Economic threshold ready reckoners

Quick reference tables for key insect pests in sorghum, chickpeas, soybeans and mungbeans

Economic thresholds are a key decision tool for growers and agronomists managing insect pests in their crops. Dynamic economic thresholds enable individual costs of control and grain prices to be included in the calculations to ensure the decision to treat or not to treat the pest infestation is as accurate as possible.

Ready reckoners have been calculated for a range of pest densities, costs of control and grain prices.

  1.  Potential yield loss tables are useful for planning a course of action and provide a basis for discussion on the economic viability of spraying a crop for a given pest density. They answer the question “How much will I lose ($/ha) if I do nothing?”
  2. Economic threshold tables provide an estimate of pest density at the economic breakeven point (where the cost of control = the cost of the spray) and are useful guides for making decisions on sampling data.

Ready reckoner tables

Sorghum

Chickpea

Soybean

Mungbean

 

Developing ready reckoners

  • To calculate a ready reckoner for the potential yield loss per pest

Potential yield loss ($/ha) = P x D x V  / 1000

Where:
P = pest number per unit of measure e.g. per 1m2 beatsheet sample; or per head; or per 10 sweeps
D = yield loss per larva per sampling unit (kg/ha)
V = crop price ($/t)

  •  To calculate a ready reckoner for breakeven economic thresholds (pest density)

Economic threshold (number of larvae/sampling unit) = C ÷ (D x V)

Where:
C = cost of control ($/ha)
D = yield loss per larva per sampling unit (kg/ha)
V = crop price ($/t)

  • Damage (D) values

D describes the relationship between the crop damage caused by an individual pest and the compensation of the crop for the damage. The resulting value (D) is an estimate of the potential yield loss if the pest were allowed to complete its development in the crop (uncontrolled).

D value for helicoverpa in:

  • Sorghum – 2.4 g/larva,
  • Chickpea –  2.0 g/larva,
  • Soybeans –  4.0 g/larva,
  • Mungbean – 3.5g/larva
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