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Department of Primary Industries and Fisheries researchers have completed a three year study to clearly define recommendations for the control of Helicoverpa in chickpeas grown under Queensland´s advanced farming systems technology.
DPI&F senior entomologists Dr Paul Grundy at Biloela Research Station and Dr Melina Miles, Toowoomba, took up the challenge to provide agronomists and growers with much-needed answers on which to base economic control decisions for this universal insect pest.
Drs Grundy and Miles said no previous research in Australia or overseas had taken a close look at the impacts of Helicoverpa pest density on chickpea yield and grain quality, timing of control, and larval feeding behaviour.
"The commercial reality for making a decision to spray has to be based on the relationship between Helicoverpa damage potential, chickpea grain prices and the control costs rather than setting a fixed number of larvae per square metre of crop," Dr Grundy said.
"From our threshold trial assessments across 25 southern and Central Queensland sites, the relationship between larval numbers and yield loss showed that cumulative larval damage based on 1 larva per square metre is 20kg/ha.
"During the development of the chickpea, one larva will eat 2 grams of grain, which is comparable to the Helicoverpa grain sorghum loss assessment of 2.4g/larva."
Dr Grundy stressed that this relationship was fundamental to calculating the break-even point for determining if it was economic to spray Helicoverpa in chickpeas.
Observational studies of larvae showed that small larvae were predominantly foliage feeders whereas the medium to large larvae caused the vast majority of pod damage.
Control decisions have to be made to prevent larvae reaching the medium to large stage.
Dr Miles said neither the small or large larvae showed any preference for flowers, which supported data that showed no yield loss was incurred when Helicoverpa were tolerated during the crop flowering.
"Loss of yield and grain quality only occurs from pod set to maturity, and it is only in rare situations were Helicoverpa pest pressure is extreme and exceeds 20 larvae/sq m that spraying may be warranted in the vegetative to flowering stage," Dr Miles said.
"It is important to determine the size and count the Helicoverpa larvae per metre of row using a 1.3m by 1.5m beat sheet to catch larvae dislodged from the chickpeas.
"Larvae less than 3mm are not included in the calculation used to assess the average number of larvae/sq m.
"It is then possible to apply a formula to effectively count the number of larvae/sq m.
"By multiplying the number of larvae/sq m by 2 grams, multiplied by the market value of chickpea ($/tonne), and dividing this figure by 100, provides the yield loss in dollars/ha if the larvae are not controlled," Dr Miles said.
Dr Miles said that a decision to spray had to consider the benefit:cost ratio, which compared the value of the crop to be saved to the cost of controlling the outbreak.
"For example, if the preferred benefit:cost ratio is 2:1, then the grower would not initiate control measures until there was a $2 saving in crop yield for every $1 spent on the control."
The research project has produced a table for a range of larval densities and crop prices to enable growers and consultants to answer the question "what will I lose if I don´t spray?", or alternatively "what will I potentially save if I do spray?"
Dr Grundy said the ground-breaking trial work showed that at an infestation range of 1 to 4 larvae/sq m, the amount of defective grain was well below the grain quality standard (6 per cent by weight) at which penalties were applied.
"Given this result, when we look at the range of larval densities, for which it is economic to control Helicoverpa, the potential cost of grain quality loss does not need to be factored into the economic threshold," Dr Grundy said.
A 4-page DPI&F colour brochure outlining the recommendations for Helicoverpa management in chickpeas was released to industry in August 2007 in conjunction with Pulse Australia and the support of the Grains Research and Development Corporation.
Editor's Note: Original news release can be found here.
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