Managing Helicoverpa larvae in chickpea crops close to dessication and harvest.

By admin | Published: November 5, 2008
Over the last week or so we have received a number of enquiries about how best to manage new egg-lays, and populations of small larvae, in chickpea crops that are close to dessication and senescing.

Of most concern are crops that still have reasonable areas of green crop in them, and what the likelihood of damage is if the weather is cool and moist rather than hot and dry.

Hot, dry weather will rapidly advance a chickpea crop which means that very small and small larvae are unlikely to survive on leaves of rapidly deteriorating quality. As the pods dry they also become more resistant to damage by small to medium larvae. In summary, this means that the major source of damage in a scenesing crop is late medium and large larvae.

Therefore, the recommended approach to managing Helicoverpa populations in the later stages of a chickpea crop is to continue to monitor both number and size of larvae. If the population of medium and large larvae exceeds the economic threshold, AND the crop is still susceptible then treatment may be warranted.

The table below gives an indication of how rapidly Helicoverpa larvae will develop at this time of year.

 
 
 
 
 
 
 
 
 
 
Predicted development times for Helicoverpa larvae (Oct-Nov 2008) – Dalby
Up to 3 November the prediction uses 2008 temperatures for Dalby. Beyond 3 November, the predictions use long term average temperatures (long term averages are generally cooler and development slower).

The predictions indicate that larvae are developing from very small to medium in around 7 days and from small to medium in 3 days.

At this stage of the crop, a wait and see approach (continue checking the crop 1-2 times a week) to is recommended principally because it is difficult to predict a week or two ahead how fast a crop will dry down, and what the Helicoverpa population will be whilst the crop is still susceptible. The alternative approach is to treat above threshold populations of small larvae when they are detected. This approach is likely to result in treatment of fields that subsequently would not have been at risk of damage, particularly if the crop dries faster, or larval mortality is higher than expected.

The options available for the treatment of Helicoverpa infestations late are limited because of withholding periods (WHP). Methomyl has a 1 day WHP while thiodicarb has a 21 day WHP. Indoxacarb (StewardTM) has a 21 day WHP, but no more than one application is permitted per crop growth cycle, and the cut-off for indoxacarb use has now passed in all regions (15 Sep in CQ, 15 Oct in warm areas, 30 Oct in cool areas). Check with others in your local area on their experience with the efficacy of options when making a choice.

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Armyworm in wheat

By admin | Published: October 20, 2008
Over the past couple of weeks there have been numerous reports of armyworm in both barley and wheat. The appearance of armyworm in wheat raises a number of questions:
1) Do they behave the same way in wheat as in barley in relation to the type of damage they cause
2) what is their damage potential and is there an economic threshold?
3) What sort of strategy can be used to monitor and manage populations?

For information on armyworm identification see previous Beatsheet postings on armyworms.
http://thebeatsheet-ipmnews.blogspot.com/2007/10/can-you-confidently-identify-armyworm.html

http://thebeatsheet-ipmnews.blogspot.com/2007/09/watch-for-armyworms-in-barley-and-oats.html

There is no reason to expect armyworm to behave differently in wheat to barley. This means you can expect to see feeding on leaves whilst the crop is still green, and then on stems as the crop dries down further.

Characteristic armyworm damage in winter cereals
During the vegetative growth phase, plants can tolerate considerable leaf feeding. Leaves may look tattered from the eaten-out leaf margins. Faecal pellets around the base of plants are another indication of armyworm infestation. Armyworm generally do not require control during the vegetative stage.

Ragged flag and other leaves on a maturing barley crop

The most serious armyworm damage in cereal crops occurs when larvae feed on the upper flag leaf and stem node as the crop matures. Larvae target the stem node as the leaves become dry and unpalatable, and the stem is often the last part of the plant to dry. Head cutting begins at this time.

 
One large larva can sever up to seven heads of barley a day. One larva a square metre can cause a loss of 70 kg/ha grain per day. A larva takes around 8-10 days to develop through the final, most damaging instars, so the crop is susceptible to maximum damage for this period.

 
 
Head cutting in barley caused by armyworm

Calculating an economic threshold
The following table shows the value of yield loss incurred by 1 larva/square m per day, based on approximate current values for wheat and an estimated loss of 70 kg/ha per larva.

Based on these figures, and the relatively low cost of controlling armyworm, populations in ripening crops in excess of 1 large larva per square metre will warrant spraying.

