To spray or not to spray? That is the question facing grain growers across New South Wales and Queensland following a recent spate of Helicoverpa armigera emergence in chickpea and canola crops.
With many early and moisture stressed crops nearing maturity and others benefiting from storm rains, estimating the rate of larvae development and therefore the need for control is posing a challenge for some growers.
A Grains Research and Development Corporation (GRDC) supported initiative, the Beatsheet Blog, is offering advice to help growers estimate the likelihood of grain damage by helicoverpa larvae in the lead up to harvest.
Queensland Department of Agriculture and Fisheries (DAF) principal entomologist Dr Melina Miles said there were some key considerations for growers in deciding on the need for a control strategy.
“Many of the very small-small larvae will die of natural causes, so populations of small larvae below or close to threshold are unlikely to exceed threshold by the time they reach damaging 4-5th instar,” Dr Miles said.
“Death rates of very small and small helicoverpa larvae are very high. Larvae are easily dislodged by wind and rain, and can be eaten by predators.
“Loss of very small-small larvae is typically around 70%. In the chickpea threshold, we include a conservative loss of 30% of small larvae. Once you have 4th instar larvae (medium), there is very little natural mortality.”
Dr Miles said very small and small larvae feed predominantly on leaves and cannot penetrate pods to feed on seed until they are at least 4th instar (medium).
“In a crop that is senescing quickly, the mortality of these small larvae will be added to by a lack of suitable food,” she said.
“Growers may still find these larvae in the greener patches of the crop, but they need to estimate what proportion of the crop is still green if considering the need to control these larvae.”
With 80% - 90% of grain damage done by 5-6th instar larvae, Dr Miles advised growers to undertake control measures before larvae reached 5th instar.
She said it was also important to note that larval growth rate was affected by temperature - the warmer the temperatures, the faster the larvae grow.
For more information on helicoverpa larvae development or to access figures showing the predicted rate of helicoverpa development for different locations, visit The Beatsheet blog.
DAF Manager (Media)
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Sarah Jeffrey, Senior Consultant Cox Inall Communications