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Cotton aphids are a regular pest of cotton.  Cotton aphids (Aphis gossypii) have the potential to affect cotton productivity in three ways;
  1. by feeding on phloem sap, thereby competing with the plant for the sugars produced by photosynthesis
  2. by spreading the disease ‘Cotton Bunchy Top’ (CBT) which can significantly reduce cotton yield and
  3. by secreting sticky honeydew onto open bolls – contaminating the lint resulting in downgrading.
The Aphid Yield Loss Estimator is designed to help growers deal with point (1) – the direct effect of aphid feeding on yield.  It allows crop managers to enter scores of aphid abundance and obtain an estimate of the likely effect on yield.  On-going entry of new counts enables this estimate to be adjusted, allowing for faster or slower development of populations.
 
Sampling Aphids
 
Aphids are challenging to sample, as they are patchy in distribution, small and too numerous to count quickly.  A simple scoring system has been developed which involves scoring the density of aphids on the underside of main-stem leaves on the 3rd or 4th node below the terminal.  A complete description of this scoring system can be found in the Australian Cottongrower (October- November 2008) Volume 29, No 6. Pages 12-16 and in the Cotton Pest Management Guide available on the Cotton CRC Website.

The Aphid Yield Loss Estimator takes the regular aphid scores and calculates the sample aphid score (SAS) which basically allows for the time between the current and previous scores.   The SAS is calculated as;

SAS = (Previous score x days since last score) + ((current score – previous score) x days since last score/2)

The SAS is then accumulated across sample dates to give a ‘cumulative seasonal aphid score’ (CSAS) which is used in the formulae to estimate yield loss (see below).
 
Using the Cotton Yield Loss Estimator
 
A simple formula has been developed that allows prediction of yield loss due to aphid feeding with a reliability of about 85%.  The equation uses the CSAS (see above), the time remaining in the season from when the aphids were first found (TREM) and the cotton growing period for a the particular region (season length, SL) to predict yield loss.  For any given CSAS a longer TREM will result in a higher yield loss.

The crop manager first needs to register with CottASSIST and set up the farms and crops they wish to monitor.  Once this is complete, crop managers can enter aphid scores for particular crops by going to the Aphid Yield Loss option and selecting Your Aphids samples.  Select the farm and field then enter the data requested (average aphid score AAS), check the boxes if necessary and click Save Observation.

Click the Analyse option.  A page showing the data and charts for yield loss, CSAS and AAS will display.  The yield loss and CSAS charts have a horizontal green line on them which indicates the nominal threshold of 4% yield loss which justifies an aphid spray.   This can be used to ‘look-ahead’ at what might happen if aphid populations continue to grow at the same rate, faster or slower.  If the crop has been sprayed to control aphids or if aphids decline to zero over 2 consecutive checks; then the TREM will be recalculated as described below.  When this occurs, a new green line will appear indicating the updated number of CSAS required to cause 4% yield loss.

If a field is sprayed to control aphids, this box should be checked when entering the data.   The Aphid Yield Loss Estimator will use aphid scores entered in the seven days after spraying to establish if the spray has been effective, using the criteria that an effective spray should reduce the AAS to less than 0.5.  If the spray has been effective the CSAS and yield loss will be reset to zero and the TREM set to the current date.  This will be indicated by a vertical red line on the charts.  This allows any subsequent build–up of aphids to be managed as a new event.  If the spray was not effective a warning is given that the crop may need to be resprayed.  The CSAS and yield loss will in be reset and the TREM will remain at the value before the spray was applied.  This assumes that the aphid population was not controlled effectively and the risk of yield loss remains.

Sometimes a low aphid population will be virtually eliminated by beneficials, then re-appear sometime later.  In this case using the TREM from when aphids were first found could result in a higher yield loss prediction than would actually occur.  So, to prevent this, if the AAS is zero for two sample dates in succession, even if there is no spray, the CSAS and yield loss are reset to zero and the TREM is recalculated using the current date.  This is defined as a 'natural reset' and is indicated with a blue vertical line on the charts.

If economically preventable yield loss is predicted a control spray may be required.  When deciding whether or not to spray the value of the crop and the cost of control should be used to determine how much yield loss can be tolerated before intervention is required.  Leaving non-economic aphid populations saves money and also allows beneficials the chance to build and control aphids and other pests.  Crop sensitivity to yield loss from aphids declines as the crop gets older.

The Aphid Yield Loss Estimator should only be used between squaring and first open bolls.   Before squaring aphid populations are often eliminated by predators or parasites.
 
Managing to avoid lint contamination with honeydew
 
Once crops have first open bolls, the risk of aphids reducing yield is low, but the risk of contamination of lint with honeydew increases and pest managers should revert to the existing threshold recommendation: First open bolls onwards: 50% of plants infested with aphids, or if honeydew is present, 10% of plants infested.
 
Managing to avoid the risk from CBT
 
Aphids can spread the disease CBT. The risk of CBT developing in a cotton field depends on:

  1. The timing of infection and proportion of plants infected.
  2. The number of aphids entering the field and the number of aphids per plant.
  3. The proportion of aphids carrying the disease.
  4. The latent period.  This is the time between an aphid carrying CBT feeding on a plant and that plant being capable of infecting new aphids.
These issues are discussed in detail in the Australian Cottongrower (October-November 2008) Volume 29, No 6. Pages 12-16.  Under most circumstances pest managers can manage aphids according to their potential to reduce yield from feeding rather than via disease transmission.  Regular checking in areas where aphids were initially found will confirm if CBT was brought in and if it has spread.  An exception would be in a field where there is a high incidence of volunteer or ratoon plants in or around the field, especially if a lot of cotton aphids are noticed in the field early season.  In this case it is likely that aphids on the ratoons or volunteers will carry CBT, and these areas should be monitored carefully and aphids controlled if necessary.
 
Cotton Pest Management Guide

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