Helicoverpa in chickpea - threshold calculator

Helicoverpa is the only major insect pest of chickpea. Research has shown that the impact of one helicoverpa larvae (per square metre) completing development on the crop is a resultant loss of 2 grams of grain. This figure is used in estimating the potential yield loss and subsequently the economic threshold. The following calculator can be used to identify potential yield loss and provide an appropriate suggestion for action.

Helicoverpa threshold in chickpea

Steps in determining if control is warranted:
  1. Sample the crop and record the number of small (S), medium (M), and large (L) larvae in each sample (e.g. 5 beatsheet samples of metre row).
  2. Average the number of each size of larvae and enter into the relevant box
  3. Enter the crop's row spacing and click the calculate button from mean larval density
  4. Add your estimate of the cost of control (including application) and expected crop value to calculate potential yield loss and break-even economic threshold. (If you have a preferred cost:benefit, enter it to get a revised economic threshold).
  5. You can then request a suggestion for action based on crop stage and selected threshold.
Number of larvae    
Very small (1-3 mm) = 1st instar  Note: due to high mortality and low damage rates, very small larvae are not included when calculating larval densities
Small (4-7 mm) = 2nd instar   Note: Final density assumes a 30% mortality rate of these larvae
Medium (8-23 mm) = 3rd or 4th instar  
Large (24-30+ mm) = 5th or 6th instar  
Row spacing (m)  
Mean larval density (per m2) after factoring in likely mortality:
     
Cost of control ($/ha)  
Crop value ($/t)  
  Potential yield loss ($/ha):  
  Economic threshold (larvae per m2):
     
Note that cost:benefit=1 is the break even point, and used to calculate the default threshold. Only when the ratio is greater than one is there an economic benefit from spraying. Often a ratio of at least 1.5 is applied before a spray decision is made.
Your preferred cost:benefit ratio  
Revised economic threshold (larvae/m2) based on preferred cost:benefit ratio above:
     
Crop stage  
or

 

 

 
 

Midge in sorghum - threshold calculator

The yield loss estimates in the  calculator assume that spraying results in a 100% kill and that there is no midge damage prior to chemical application. It also assumes that you will receive the same average midge pressures 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.

Collecting data to use in the calculator

Generally, peak midge activity occurs between 9-11am, and this is the best time to look. Sorghum heads are most attractive to midge at mid flower. Midge flies are only 1-2 mm long  and the easiest way to ‘get your eye in’ is to look at the top half of mid flowering panicles and look for  movement of the small red flies against a still sorghum panicle looking from side on and slightly above side on one section of the sorghum panicle at a time. Keep your eyes focused over a couple of branches of florets for several seconds at a time to detect female midge walking around the branch or bobbing up and down probing their ovipositor into each floret. On windy days shelter the panicle for 10-20 seconds before scanning each panicle,  to allow you to more accurately see midge movement.  Monitor for midge over 10 metres of row in at least 4 different locations in your crop. It may be necessary to spray only one section of crop at a time, or the whole crop accordingly.  
Number of midge per metre row
Midge rating of sorghum hybrid used  
Row spacing cm
Crop value $/tonne
Cost of control $/ha
Residual life of chemical used days
Suggested action:

 

 
 

Helicoverpa (corn earworm) in sorghum - threshold calculator

One corn earworm larva is estimated to consume 2.4 grams of sorghum during its lifetime. The economic threshold (that is, the number of larvae per head where the cost of control is equal to the value of the grain saved) can be calculated below:

Helicoverpa threshold in sorghum

Estimated cost of control ($/ha)
Row spacing (cm)
Estimated crop value ($/t)
Average number of heads per metre of row
   
Break-even threshold for this example: larvae per head

Calculating potential yield loss

A cost:benefit comparison of potential yield loss versus the estimated cost of control is another way to determine action thresholds.
Using the above row spacing, average heads per metre and expected crop value,
Actual counts of larvae (per head)
Potential yield loss: $/ha
   
Using the estimated cost of control and calculated potential yield loss,
Cost:benefit ratio = (target should be 1.5 to 2.0)
  Note that cost:benefit=1 is the break even point. Only when the ratio is greater than one is there an economic benefit from spraying.
 

Using these calculators

The calculators can be used on-line, and users with the latest browser versions of Internet Explorer (9+), Chrome, Firefox or Safari, the calculator can also be used offline. This means that after using the calculator on-line, you will still be able to access and use them offline without an internet connection.

Each calculator includes a brief introduction and instructions. Entry fields for your sampling results and other inputs are highlighted with beige backgrounds, and cells automatically calculated for you have a green background. Some cells may automatically update depending on the cursor placement, but to ensure totals are correct, click on the relevant 'Calculate' button each time you adjust an input value.