Over the past couple of weeks there have been numerous reports of sunflower crops being defoliated by caterpillars.  

High soybean looper (Thysanoplusia orichalcea) populations (20-40/m²) have been reported in many regions in flowering and podding soybeans. While soybeans can tolerate 33% defoliation with no yield loss during the vegetative stages, their defoliation tolerance slips to 16% during podset/early podfill. Consequently, consider taking action if high looper populations are threatening your crop.

Damaging bean podborer (Maruca vitrata) populations of up to 100+ larvae/m² have been observed in flowering and podding mungbeans in the South Burnett, Dawson Callide and Central Highlands. Although podborer is not an uncommon pest in these regions during wet summers, very high populations can inflict devastating damage with zero pod set observed in some crops where the pest is uncontrolled. In wetter seasons, the pest has also been reported on the Darling Downs, albeit usually in lower numbers. Other crops at risk from podborer attack are adzuki beans, navy beans and pigeon pea.

Typical bean podborer damage to flowering mungbeans. Note the webbing surrounding the damaged buds and flowers.

Damage

Early warning signs are large numbers of the distinctive moths flying in the crop. As the moths are flighty and often difficult to see when at rest, consider using a sweep net to catch them and to confirm the species. Flowers and buds that are webbed together are often the first visible sign of larval damage. However flowers infested with small larvae (<5 mm) may show no visible signs of damage until they are cut open. After initially feeding inside the flowers, larvae move to adjacent pods. Larvae are a pale translucent cream with rows of distinctive black spots.

2)      Then multiply the mean number of infested racemes per plant by the number of plants per square metre, assuming one larva per infested raceme.

 Many severely infested crops found by Entomologist Hugh Brier have populations up to 50 times the current nominal threshold.

Even if SLW numbers are below threshold, it is possible for the team to make collections of either nymphs (on leaves) or adults (using the suction machine). We are currently looking for fields with SLW in any of the cotton-growing valleys to include in the screening for insecticide resistance.

Because the cotton industry is so reliant on 1-2 insecticides to control SLW, it is important that the any changes in the susceptibility of the populations be detected early. Every season since 2008, DEEDI has been screening SLW populations in laboratory bioassays to determine if there is any evidence of resistance developing to pyriproxyfen (Admiral^®), diafenthiuron (Pegasus ^®) bifenthrin (Talstar^®), and more recently spirotetramat (Movento^®).

To date, there has been no change detected in the susceptibility of SLW field populations from cotton-growing regions.

Watch the video to see how the field sampling is done and get a glimpse of a bioassay underway in the lab.     

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 eg, 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

Action thresholds for GVB in cotton are

• 1 bug per metre when sampling with a beatsheet
• 0.5 bug per metre for visual sampling     

Parasitism by Trichopoda – do parasitised GVB cause any damage?

Research shows that a GVB parasitised by Trichopoda can continue to cause damage for up to 2 weeks after being parasitised.

              Parasitised GVB adult           

  Noticeable numbers of grass blue butterfly larvae (Zizina labradus) have been observed in young soybean crops in the Wide Bay Burnett. While the small green larvae (maximum length 10 mm) may be difficult to see in the crops, the damage they cause is very obvious.

Grass blue butterfly larvae feeding results in windowing of the leaves (this is mostly cosmetic) and destruction of vegetative terminals.     

The latter damage is of greater concern. Terminal death (tipping) can result in crops setting pods low to the ground where they are more difficult to harvest. Damage in most crops has been uneconomic to date. 

Thresholds are determined at 25% terminal loss.   

However after the current rain event dies down, growers should re-inspect crops for any terminal death symptoms. Other early signs of infestation of this pest include the small blue-winged adults (butterflies) flitting over the crop, and the presence of their relatively large (0.6mm diameter) pale blue eggs.

Grass blue larvae are often confused with hoverfly larvae. They are of similar appearance and size but hoverfly larvae are key aphid and whitefly predators.. Hoverfly larvae are ‘true fly’ larvae or maggots and they differ from grass blue larva in that they do not have a head capsule, true segmented legs and fleshy prolegs.  

To confirm that you have pest (the grass blue larvae) and not the beneficial, flip them over to check for the (normally hidden) head capsule, 3 pairs of proper legs (at the head end) and 4 pairs of ventral prolegs.  These legs are positioned towards the larva’s mid-line and are not visible from above. Another way to confirm identity is to place the larvae in a jar with some leaves – the hoverfly larvae will not eat the leaves but instead rear up to look for prey while the plant pest will eat the leaves.

Hoverfly larvae                                Grass blue butterfly larvae

 Management

There have been several reports of high numbers of apple dimpling bug (ADB) in early squaring cotton throughout the major cotton growing valleys. Also known as the yellow mirid, ADB adults (about 3mm long) are about one third the size of green mirid adults.  They are yellow-green, have dark spines on the legs and hairy wings that are folded flat on the back. Apple dimpling bugs are capable of moving quickly and have a distinctive apple smell when squashed.