Silverleaf whitefly in cotton – an update

By admin | Published: January 23, 2009

Silverleaf whitefly (SLW) is a serious pest of cotton. It reduces yield and quality of cotton due to feeding damage and excretion of honey dew. It is a difficult pest to manage due to its ability to rapidly increase in numbers and the development of resistance to many insecticides.

Resistance testing for the 2007-08 season indicated no alarming results for Admiral® (pyriproxyfen) or Pegasus® (diafenthiuron) in cotton areas. The IRMS guidelines for Admiral® require that only 1 spray may be applied per season. Apart from the cost involved, more then one Admiral® spray has been shown to increase the development of resistance.

As part of the management strategy for whitefly it is important to know what species or biotypes are present as these will significantly impact on the management decision that is required. Refer to past beatsheet articles to read more on identifying the different species and biotypes of whitefly.

We are seeing mixed populations of whitefly across Queensland. On the Darling Downs, greenhouse whitefly (GHW) has made up >90% of the population in the Norwin area. A sample from Theodore showed the whitefly population was made up of 70% SLW and 30% GHW.

In Emerald, a limited number of Pegasus® sprays have been applied for SLW. Pegasus® is best used for early season suppression of SLW at low insect densities or as a late season knock down to prevent honey dew contamination of open bolls.

Very few (if any) Admiral® sprays have been applied so far this season. Admiral® may be applied after 1450 day degrees if SLW numbers reach high densities. Remember that only one Admiral® spray may be applied per season to limit the potential for resistance to develop.

An excellent publication has been produced through the Cotton CRC by Richard Sequeira and Tracey Farrell That outlines thresholds and sampling methods for SLW in cotton in more detail (www.cottoncrc.org.au/files/5743fee2-f978-4a79-a9d1-9b1800e899cf/Whitefly_Management.pdf). This should be referred to when making management decisions for this pest.

In Emerald, there were high levels of natural control of whitefly from the parasitic wasps, Eretmocerus hayati and Encarsia formosa. Parasitism levels of 40% and 75% were recorded in two fields in the Emerald Irrigation Area. In a field at Biloela, parasites were so abundant that the tiny wasps were clearly visible walking around on leaves.

It is likely that the high parasitism levels recorded in Emerald and Biloela are due, in part, to very limited mirid sprays earlier in the season. This has avoided flaring SLW and allowed beneficials to multiply and offer a free service to growers and consultants in controlling whitefly.

Whitefly numbers are reportedly building up at St George/Dirranbandi. DPI&F entomologists will be visiting the area next week to collect samples for resistance monitoring and check parasitism levels.

While whitefly are definitely starting to make their presence felt in cotton fields across Queensland, their presence does not necessarily warrant action. Monitor fields often as whitefly can build up exponentially, identify what species/biotypes are present, use the available thresholds and avoid flaring whitefly by minimising the use of disruptive insecticides and maintaining beneficials in the system.

Article by Zara Ludgate

Posted in Cotton, Silverleaf whitefly, whitefly | Leave a comment

New Helicoverpa thresholds in vegetative soybeans

By admin | Published: January 15, 2009

The new economic threshold for Helicoverpa in vegetative soybeans is 8 larvae per sqare metre and replaces the old 33% defoliation threshold. The new threshold is based on field trials conducted by John Rogers (formerly with DPI&F at Kingaroy). These field trials show that approximately 7.5 larvae per square metre can be tolerated with no yield loss, but that severe yield losses can occur once this critical population (the inflection point) is exceeded.lds were based on the maximum defoliation (33% and widely cited in the scientific literature) that can be tolerated without reducing soybean yield. In John Rogers’ trials, Helicoverpa populations equivalent to the new threshold (8/m2) inflicted significantly less than 33% defoliation. Note that the threshold may be influenced by crop size, with fewer larvae tolerable in very early or very small crops, and more larvae acceptable in larger more vigorous late-vegetative crops.
Immediate intervention with a more robust larvicide may be required against extremely high populations (e.g. > 20/m2). While indoxacarb (Steward®) could be used at this stage, only one application is allowed per field per crop growth cycle, and this product is best saved for later in the season when it is most needed.
loopers and cluster caterpillars which are primarily foliage rather than bud feeders. However, cluster caterpillars are more likely to attack soybean pods than loopers, but not as savagely as Helicoverpa.
Helicoverpa damage in soybeans

