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Using your phone to hunt SLW

In part one of our focus on IPM, we looked at some of the more sporadic pests encountered this season. This week, we take a closer look at that familiar foe - SLW - and the new tools now available.  

Silverleaf whitefly (SLW) is a pest that must be factored into management considerations for the whole season. Insecticide usage earlier in the season for other pests such as mirids or green vegetable bugs (GVB) will likely influence the activity of SLW and their natural enemies later in the season. The consequences of honeydew contamination in harvested lint can be substantial, both for individuals and the industry as a whole.

1. Prevent honeydew contamination on open cotton 

A wetter and cooler season has slowed the rate of SLW population development across most regions compared to previous years. Despite the delay, sampling of crops across many regions indicates that SLW are established and populations could rapidly increase, particularly if conditions become hot and dry during late February or if spraying for GVBs has disrupted natural enemies.
The key objective for SLW management is to prevent honeydew contamination of open cotton bolls. A light sparkle on the top sides of the lower canopy leaves is a warning sign that honeydew is starting to accrue within the crop. If this sparkle develops from pin prick spots to pencil sized dots within a week and is progressing towards a leaf sheen you need to consider your control options for SLW (taking into account when boll opening might commence). If you are near or have open cotton, treat the SLW immediately with a product appropriate for the crop stage, population density and time to defoliation. Crops that develop a honeydew sheen on the lower canopy leaves and receive no rainfall between boll opening and picking are at the greatest risk for lint contamination.
The primary focus should be to avoid this level of contamination before it occurs and this season there are new tools at hand to help you avoid a sticky situation.

2. Use the NEW Decision Support Tool to guide decision making

The SLW control matrix in the Cotton Pest Management Guide has been superseded by a new Decision Support Tool (DST). Developed by Dr Richard Sequeira of QLD DAF and collaborators, this new method relies on nymph counts from leaves in the mid-lower canopy. Nymph sampling can more accurately predict where the early trajectory of a SLW population is heading, enabling more timely management decisions.
A detailed description of this method and the Excel-based calculator can be accessed from the CottonInfo website.

Key steps of this sampling strategy are:
  1. Sample main stem leaves from the middle/lower canopy (11-14 nodes from the top of the plant). Collect up to 30 leaves from spaced locations along a transect that is representative of the crop.
  2. Examine the underside of leaves for large red-eyed SLW nymphs. A proportion of these may be parasitised, therefore use a hand lens or other form of magnification to distinguish between viable nymphs and parasitised/dead nymphs, ignoring empty cases. A video on detecting parasitised nymphs can be found here.
  3. Enter your data (viable vs total nymphs) and day degrees into the DST so that you can track the SLW population trajectory within your field against the colour coded risk matrix.
  4. If a spray decision is warranted, balance the range of factors that can influence insecticide choice. Things to consider include:
    • Severity of SLW infestation.
    • Product mode of action and expected time to achieve control.
    • Crop stage, open bolls & time to maturity.
    • Natural enemy activity.
    • Resistance management considerations and/or restrictions.
As determining nymph status (step 2) takes time, focus more on this step as nymph numbers increase and the DST tool suggests that the time for control may be near. This new video explains the basics for field sampling nymphs for use with the DST.

3. Use your smartphone to join the hunt for SLW

The Cotton PestDetect App is a digital tool being developed to assist with sampling for SLW nymphs by providing image-derived insect counts using your phone’s camera. Following an initial test run last season, researchers at USQ and QLD DAF improved the accuracy of the camera app which now includes a range of new features.

The biggest of those new features is the automatic plotting of results for the SLW DST. The app automatically records the accumulated day degrees for each field based on the provided GPS location, so you can see the latest results for that field as soon as you are finished taking photos.

As part of ongoing development, a beta version of the app has been released for this cotton season and is available for interested growers and consultants who will be sampling whitefly and aphids in the coming weeks. To get started, contact Derek Long from USQ, who will create an account for you and send you a link to the app.
CRDC, USQ and QLD DAF are currently looking for a commercial partner to deliver this exciting new technology to growers/consultants. For more information, click here.

4. Consider resistance and your local pyriproxyfen usage window

Resistance to pyriproxyfen was first detected 4 years ago but continues to fall thanks to current mitigation tactics that limit the chances of SLW being treated more than once in each valley (Fig 1). This season’s 30 day usage window dates for pyriproxyfen for each valley can be found here.

The registration of buprofezin for SLW late last year now provides the cotton industry with another unique mode of action that does not have cross-resistance with pyriproxyfen. The availability of an increased number of products with alternative modes of action over the last 5 years combined with ongoing stewardship guidelines means the industry is well placed to manage SLW without further exacerbating resistance.
Figure 1. Percentage of SLW populations collected from Australian cotton that were positive for resistance to pyriproxyfen. Number of populations collected and tested each season is given above each column (Source J. Hopkinson QLD DAF).

5. Look after your friends.

The management of SLW is a lot easier in a farming system that has not been disrupted by broad spectrum insecticide use for other pests such as mirids. One of the reasons why highly selective products such as Insect Growth Regulators including pyriproxyfen and buprofezin work well is that these products leave the majority of SLW natural enemies in place after application. These natural enemies are very effective at 'mopping up' any SLW that survive treatment (including potentially resistant individuals), minimising the chance of SLW resurgence and potentially helping delay resistance.

6. If you have honeydew on open bolls, what should you do? 

At the end of the day we are all human and while we are doing our best to manage a dynamic biological system, things may not go to plan. If you suspect honeydew contamination might be affecting the open bolls in your crop there are steps that you can take that may help: 
  1. Delay picking for as long as you can to increase the likelihood of rainfall exposure, as weathering can significantly reduce stickiness levels. While this might increase the chance of a lint colour downgrade, it is preferable to honeydew detection by a spinner, which poses the risk of larger financial penalties and may tarnish the reputation of Australian cotton as being honeydew free. 
  2. If rainfall is unlikely or harvest cannot be delayed, talk to your ginner and/or marketer to explore alternate handling and marketing options.
For more information: contact your local CottonInfo REO or CottonInfo IPM Technical Lead, Paul Grundy, and download this fact sheet on managing SLW.
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