adults of both species are small (<4 mm) dark greyish coloured beetles that are covered in white hairs and feed on brassicas.

Adults could be confused with other small weevils, such as the spotted vegetable weevil and the small lucerne weevil, however the larvae of these species do not affect canola stems.

See the fact sheet

Early detection

Stuart Kearns is Manager of Farm Biosecurity Programs at Plant Health Australia (PHA).

“Early detection of any of these pests can greatly increase the chance of successful eradication and reduce the cost and social impact of an incursion” says Mr Kearns.

“Remain observant for anything unusual in your crops and storage facilities. If a pest is found that is not normally present on your farm, it may be new not only to your farm, but to the region, state or even Australia.”

Report anything unusual to the Exotic Plant Pest Hotline (1800 084 881).

Resources that will assist in preventing incursions

Free copies of the Biosecurity Manual for Grain Producers and all of the Exotic Pest Factsheets are available online from the Plant Health Australia website or in hard copy from Jim Moran at Agriculture Victoria in Epsom (03) 54304479. Jim can also provide:

  • free biosecurity gate signs to aid control of traffic on-farm.
  • a small clip-on phone lens that enables magnified images of pests and diseases to be taken and sent by phone to PHA, researchers or industry experts for rapid identification and without touching the suspect plant. 

The knowledge in these publications, monitoring tools, and ensuring you follow good biosecurity practices are your best defence against the spread of pests and disease into growing regions. Early detection and identification will support rapid exotic pest responses and increase the likelihood of eradication.

Cover image: Photo by John C. French Sr., Retired, Universities: Auburn, GA, Clemson and U of MO,, CC BY-NC 3.0 US

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PestFacts south-eastern keeps growers and advisers informed about invertebrate pests and beneficials in broadacre crops and pastures during the winter-cropping season in Victoria and southern New South Wales.


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