Growers and consultants need to revise the thresholds used for important insect pests of grain sorghum such as corn earworm and sorghum midge in light of lower prices currently being offered.
With new crop grain sorghum prices below $200 per tonne, the break even cost of control means that higher pest numbers (density) are needed before control becomes economic, compared to thresholds used last season when grain values were much higher.
The use of a benefit:cost ratio is also an important consideration. The break even point is where the cost of control is equal to the loss caused by the pest. The benefit:cost ratio will vary according to individual preferences, and needs to be factored in to calculations.
One corn earworm larva is estimated to consume 2.4 g of sorghum during its lifetime. The economic threshold (i.e. the number of larvae/head where the cost of control is equal to the value of the grain saved) can be calculated using the formula:
No. larvae/head = (C x R) ÷ (V x N x 2.4)
C = cost of control ($/ha)
R = row spacing (cm)
V = value of crop ($/tonne)
N = number of heads/metre of row
2.4 = weight of sorghum (grams) lost/larva
The value of crop loss caused by corn earworm larvae in grain sorghum, for a range of larval densities and grain prices and based on 10 heads/metre of row on 1 metre row spacing, are presented below.
Listed below are estimates of midge damage at different grain prices without chemical treatment.
The yield loss estimates in the table assume that spraying results in a 100% kill and that there is no midge damage prior to chemical application. It also assumes that the same average midge pressure persists over 4-5 days. In reality research has shown that one well timed insecticide for midge (put on from panicle emergence and before midge even enter the crop) will still only prevent 70-80% damage protection in lower rated sorghum hybrids. In 8+ rated hybrids, yield losses can be reduced by over 90% with this spray timing.
If the total cost of applying a synthetic pyrethroid by plane is around $20/ha, we can see that at a grain price of $150 per tonne, it is simply not economic to spray mid to high rated hybrids at 1 midge/head and 8+ hybrids at 3 midge/head. It should be emphasised that 8+ is the highest rating that can be assigned to midge resistant hybrids. There are some 8+ lines that would have a considerably higher rating if the scale was extended, and are practically immune to midge damage.
The above information shows the importance of calculating thresholds for the current situation, rather than relying on a fixed value from one year to the next.