A- vegetative damage
B – damage to terminals results in
C – reduction in pods and yield

Article by Hugh Brier (DPI&F Kingaroy), John Rogers (formerly DPI&F Kingaroy and Kate Charleston (DPI&F Toowoomba)

The new threshold (8 larvae/m2) is based on the maximum number of larvae that can be tolerated before there is an economic reduction in yield. The closeness of the threshold and the inflection point is a measure of the severity of the yield losses that can occur once this critical population is exceeded.

Previous thresho

The reason yield loss occurs below 33% defoliation is because of Helicoverpa’s feeding behaviour – they are not called budworms for nothing. As well as feeding on leaves, they also feed on the soybean plant’s vegetative terminals and auxiliary buds, the latter which are the precursors to floral buds.

Previous vegetative thresholds allowed for vegetative terminal loss (tipping) with 25% terminal loss the cited critical level above which action was required. The new thresholds are below the old terminal-loss guidelines as populations of 8 larvae/m2 destroyed fewer than 25% of terminals in John Rogers’ trials.

The crop’s ability to tolerate 7.5 larvae/m2 during the vegetative stage without yield loss, means that Helicoverpa nucleopolyhedrovirus [NPV] (e.g. VivusMax®) can still be safely used prior to flowering, provided it targets appropriately small larvae (<7 mm long). This is because NPV only has to keep populations below this critical level, rather than achieving ≥90% control that would be required if yield loss commenced as soon as populations exceeded 0/m2.

Until data to the contrary is available, the 33% defoliation vegetative threshold is still valid for

John Rogers’ studies illustrate the link between a pest’s feeding behaviour and its impact on crop yield. The studies also highlight the importance of having ‘species specific’ data, and that a ‘one threshold model fits all’ approach is not always appropriate. Further trials are planned to study the feeding behaviour and damage potential of cluster caterpillars and all the major looper species attacking soybeans. However, such detailed research is likely to take at least 3-4 years to complete.

Posted in helicoverpa, soybeans | Tagged | 6 Comments

Insurance spray for mirids in Bollgard II

By admin | Published: December 19, 2008

With the Festive Season almost here and an irrigation pending, is there value in applying a spray for mirids for peace of mind? This decision to apply an insurance spray needs to be carefully considered because it has the potential to cost much more in the long run.
Where do mirids come from?
Mirids overwinter as adults on wild hosts. In the spring with rising temperature, mirid populations start to build up on alternative hosts around cotton growing areas. Wet winters usually contribute large reserves of alternative hosts which support mirids. Once these hosts hay off and lose their suitability, the mirids can invade seedling or squaring cotton. Mirids sometimes build up in western Queensland, from where they allegedly move on north-westerly winds and invade cotton areas. Mirid adults also continually move into cotton from surrounding hosts and move out of the cotton to alternative hosts throughout the season.

In order to make this decision, we need to understand the population dynamics of mirids, their damage potential and what is currently happening to the crop.

What is happening this year?
Although the past winter was relatively wet, mirid numbers have been very low across the cotton growing areas so far this year. The reason for this phenomenon is not well understood. It could be that alternative hosts are still fresh enough to continue to support mirids, or the severe and prolonged winter may have had a detrimental effect on mirid populations.

Do we need to be worried about square loss at this stage?
If square retention is above 70% and mirid numbers are very low (as is the case for many crops this year), there is no need to be unduly concerned. Squares may be lost for various reasons e.g. insect feeding and physiological reasons.

