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.