Thresholds and changing sorghum crop value

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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.

Corn earworm
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.

When sorghum is valued at $300/tonne, one larva/head could cause $72 crop loss/ha. If the price drops to $150 per tonne, one larva/head causes just $36 crop loss/ha, or 50% less economic damage. This example demonstrates just how important it is to consider each case on its merits, and in particular to consider the cost of control, as it too can vary widely depending on whether aerial or ground spraying is used.

Sorghum midge

As with corn earworm, decisions to spray for midge are greatly influenced by crop value and it is possible to calculate theoretical yield loss estimates for a particular crop scenario (see table). These yield loss estimates are based on extensive field trials by DPI&F that determined the average yield loss/midge/day on different rated midge hybrids. For a susceptible hybrid, one midge is estimated to cause 1.4 g yield loss/day. (Photo: D. Ironside)

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.

2 thoughts on “Thresholds and changing sorghum crop value

  1. BigBug

    The short answer is yes, the value of 2.4 grams/larva takes compensation into account.

    The estimate is based on a series of five glasshouse and field trials where a regression relationship was developed between grain loss/head and larval numbers.

    Measuring yield loss in sorghum trials by weighing grain from individual heads is subject to major sources of variation. While the damage relationship in these trials was highly significant (P<0.001), it explained only 11% of the variation.

    The value of 2.4 grams/larva is within the range from previous published research (1.6 to 8.4 grams/larva).

    Results of the field trials showed that when the grain number is reduced by corn earworm, the remaining grains partially compensate by increasing weight. Percentage compensation at 50% seed set ranged from about 20-40%, which is similar to the amount of compensation after damage by sorghum midge.

    Another key point when pest densities are low is that natural enemies can greatly reduce survival of larvae. At this time of the year it is not uncommon to find 30-50% of small-medium larvae parasitised by the small wasp, Microplitis demolitor. At low pest densities, the activity of this biocontrol agent combined with other mortality factors e.g. natural NPV, may be enough to tip the balance against applying a treatment to control grubs.

  2. Glenn

    Growers keep asking if the 2.4 gm figure when used in the calculation, takes into consideration compensation of the head. I realise compensation may only be 10%, however this may be significant if you have only one grub per head. I would live to hear from the experts. Cheers

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