Managing silverleaf whitefly and mealybugs

SLW have become more numerous in most regions during February and many crops have been treated with pyriproxifen during the 30 day resistance management window. Now that cotton is opening and crops are moving towards defoliation, it is vital to continue to keep a close eye on populations to avoid a sticky situation.

Top 4 tips for managing SLW mid-season:

The main game for SLW management is to avoid honeydew contamination of open bolls. Good management is underpinned by season-long IPM, effective monitoring and taking action when required. Control actions need to be mindful of SLW numbers, honeydew presence, crop stage, insecticide efficacy and resistance management. As the pyriproxyfen window closes for most valleys this week, here are some tips for managing SLW through until defoliation:

1. Allow sufficient time for pyriproxifen to work. Unlike most insecticides, pyriproxyfen doesn’t immediately kill individuals, but breaks the lifecycle of SLW by preventing adult females from laying viable eggs and late instar nymphs from emerging into new adults. It is not until the current batch of adults die from old age and are not replaced by nymphs that a drop in numbers becomes evident. This will take 16-20 days depending on temperatures, the number of SLW present when the spray went on, and whether there are good numbers of natural enemies present. During this period, honeydew will continue to accumulate so it is important that pyriproxyfen is applied early enough to allow time for SLW control before boll opening. Remember that along with the 30 day window, pyriproxyfen can only be applied to fields once per season. 
2. Estimate the level of honeydew contamination. Once your cotton starts to open, you will need to keep a close eye on the level of contamination (none, light speckling, moderate speckling or leaf sheen). As the open bolls will be exposed to similar amounts of honeydew as the leaves, the development of a leaf sheen whilst there are open bolls can be used as an indicator of potential lint contamination, particularly if conditions are dry (no rain or heavy dews). To do this, hold the leaf up to the sun to see the reflection of SLW honeydew spots. Crops should be managed to prevent the occurrence of a sheen on the lower canopy leaves (see image). 
3. Calculate the time until leaf drop as opposed to the defoliation date. SLW only stop posing a honeydew threat when the leaves are on the ground. If you have large SLW populations prior to defoliation, be sure to account for the time between defoliant application and actual leaf drop. In southern valleys this might take 2-3 weeks, meaning a crop 7-days out from defoliation might still have 3 or more weeks during which SLW can produce new honeydew. If honeydew is increasing or SLW have emigrated from other defoliated fields it may be prudent to apply a knockdown product just prior to defoliation. This is particularly important if the weather outlook is for dry conditions.
4. Don’t panic if the pyriproxifen window has closed. There are a number of different products that have a place for SLW management during the later stages of the season. Firstly, if populations are at threshold and creating problematic honeydew prior to boll opening, don’t be tempted to try and manage populations with knockdown products alone if you are still in the 30 day window. Pyriproxifen is the best choice during the window period at this crop stage. The use of knockdown products at this early stage may result in 2-3 applications being required prior to defoliation as SLW can bounce back due to the disruption of natural enemies. If the window has closed and the crop is at very early boll opening, Spirotetramat (Movento®) can also be an ideal fit. Less disruptive to natural enemies than some other options, Movento’s systemic activity should provide reasonable control of SLW that were not at threshold levels during the window. For problematic populations closer to defoliation, Diafenthiruon (Pegasus®) and several of the registered neonicotinoid based (acetamiprid, dinotefuran) products can provide knockdown relief. These products can effectively reduce SLW numbers in the short term but resurgence is possible within 2-3 weeks of application. The other relatively new suppressive product registered in cotton is Afidopyropen (Versys®). This product stops SLW feeding (resulting in later mortality) soon after application and therefore may have a role for managing honeydew accumulation in certain situations.  It is important to also consider efficacy on other pests also present in the crop. Refer to the Cotton Pest Management Guide for more information on product choices.

For more information, see the CottonInfo booklet Managing SLW in Australian Cotton, & the series of CottonInfo videos on SLW management.

Mealybugs have continued to turn up in new locations this season, highlighting the importance of IPM practices that seek to conserve natural enemies where possible.

Beyond strict on-farm hygiene, once mealybug become present on farm, naturally occurring lacewings and ladybirds are a key line of defence against this pest as chemical control options for this pest are limited. If you find a hot spot of this pest, mark its position, take a note of the natural enemies that are present and revisit the site on consecutive occasions to assess the effectiveness of predators and parasitoids over time. Use this information to your advantage if you need to choose an insecticide for a spray decision, to better inform management decisions going forward.

Controlling mealybugs with insecticides is challenging. Insecticides currently registered for mealybug control may not always give adequate control and should be used only as a last resort. These products will suppress an outbreak of mealybug if natural enemies are providing insufficient control but efficacy is dependent on very specific application techniques that are best conducted with a ground rig to deliver high application volumes with the use of nozzles on droppers to achieve within canopy coverage. A decision support tool for determining whether or not an insecticide might be viable and application requirements can be found here.

Mealybugs are not like most other cotton pests due to their ability to limit insecticide contact by inhabiting concealed places (inside bracts on bolls or underneath leaves) combined with a waxy water repellent coat. The management of this pest the world over depends on good IPM where hygiene and biological control are given priority.

If you have mealybugs be mindful of the movement of people and machinery in and out of any infested areas to limit the on-farm movement of this pest. Ensure that your crop destruction at season’s end is 100% to minimise mealybug overwintering and consider not planting back-to-back cotton in affected fields.