Monitoring and management strategy
For insecticide application to be economic, check or scout the crop and assess the problem before head cutting starts. Check for larvae on the plant and in the soil litter under the plant. Late in the day, when the larvae are becoming active, use a sweep net (or swing a bucket through the crop) to make a quick assessment of whether armyworm larvae are present in the crop. Infestations are often patchy, so check a number of sites across the field.

Some judgements will need to be made about how quickly the larvae will reach damaging size and when this will occur in relation to the crop’s development.For example, if the crop is nearing full maturity/harvest, and the grubs are still small, then there is most likely no need to spray. Small larvae take 8-10 days to reach a size capable of head lopping.The other extreme would be a late crop that is still very green and at early seed fill. In this case, any small larvae present will most likely reach their most damaging size in time to significantly reduce crop yield, and so a spray is more likely to be required.

I you are unable to monitor the crop on a regular (daily) schedule during the critical period of drying down, and armyworm are present, it may be better to spray just in case. This is not the preferred option, but provides peace of mind in a year like this where armyworm are abundant.

Early recognition
It is essential to recognise the problem early and be prepared to spray when economic damage is imminent. A cereal crop can be almost destroyed by armyworm in just a few days. Whilst large larvae do the head lopping, controlling smaller larvae that are still leaf feeding may be more achievable.

Control
Many chemicals will control armyworms. However their effectiveness is often dependent on good penetration into the crop to get contact with the caterpillars. Control may be more difficult in high-yielding thick canopy crops, particularly when larvae are resting under leaf litter at the base of plants. As larvae are most active at night, spraying in the afternoon or evening may produce the best results.

If applying sprays close to harvest, be aware of relevant Withholding Periods. Always read the label.

Biological control agents may be important in some years. These include parasitic flies and wasps, predatory beetles and diseases.

Posted in Winter cereals, armyworm | Leave a comment

Helicoverpa management in chickpea – a refresher

By admin | Published: October 8, 2008

A comprehensive overview of Helicoverpa management in chickpea can be found in the DPI&F brochure Helicoverpa management in chickpea (2007). You can read or download a copy of the brochure at the DPI & F website at www.dpi.qld.gov.au/fieldcrops. Click on the link to Helicoverpa management in chickpea where you will find the brochure.
Key management decisions
The following is an excerpt from the Helicoverpa management in chickpea brochure, and deals specificially with determining whether an infestation of helicoverpa warrants control – based on the economics of potential yield loss vs cost of control.


If control is warranted, which product?
There is a range of products registered for helicoverpa control in chickpea. However, the use of synthetic pyrethroids is really only an option in regions where H. punctigera dominates, or where the population is predominantly made up of larvae smaller than 5 mm in length. The use of SPs against a predominantly H. armigera population is likely to deliver a poor result in terms of control.
NPV (VivusMax) and Bt (e.g. Dipel) are two options which are effective against both species of Helicoverpa. They are most efficacious when deployed to control populations of small larvae (less than 7 mm in length), and lower pressure infestations.
Thiodicarb (Larvin) is another option, particularly where efficacy of this product in the local area is known to be high. Methomyl (Marlin®) could be considered whare large larvae are present close to harvest as it has a 1 day withholding period
Spinosad (Tracer II ™) and indoxacarb (Steward ™) are both effective against both H. armigera and H. punctigera. Remember that Steward has a cut-off for use in chickpea (15 September in CQ, 15 October in warm areas, 30 October in cool areas).
One strategy for the management of mixed age populations of helicoverpa is to use Steward™ first, if prior to cut-off, and then one of the other products if the crop needs to be sprayed again.
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What are those grubs in winter cereals?

By admin | Published: October 1, 2008

Grubs in winter cereals are not unusual at this time of year, and already there have been reports of high numbers (up to 20/m2) in Central Queensland wheat (Figure 1). More grubs can be expected in southern districts as the season warms up.

The two most likely larvae (grubs) found in winter cereals are the corn earworm, Helicoverpa armigera, and the common armyworm, Leucania convecta. See previous blog postings for more information on these pests.
http://thebeatsheet-ipmnews.blogspot.com/2007/10/can-you-confidently-identify-armyworm.html


Figure 1. Large corn earworm larva on a wheat head. (Photo: R. Lloyd)

All Helicoverpa larvae found feeding in wheat, barley or triticale crops will be corn earworm. The native budworm, H. punctigera, is not normally found on monocots (grasses). This is important to know, because the corn earworm has developed resistance to pyrethroids, and unless the larvae are small, a pyrethroid spray is unlikely to control them.

