Management of whitefly this season will be challenging as a result of the high rainfall and flooding in November and again in February which has resulted in a wide spread of crop maturity both between and within each region. While some cotton crops will mature on time, others could be set back by as much as six weeks by the cool overcast and wet conditions.
Whitefly numbers increased in most cotton growing regions during February although infestations reported vary greatly from zero to 80% of leaves infested. The recent warm temperatures will likely lead to an increase of whitefly numbers in crops as well as through migration. Whitefly that migrate into cotton crops come from hosts sustained by regular rainfall but which are now becoming less palatable to whitefly.
So given the weather conditions, crop maturity and increasing whitefly numbers – how do we manage SLW? The following scenarios illustrate how SLW may be managed in a range of situations.
When crops are on time
The threshold matrix (below) should be used to guide whitefly management decisions for crops that are on time, or close to it. This matrix provides an excellent measure of projected population build up and action thresholds for crops that are on time and not subject to mass SLW migrations from surrounding areas. The day degree calculator found at http://CottASSIST.cottoncrc.org.au helps to assess whether crops are maturing on time. The 2011/12 Cotton Pest Management Guide (p 26-28) provides control recommendations using the threshold matrix.
When crops are late
Whitefly populations in late crops are likely to be a combination of resident and immigrating whitefly. In this situation, the threshold matrix may not be accurate. as the matrix is based primarily on SLW populations building up in crop without migratory influxes. In addition, the day-degrees that a crop accumulates may no longer align well with the matrix due to flood damage or waterlogging.
A crop manager faced with this scenario should aim to avoid honey dew contamination of open bolls. This means that prior to open bolls, influxes of whitefly can be tolerated.
Delay treatment?
The delay of treatment on late crops may mean that cooler temperature in March/April slows whitefly activity and honeydew production. It also allows more settling time for migratory whiteflies before treatment is applied. Treating too early maybe partially ineffective because of further crop re-invasion by adult whitefly as the treatments efficacy declines. Treating too early increases the risk of having to retreat.
Control decisions for late crops and/or where there are influxes of SLW, should be based on:
- Origin (e.g. local build-up or mass immigration) and numbers of SLW
- Presence of open bolls
- Expected time until defoliation leaf drop
- The rate and level of honey dew accumulation on the crop canopy and lint
- The likely efficacy and residual impact of the chosen insecticide.
- Relevant product withholding periods (WHP’s).
>Product choices for SLW are primarily limited to knockdowns such as Pegasus (Diafenthiuron) and Pyrethroids (Bifenthrin) or the slower acting IGR’s such as Admiral (Pyrproxifen) and Movento (Spirotetramat). Resistance is a threat to these products, so follow the IRMS. Admiral can only be applied once.
Late crops with low SLW numbers
Crops with very low whitefly populations, <10% infested, and little honeydew at boll opening should not require control. However numbers and honeydew should continue to be monitored and if there is a sudden increase in whitefly, due to an influx from neighbouring fields, control with a knockdown may be warranted
Late crops with moderate to high SLW
If numbers are moderate to high and defoliated leaf drop is 3 or more weeks away an IGR may be the best control option. Pyrproxifen has excellent residual and will mostly prevent the continued build up of resident and immigrant SLW. The Central Queensland experience strongly suggests that Pyriproxyfen (Admiral®) works even better at lower autumn temperatures than it does in summer and is capable of cleaning up dense whitefly populations on late cotton effectively with a single application. Continue to monitor whitefly numbers and honeydew and if adult numbers begin to rebuild – a knockdown may be required.
Crops less than 2 weeks from defoliation
If the crop is less than 2 weeks from defoliation and an influx of adult SLW occurs, a knockdown type product may provide better value, keeping in mind relevant WHP’s. Continue monitoring of honeydew and if whitefly numbers start to recover and cause honeydew close to defoliation – consider defoliating a few days earlier. Once defoliant is applied adult whitefly will generally leave the crop and falling leaves will take the nymphs with them.
Monitor for honeydew
It is difficult to determine at exactly what point, levels may become problematic once bolls begin to open. In CQ and during later outbreaks on the Darling Downs, honeydew was considered problematic when leaves on the lower canopy became heavily speckled with honeydew. If the leaves are at or get beyond this level and develop a honeydew “sheen” then corrective action is required immediately.
Other considerations
Some crops will have an earlier and later maturing phase of bolls. It is important to manage whitefly to reduce the risk of contamination of the earlier bolls. The basic strategies outlined above should be used in relation to the earlier bolls e.g., delaying the first treatment as late as possible, but not later than 5% open bolls. However, with ongoing monitoring of whitefly and honeydew as a guide, a second application of an insecticide from a different group may be required. Earlier defoliation may also be beneficial.
If, despite efforts to manage whitefly, crops end up with significant honeydew contamination – e.g. lower bolls dark with sooty moulds – then picking should be delayed as long as possible to expose the lint to maximum weathering, especially rainfall, which will help reduce honeydew levels. The long range forecast indicates a high probability of rainfall late February and at various intervals throughout March. In general however, the costs of preventing the problem will be far less than the potential grade penalties for weathered lint – so proactive management is the best option.
Article by Ian Taylor, Richard Sequeira, Paul Grundy and Lewis Wilson.
This article first appeared in the Namoi Valley Cotton tales