FABA bean crops on the Darling Downs have come under pressure from helicoverpa larvae this season, potentially slashing returns if grain does not meet stringent quality standards.
Pulse Australia northern industry development manager Gordon Cumming said controlling pests such as helicoverpa was a priority if growers were aiming for the premium human consumption market for their faba beans.
"There is very low tolerance of damaged grains for this export market," he said.
"Affected consignments would incur additional grading costs to meet the quality standards or be downgraded to significantly lower price markets."
The problem facing growers is that the economic threshold of two larvae per square metre, used to manage helicoverpa in southern Australia, does not seem to be sufficiently restricting grub damage in northern crops. The Department of Agriculture, Fisheries and Forestry entomology team, led by principal entomologist Dr Melina Miles, has responded to the situation immediately with field-sampling trials to determine more accurate economic thresholds.
"Helicoverpa larvae feeding on faba bean pods usually damage every grain, and these partially damaged grains go through into the sample," she said.
"As a result, the yield may not be greatly affected, but the whole crop might be downgraded. This means the economic threshold for helicoverpa in faba beans is low."
The first step the entomologists took to unravel the problem was to determine whether the beatsheet sampling method was effectively sampling the larvae - the concern being there may be more larvae in the crop that are not being dislodged with the beatsheet, therefore underestimating population size.
"We have completed one sampling trial comparing the efficacy of the beatsheet and sweep nets as sampling tools for helicoverpa in faba beans.
"What we have found is that the beatsheet method collects about half of the larvae that are actually present on the sampled plants, and the sweep net method collects only one-third of the total present.
"What is not effectively sampled with the beatsheet or sweep net are the small larvae in the buds, terminals and flowers."
The implication for this season is that growers and agronomists need to conduct a visual inspection of flowers and growing tips in the sampled plants, as well as continuing to use the beatsheet.
Dr Miles suggests that, until more scientific information is available, spraying decisions should still be based on the current economic threshold of two larvae per square metre, using both the beatsheet and visual inspection.
"By next season we will have a better idea of whether the economic threshold needs to be investigated, or if effective sampling is the main issue."
The other concern that Dr Miles and her team are exploring is the efficacy of insecticides to control helicoverpa.
Helicoverpa armigera is common in northern faba bean crops, so synthetic pyrethroids are not recommended.
"We got good control of small and large larvae in the trials, showing that with good coverage, it is possible to control individuals in the growing tips and flowers."