If large larvae are present, identification becomes a somewhat academic issue. However, large H. armigera larvae can be identified by the white hairs behind the head (Figure 2). In contrast, the hairs on large H. punctigera larvae are black. These compare with armyworm larvae which have three pale stripes just behind the head, and smooth skin, without any hairs or bumps.

Figure 2. White hairs behind the head of corn earworm larva. (Photo: R. Lloyd)

If corn earworm infestations are detected early and larvae are small, preferably less than 7 mm in length, Helicoverpa nucleopolyhedrovirus (NPV) sold as Vivus Max could be considered as it will not harm beneficials (predators and parasites) in the crop. Some caution is needed as NPV will not kill corn earworm larvae greater than 13 mm in length, and will have no effect on armyworms.

Invariably when larvae are found on cereal crops, they are medium or large (>13 mm in length) and a more robust option is needed to control them. Both corn earworm and common armyworm are usually present in winter cereals, and control measures will be influenced by the relative abundance of each.

Follow the link below for more information related to thresholds and control options.
http://thebeatsheet-ipmnews.blogspot.com/2007/10/are-corn-earworm-problem-in-winter.html

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Cereal Aphids in wheat and barley Spring 2008

By admin | Published: September 25, 2008

Cereal aphid numbers have increased rapidly over the past 3 weeks as the temperatures have increased. Whilst low numbers of aphids have been present in many crops (wheat and barley) for some time, it was not until a couple of weeks ago that numbers reached levels of concern to agronomists and growers.

Until the last few seasons, cereal aphids have not been considered a major pest in winter cereals. However, higher grain prices mean that the value of any yield loss is higher than it was and control may be economic at the densities we are experiencing.
Which species are in crops this season?
There are several species of aphid that occur in winter cereals (oats, wheat and barley). The most abundant, and the species that has been present in low numbers through winter are the oat aphid (Rhopalosiphum padi – it sounds like Row-pal-o-si-fum pad-i). This species tends to colonise the lower portion of the plant, mature adults are a dark green and rounded. Juveniles are paler and smaller.

On the Downs, the oat aphid is currently the dominant species, with infestations extending from around the base of plants up on to leaves and stems as the crop starts elongation. Smaller number of the rose-grain aphid and corn aphid are also present.
The rose-grain aphid (Metopolophium dirhodum – sounds like meto-pal-o-fee-um di-road-um) is a large, pale aphid with a dark stripe down the midline of the back. This species tends to colonise leaves higher on the plant, and is often very obvious. Clusters of juveniles are common on upper surfaces of leaves.

The corn aphid (Rhopalosiphum maidis – sounds like Row-pal-o-si-fum may-dis) is rectangular in shape rather than round. Legs and antennae are typically dark, the body green-blue, and they may look waxy.

(line drawings from “Insectopedia” Agriculture Victoria, 2000)

