Rare fly outbreak hits soybean crops hard

Stem fly has been causing serious problems in soybean crops in northern NSW.
Stem fly has been causing serious problems in soybean crops in northern NSW. Contributed.

 AGRONOMISTS estimate 4000 hectares of soybeans near Casino, NSW have been damaged by soybean stem fly (Melanagromyza sojae), a pest rarely seen in damaging numbers in Australia.

Department of Agriculture, Fisheries and Forestry (DAFF) senior entomologist, Hugh Brier, of Kingaroy, Queensland, says the only other significant soybean stem fly outbreak occurred in 2009 in the Mackay region of Central Queensland.

Mr Brier says experience overseas suggests yield losses of 20 to 30 per cent when crops are infested in the early vegetative stage as damage causes reduced pod and seed set.

However overseas literature is divided as to the pest's impact on more advanced crops, he said.

"In many crops at Casino, stem fly damage was confounded by poor root development due to water logging, and the fungal disease, charcoal rot," he said.

Dr Malcolm Ryley, DAFF principal plant pathologist, Toowoomba said charcoal rot, a disease which blocks the plant's vascular (xylem and phloem) tissue, causes similar damage such as leaf wilting and plant death.

Mr Brier said plant stress may exacerbate the impact of SSF, and visa versa.

"Fortunately recent rain does seem to be easing symptoms in some crops as they are better able to meet water demands, despite damage to vascular tissue in the stems, be it due to stem fly, charcoal rot or earlier water logging," he said.

"However reports are coming in from agronomists in the Richmond Valley of new instances of damage including leaf death in stem fly-infested crops with healthy root systems."

Soybean stem fly has also been reported this summer in the Clarence Valley by Dr Natalie Moore, NSW Department of Primary Industries (NSW DPI) senior research agronomist, Grafton who reports the pest is not yet having a noticeable impact on crops in terms of leaf death/wilting symptoms.

While soybean stem fly occurs in Australia, it is more common in Africa and Asia, where it is a common pest of soybeans.

Stem fly activity is favoured by warm temperatures, high rainfall and high humidity but it unusual to see this pest active so far south, Mr Brier said.

"Management of this pest is difficult as the damaging larval stage feeds inside the petioles and stems," he said.

"Consequently its presence is usually not detected until plants display noticeable symptoms such as leaf wilting and death."

Once damage is observed, spraying with dimethoate (Permit PER14121) is unlikely to be effective as the damage is done and other than the emergency permit there are no insecticides registered for the control of soybean stem fly, Mr Brier said.

"Significant parasitism (by as yet unidentified wasps) was observed in soybean stem fly infested plants collected from Casino," he said.

"It maybe that in most years that the pest is kept in check by these and other natural enemies, and that, in the absence of noticeable damage as evident this season, stem fly has been present previously without drawing attention to itself."

The Grains Research and Development Corporation (GRDC) funds research into crop pests and advocates an integrated pest management (IPM) approach.

This includes conserving beneficial insects by using more selective pesticides, minimising pest damage by the best practice timing and application of pesticides, and maximising the crop's ability to compensate for damage by improved agronomic practices that boost crop health.

Mr Brier says adult stem flies are shiny black, about two millimetres long and look very similar to the bean fly (Ophiomyia phaseoli) which is a major pest of navy bean seedlings.

"The damaging stage is the white larvae (maggot) which feeds inside the stem.

"Soybean stem fly larvae can be distinguished from other bean fly species by the "blunt, somewhat atrophied (shrunken) central horn" in each posterior spiracle. The pale brown pupae are cylindrical in shape with rounded ends.

"Infected stems are often red inside (sometimes pale) and a distinct zig zag tunnel may be observed - with maggots or pupae inside."

He says apart from the exit holes, the soybean plants will initially appear healthy on the outside.

"Large infestations (three or more maggots per plant) may cause wilting and may even cause plant death, especially in younger plants particularly if damage occurs in the plant's basal stem."

Please report any suspected soybean stem fly infestations to: Hugh Brier, DAFF Kingaroy, ph: 07 41 600 740 or; Natalie Moore, NSW Department of Primary Industries (NSW DPI), ph: 02 6640 1637 or; or contact your local agronomist.

For more information on GRDC-supported research, visit

Topics:  biosecurity soybean

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