A major pest usually found in coastal and subcoastal mungbeans has moved inland over the past two summers, with severe and sustained outbreaks of more than 100 larvae per square metre reported in many crops.
Hugh Brier, Department of Agriculture, Fisheries and Forestry (DAFF) entomologist, Kingaroy, says the podborer (Maruca vitrata) can dramatically affect crop yield by reducing pod set, severely damaging pods that are set, and delaying harvest maturity.
“Bean podborer larvae feed initially inside buds and flowers, before attacking pods,” Mr Brier said.
“The basic management strategy is to control larvae before they leave the flowers to bore into the pods.”
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.
Dr Rohan Rainbow, GRDC senior manager, plant health says the northern region has a strong history of successful IPM research, extension and application across a diverse range of grain crops.
“To help spread this success and develop parallel models in the other cropping regions GRDC initiated the National Invertebrate Pest Initiative (NIPI),” he said.
“A central objective of NIPI is the expansion of IPM expertise over a broader range of crops and farming systems.”
In response to industry concerns, DAFF initiated a series of GRDC-supported trials in 2011 and 2012 to evaluate a number of pesticide options in response to industry concerns regarding difficulties in controlling bean podborer.
Mr Brier says podborers are normally a major coastal/subcoastal mungbean pest but last year, damaging numbers were reported as far west as Surat.
“Podborer pressure was so sustained and severe in some areas that a number of crops were sprayed three to four times,” he said.
“This repeated spraying raised questions about pesticide efficacy against this pest.”
Mr Brier said the trials generated efficacy data that secured now-expired emergency use permits for indoxacarb and methomyl against podborer in mungbeans last summer.
Both products gave good podborer control, and questions about product efficacy in commercial crops may have been due in part to poor spray coverage, as well as sustained pest pressure.
Renewal applications for indoxacarb and methomyl have been submitted to the Australian Pesticides and Veterinary Medicines Authority (APVMA) for 2012/13.
Mr Brier said the trials also identified new very effective but very selective (IPM friendly) products with registration potential in summer pulses.
He says monitoring remains the first line of defence against podborer as the first sign of an impending infestation are the distinctive moths which fly away when disturbed in the crop.
“The first visible sign of damage is the webbing of buds and flowers, however in the very early stages of an infestation, small larvae are out of sight inside buds and flowers with no external evidence of their presence.
“Infested pods have a well-defined entry hole (mostly one per larva), usually ringed with distinctive frass.
“The most reliable way to assess podborer activity is to open up buds and flowers from as many racemes as possible, ideally from at least 30 random flowering racemes across a crop.”
Mr Brier says dividing the total number of podborers detected by the number of racemes sampled, and then multiplying this figure by the number of racemes per square metre, gives an estimate of the number of podborers per square metre.
This value can be compared with the new provisional economic threshold ranging from five to nine podborer larvae per square metre (as detected in flowering racemes), depending on mungbean crop value and cost of control.
Alternatively, bean podborer larvae can be sampled with a beatsheet. However as the beat sheet only detects one seventh of the podborer larvae actually present, multiply the beat sheet counts by seven to compare to the new threshold.
For more information about bean podborer management and thresholds, please contact Mr Brier on (07) 4160 0740 or firstname.lastname@example.org.
PHOTO CAPTION: The podborer, a major pest usually found in coastal and subcoastal mungbeans has moved inland.
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