In northern NSW, the corn aphid is abundant higher in the canopy, particularly in crops that are booting. Corn aphid is reputed to decline in number as the crop comes out into head.
The photo illustrates a typical corn aphid infestation in a crop of barley prior to head emergence.
How much damage can aphids cause?
There has been surprisingly little work done on cereal aphids in Australia to establish the relationship between aphid numbers, the duration/timing of infestation, aphid species, and ultimately the impact on yield.
Direct aphid damage, as a result of feeding, is difficult to detect. In moisture stressed crops you may see yellowing with high aphid populations. Otherwise, there are generally no early signs of how much impact the aphids are having on the crop.Western Australian recommendation are to check crops regularly from late tillering, and consider control if the aphid population exceeds 15 aphids/tiller on 50% of tillers.
The WA research showed yield losses of up to 10%, and reduction in seed size, with aphid infestations (this was without any impact of barley yellow dwarf virus).
Queensland DPI&F recommendations have been to:
Check 5 plants at 6 sites within the field. If 27/30 (90%) of plants are covered with aphids, and there are less than 2 natural enemies per plant, then consider treatment.
A 90% infestation level would be indicative of a well established population. Early infestations tend to be patchy, and become more uniform as the population builds up.
Checking a crop for aphids
Sample away from the edge of a field. Aphid numbers tend to be higher around field margins because this is where initial infestations start. The rest of the field will be more representative of the infestation in the majority of the field.
It is simpler to base estimates of infestation on tillers rather than whole plants. It can be difficult to determine where an individual plant starts and stops, and the number of tillers per plant can be variable.
Taking a representative sample of individual tillers from across a field will provide information on the number of aphids, and the proportion of the tillers infested. The lower the infestation the more tillers you will need to sample (e.g. 30 per management unit). The more established the population the more uniform the infestation will be and the number of tillers sampled can be reduced (e.g. 10-20 tillers may be sufficient). Record the number of aphids per tiller and see how consistent numbers are as you go. Lots of zeros means the population is patchy.
If numbers are high, you may want to use a rating system for estimating density rather than actually counting aphid numbers.
For example: 0= no aphids, 1= 1-10 aphids, 2= 10-20 aphids, 3= 20-50 aphids, 4= more than 50 etc. Once you have your eye in, a rating system is quicker than counting aphids.
It may be useful to rate the number of aphids above and below the flag leaf separately. This will be particularly useful for assessing how effective a spray has been, and determining if surviving aphids are those that may have simply not been contacted.

Information from overseas research (Canada, US) suggests:

  • that significant yield loss occurs when aphids are present from the flag leaf stage through to milky grain – no yield loss occurs with infestations later than milky grain
  • infestations of aphids on the flag leaf, and upper portions of the crop, including on the heads, cause greater yield loss than infestations lower in the canopy
Other considerations when making a decision about cereal aphids
  • Corn aphids may disappear by themselves. Corn aphids, the species that colonises the upper canopy, reputedly decline in number when the crop comes into head. This may be because they tend not to survive as well on leaves as they do on the flag leaf or in the whorl.
  • Natural enemies (lady beetles, hoverflies, parasitic wasps) can have a big impact on aphid populations, reducing them to very low levels in many instances. This is particularly important in managing the resurgence of any aphids that survive a spray.
  • Dimethoate and synthetic pyrethroids (e.g. Bulldock®) are highly disruptive to natural enemies. The application of these insecticides early may result in a later reinfestation of the crop because small numbers of surviving aphids are no longer controlled by natural enemies. The impact of these products on natural enemies can persist for some days.
  • Pirimicarb (e.g. Pirimor®) is a soft option for cereal aphid control, but be aware of the with-holding period.
  • there is no Australian data on resistance to any of the registered insecticides in cereal aphid populations.
  • Oat aphids, at the base of the plant, can be difficult to contact in a dense crop, and with aerial application.
  • Rain will reduce aphid populations by knocking/washing individuals of plants, particularly if the rain is high intensity (storm) rain. When washed off, aphids tend not to get back on the plants. Often ground predators, like carabid beetles, ants etc will eat aphids on the ground. It may be worth re-checking numbers if you get a storm between checking and applying a spray.
Posted in Winter cereals, aphids, barley, wheat | Leave a comment

Farm hygiene important in pest management

By admin | Published: April 17, 2008
David Murray, Toowoomba

Farm hygiene is an important component of integrated pest management (IPM), particularly when it comes to managing pests such as cotton aphids and Cotton Bunchy Top (CBT) disease.

Recent surveys of cotton-growing areas indicate the presence of aphids and CBT, and wetter conditions through the coming winter could favour the growth of weeds that are hosts for aphids and also increase survival of cotton volunteers that carry CBT to the next season.

The photo (right) shows volunteer cotton with CBT symptoms adjacent to the current season cotton crop (Photo: Lewis Wilson, CSIRO)

CSIRO and Cotton CRC entomologist Dr Lewis Wilson suggests that growers maintain good farm hygiene to reduce the risk of aphid or CBT problems next season.

CBT can stunt the growth of cotton plants and, if plants are infected when young, dramatically reduce yield.

CBT is spread by cotton aphids when they feed. Both cotton aphids and the disease need a host plant for survival through winter.

Cotton is a good host and volunteer or ratoon cotton plants can be found on farms all year. These plants can carry the disease and aphids through winter. Aphids can then move to cotton crops in the following spring and infect plants with the disease.

Photo: Fallow field with a high number of volunteer cotton plants, a potential resevoir for cotton aphid and CBT (Lewis Wilson, CSIRO).

