Heliothis Threshold Calculator
Sorghum head and close-up of brown anthers (right)
The formula below calculates the economic threshold for heliothis larvae in sorghum. The threshold is calculated by knowing the amount of grain a grub eats and multiplying this by the number of grubs per hectare. This allows a measure of the estimated yield loss if the grubs aren't controlled. The most recent research by the QDPI&F has shown that in sorghum, one heliothis larva will consume 2.4 grams of grain.
Proper monitoring during flowering is important to accurately estimate grub density in your crop. To calculate the number of larvae per head in a sorghum block, vigorously shake 10 randomly selected heads into a bucket in each of 4 locations in the block. After shaking the 10 heads, empty the bucket onto an even light coloured surface and carefully count the number of heliothis larvae. Divide by 10 to give the average number per head in that location.
If the average number of larvae per head in your crop is greater than the threshold value calculated below, spraying with ViVUS is warranted. Application in sorghum should be timed for three days after 50% of heads in the field have completed flowering (brown anthers to the base of the head – see picture). Be sure to follow directions on the ViVUS label.
Please contact Anthony Hawes (ph. 0425 232 052) if you require further information.
Calculator
Threshold calculation formula courtesy of the Queensland Department of Primary Industries and Fisheries