While it is likely that CBT will survive on alternative weed hosts, these relationships have not been studied.

In recent field inspections on the Darling Downs, cotton volunteers were found on virtually every farm.

Many of these volunteers showed clear symptoms of CBT, such as angular leaf mottling and small leaves (see photo below, Photo: Lewis Wilson, CSIRO).

Cotton CRC extension officer Rod Gordon said that regular rainfall through the summer had encouraged growth of cotton volunteers, many of which were glyphosate-tolerant and difficult to control with herbicides. Wet fields have also limited opportunities to control volunteer cotton, with some fallow fields and field margins containing quite high densities.

The importance of controlling volunteer cotton in fallows can not be stressed enough. Minimising host availability is critical to ensure that we do not see a repeat of the 1998 CBT outbreak.

More information on cotton aphid and CBT can be found on the Cotton Catchment Communities CRC website at
http://www.cottoncrc.org.au/content/Industry/Publications/PestsandBeneficials/CottonBunchyTopCBTCharacteristicsandModes.aspx

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Accessing the Diapause Tool – An alternative address

By admin | Published: March 27, 2008

If you are having trouble with the link to the Diapause tool provided in the story below, try the following:

http://tools.cottoncrc.org.au/xl2/diapause/index.aspx

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Diapause tool to identify helicoverpa risk

By admin | Published: March 20, 2008
While there was a hefty presence of corn earworn or cotton bollworm, Helicoverpa armigera, in the early and middle part of the 2007-08 season, pest activity has declined in recent weeks and for the most part they appear to pose no major risk.

What is diapause?
This is the time of the year when a proportion of mature larvae going to ground to pupate enter a hibernation phase termed diapause or overwintering. This dormancy strategy allows the pest to survive the winter months in temperate regions when host plants are scarce and temperatures are generally too low to allow successful development. The triggers to enter diapause are decreasing daylength and temperature as experienced during late summer and autumn.

Picture of helicoverpa pupa in earthen cell.

The proportion of pupae entering diapause increases from low levels in March, to high levels, almost 100%, by late April. The rate of diapause induction varies from season to season, and region to region. Knowing when diapause is induced is useful for identifying ‘high risk’ fields i.e. those fields most likely to have diapausing pupae.

A web tool is available on the Cotton CRC website to help calculate the likely rate of diapause induction for your area, based on local climate data. The tool is also able to compare the results for the current season with the long term average and hotter than average and cooler than average seasons. Follow this link:
http://tools.cotton.crc.org.au/cl2/diapause/index.aspx

How are diapausing pupae controlled?
Overwintering pupae are very important because they contribute to the spring population and may take with them the resistance genes enabling them to tolerate conventional insecticides and the Bt transgenic toxins found in Bollgard II®.

It is for this reason that full soil surface cultivation to 10 cm depth (also known as pupae busting) is so important. When carried out properly, pupae busting can reduce survival of overwintering pupae to less than 5%.

Pupae busting is mandatory for all Bollgard II® fields; it is a requirement of the Bollgard II® licence.

Some relaxation of pupae busting requirements has been introduced for conventional cotton fields. Sprayed conventional cotton crops defoliated after 9 March are more likely to harbour insecticide resistant H. armigera pupae and should be pupae busted as soon as possible after picking and no later than the end of August.

The same holds true for the majority of grain crops on the Downs. Crops that were mature on or before 9 March are unlikely to harbour overwintering pupae. Conservation tillage (zero till or min-till) can be confidently used on these fields. Late season grain crops that could support development of larvae after 9 March are ‘high risk’ of harbouring overwintering pupae, and each field should be judged on its merits. The decision on whether to pupae bust will be influenced by the density of larvae present in the crop, the age of the larvae, and the timing of their presence in the crop.

Parasites will also influence survival of overwintering pupae, with estimates from a 5 year study on the Darling Downs showing 44% of overwintering pupae were parasitised. The two-toned caterpillar parasite, Heteropelma scaposum, was the most abundant parasite species recorded.

What is the current seasonal outlook for diapause in cotton and grain crops?
Output for the current season at Dalby, which is cooler than average, indicates a higher than average proportion of pupae have entered diapause.


All cotton crops, and a large proportion of the grain crops on the Downs, and in most other areas, will be harvested later than mid March. This means it is worth checking and recording whether these crops are hosting larvae that will diapause. Crops determined to be ‘high risk’ will warrant pupae busting.

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Whitefly management options

By admin | Published: February 29, 2008

In the last week, reports from the Downs are suggesting that silverleaf whitefly (SLW) numbers have increased rapidly, and now there are a number of fields that have a population at or exceeding the treatment threshold.

This posting has been compiled from information and discussion with Richard Sequeira (Principal Entomologist, Emerald) and Paul Grundy (Senior Entomologist, Ayr) who have considerable experience in managing SLW. The aim is to provide information that may help in making decisions about the need for and timing of control, and the appropriate control option.

It appears that there is a larger than predicted population of SLW in cotton this season, especially as the season is average in terms of temperature. Why is this so particularly on the Downs where it is predicted that outbreaks would only occur in hotter than average seasons?
The answer to this is that outbreaks of SLW are driven not only by temperature, but by two factors, the size of the initial population in spring and the summer temperatures. In the 2007-08 season we have experienced temperatures that are close to the long term average, but we would have started the season with a large carryover from the outbreak in 2006-07. The rainfall and weed growth in winter and spring would have provided hosts for the SLW to carryover from last season to this one.

Over the last week or so, SLW populations seem to be increasing with the percentage infestation rising rapidly. In Central Queensland (CQ) it is usual to see a rapid increase in the percentage infestation at the 5th node once the crops cut out. It looks as though the population is increasing in size, but what is actually happening is movement of the population up towards the top of the plant as it stops putting on nodes. In CQ the quality of the leaves lower on the plants also starts to decline around this time and becomes unsuitable for SLW, forcing them higher in the canopy. Paul suggested that there are visual clues to whether the population is increasing in number. Observing the amount of honeydew on the lower leaves can be instructive. Over a week, an increase from a light speckling to larger droplets is indicative of a population increase.

As a SLW population approaches threshold natural enemies will not contain it. It is interesting to note that there has been very little parasitism recorded from samples taken in St George, and on the Downs this season. Parasitism levels in CQ are also low. Richard’s interpretation of this is that wet weather and high humidity has a negative impact on the parasitoids.

As SLW populations approach, or exceed, the treatment threshold, questions arise around the issues of whether to control, when and what product to use. The SLW thresholds are available in the Cotton Pest Management Guide, and on the web at
http://www.dpi.nsw.gov.au/agriculture/field/fibres/cotton/cotton-pest-management-guide Section 2, pages 11-13

To use the threshold recommendations it is necessary to calculate the day degree (DD) accumulations for the season, this will enable you to match the crop stage to the appropriate threshold and management options. The current DD data for a range of cotton-growing regions is presented in the table below.

Weighing up the options
In making a decision about SLW control, it is important to consider not only the level of infestation but also the age of the crop (DD accumulation) , how long it has to go until open cotton or leaf drop, and what is going on in neighbouring crops.

The situation on the Downs currently is that there are crops at SLW threshold that do not yet have open cotton. In this situation, Paul suggests that a second check be made on the population a week after it is first recorded to be at threshold, just to make sure the population really is at that level. If it is, it is necessary to weigh up the options for control.

Richard’s sense was that without open cotton, there is no urgency to control the population. Potentially you can wait a week or two until closer to the first open boll before applying Admiral®. Certainly, the application of Admiral® should be closer to 1600 DD than 1500 DD (first open boll is at 1650 DD). The aim of this strategy to decrease the chance that there could be a resurgence of the population post treatment, still with time to contaminate open cotton. However, on the Downs, the expectation is that the temperatures will be cooling through March and into April, and the risk of a second large population is unlikely.

Another approach is to treat populations at threshold now with Admiral®, particularly knowing that it has 3 week residual, and that SLW will not start to breed in the crop until April. With this timeframe, particularly in cooling conditions, it is unlikely that SLW will build up to threshold before the end of the season.

Pegasus® will knock a population of SLW, and control aphids in the crop. However, given the length of season many crops still have to go, the use of Pegasus ® now may require a re-treatment with Admiral ® later. Whilst Pegasus ® is the less expensive option, it is more likely to require re-treatment if used now than the application of an Admiral ®. Pegasus ® is an option for late crops where SLW populations do not reach threshold until there is open cotton and a quick knockdown is needed late in the season. If using Pegasus® remember that it is a contact product and good coverage is essential for good control.

Posted in Cotton, Silverleaf whitefly | 1 Comment

Whitefly Update

By admin | Published: February 22, 2008

Reports of increasing numbers of whitefly are filtering in from across the Downs. In some cases, numbers are sufficient to suggest that control of Silverleaf Whitefly (SLW) may be warranted. Correct identification and regular monitoring of populations is essential to determine if intervention is necessary.

Which whiteflies are out there?
Results of whitefly samples identified from Downs cotton crops this week indicated variable incidence of SLW. In all cases except St George, identifications were based on immature (scale) stages used to differentiate Bemisia tabaci (SLW and Native Bemisia) from Greenhouse Whitefly (GHWF). These results infer that all Bemisia were SLW. Very few parasitoids were found in collections.

Anecdotal evidence supports the above findings; that in some cases the whitefly infestations have been predominately GHWF. Where pyrethroids have been applied to control stink bugs (green vegetable bug and cotton stainers), whitefly numbers have been suppressed. While GHWF is susceptible to pyrethroids, SLW is likely to have some level of resistance to pyrethroids and will survive treatments. The risk is that use of pyrethroids and other broad spectrum insecticides used to control bugs may well flare SLW by disrupting natural enemies.

The Downs is considered a marginal area for problems with SLW because in an average season there is not enough heat and generally too few generations to create a problem. There is a perception that the 2007/08 season has been cooler than normal. Day Degree accumulations indicate that 2007/08 has been marginally cooler than average (see Table 1). Surprisingly, it has not been as cool as many have expected.


Management of SLW
Cotton crops are approaching the stage where decisions must be made on whether to control whitefly present in crops. Firstly it is important to know which whitefly is present. Is it SLW?

Given positive SLW identification, are they at levels that will present a problem later on in the crop cycle? To answer this question, careful monitoring of adult numbers is required.

Monitoring SLW infestations
Percentage infestation is based on a fixed sampling protocol, where a leaf is considered infested if 2 or more adult whiteflies are present. Full details of the sampling protocols and threshold values are available in the Cotton Pest Management Guide or the Cotton CRC website http://www.dpi.nsw.gov.au/agriculture/field/fibres/cotton/cotton-pest-management-guide

Control options
The objective of SLW control is to manage the population early to minimise exposure of open cotton to honeydew contamination.

Management options as defined by crop stage, accumulated Day Degrees and corresponding whitefly thresholds – include CONTROL, KNOCKDOWN and SUPPRESSION.
The CONTROL option involves application of an Insect Growth Regulator (IGR) and is appropriate for moderate to high densities after 1300 DD.
The KNOCKDOWN option involves application of conventional chemistry and may be used on low to moderate densities after 1600 DD to minimise contamination of lint from lower bolls.
Low to moderate densities after peak flowering (1300 DD) may also be treated with conventional chemistry for population SUPPRESSION so as to avoid the need for a later application of IGR.

Use of Insect Growth Regulators (IGRs)

  • Where % infested leaves is consistently equal to or above the corresponding threshold value (> 1 adult/leaf after 1300 DD), use IGR from 1500 DD but no later than 1600 DD (first open boll).
  • IGR – Pyriproxifen (Admiral®) – is highly effective against SLW, gives excellent control across a broad density range and is the corner stone of effective SLW management in cotton as it is also very selective, allowing survival of predators and parasites.
  • Ensure only a single application of Admiral® occurs within a season.
  • Delaying the IGR application beyond 1600 DD and > 2 adults/leaf could result in yield loss, lower efficacy of the IGR, substantial lint contamination or all of the above.

Use of Conventional Chemistry

  • Adult SLW densities of 0.5-1 adult/5th node leaf beyond 1600 DD (first open boll to leaf drop) are sufficient to contaminate lint from earliest open bolls.
  • After first open boll (>1600 day degrees) and densities <>
  • Single applications of Pegasus® or endosulfan (if window permits) used early (1300 – 1450 DD) on SLW densities <>
What contribution might natural enemies make to SLW control?
Natural enemies make an important contribution to the suppression of SLW when populations are low. Natural enemies include ladybeetles that will feed on scale, the parasitoid wasps, and probably a number of predatory bugs. However, their potential impact is not significant enough to be taken into consideration once the SLW population gets to the point that in-crop control (IGR application) decisions are being made.
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