If mirid numbers are very low and retention is less than 70%, square loss may be due to reasons other than insect feeding and a spray will not help. Square loss at this stage will be compensated fully provided plants do not further suffer from water or nutrient stress or from any other insects.

Figure 1. Compensation for mirid damage at squaring stage

Figure 1 shows plant mapping data in Bollgard® II..

 

Observation 1 shows the percentage fruit loss for 3 treatments at the squaring stage while observation 2 shows the same 3 treatments at cut out. The treatments consisted of the following:

In treatment 1 (blue) plants were sprayed on a regular basis starting from when 60% of plants had their first flower until cut out. Treatment 2 (red) was not sprayed for mirids at any time during the season. Treatment 3 was sprayed regularly throughout the season to provide total protection against mirids

In observation 1 – the squaring stage – there is a significant difference between treatment 1 (sprayed from squaring) and treatment 3 (sprayed all season) with 18% fruit loss for treatment 1 and 8% loss in treatment 3.
Consequences of unwarranted mirid sprays at this stage
There are several consequences of an insurance spray.
- It is an unnecessary expense.
- Disruption of benefcial communities at this early stage could flare secondary pests such as aphids, mites and whitefly.
- Sprays at this stage, whether high or low rate, will not provide long residual protection. Mirids could move in from alternative hosts at any time, and it is best to save the spray for when it is really warranted.

Article by Dr Moazzem Khan

However if we now look at what happened in these treatment at cut out, we find that there is no significant difference between treatment 1 and 3. What this shows is that in this experiment the plants had the ability to make up for the early season fruit loss.

Posted in Cotton, mirids | Leave a comment

Thresholds and changing sorghum crop value

By admin | Published: December 16, 2008

Growers and consultants need to revise the thresholds used for important insect pests of grain sorghum such as corn earworm and sorghum midge in light of lower prices currently being offered.

With new crop grain sorghum prices below $200 per tonne, the break even cost of control means that higher pest numbers (density) are needed before control becomes economic, compared to thresholds used last season when grain values were much higher.

The use of a benefit:cost ratio is also an important consideration. The break even point is where the cost of control is equal to the loss caused by the pest. The benefit:cost ratio will vary according to individual preferences, and needs to be factored in to calculations.

Corn earworm
One corn earworm larva is estimated to consume 2.4 g of sorghum during its lifetime. The economic threshold (i.e. the number of larvae/head where the cost of control is equal to the value of the grain saved) can be calculated using the formula:

No. larvae/head = (C x R) ÷ (V x N x 2.4)
where
C = cost of control ($/ha)
R = row spacing (cm)
V = value of crop ($/tonne)
N = number of heads/metre of row
2.4 = weight of sorghum (grams) lost/larva

The value of crop loss caused by corn earworm larvae in grain sorghum, for a range of larval densities and grain prices and based on 10 heads/metre of row on 1 metre row spacing, are presented below.

When sorghum is valued at $300/tonne, one larva/head could cause $72 crop loss/ha. If the price drops to $150 per tonne, one larva/head causes just $36 crop loss/ha, or 50% less economic damage. This example demonstrates just how important it is to consider each case on its merits, and in particular to consider the cost of control, as it too can vary widely depending on whether aerial or ground spraying is used.

Sorghum midge

As with corn earworm, decisions to spray for midge are greatly influenced by crop value and it is possible to calculate theoretical yield loss estimates for a particular crop scenario (see table). These yield loss estimates are based on extensive field trials by DPI&F that determined the average yield loss/midge/day on different rated midge hybrids. For a susceptible hybrid, one midge is estimated to cause 1.4 g yield loss/day. (Photo: D. Ironside)

Listed below are estimates of midge damage at different grain prices without chemical treatment.

The yield loss estimates in the table assume that spraying results in a 100% kill and that there is no midge damage prior to chemical application. It also assumes that the same average midge pressure persists over 4-5 days. In reality research has shown that one well timed insecticide for midge (put on from panicle emergence and before midge even enter the crop) will still only prevent 70-80% damage protection in lower rated sorghum hybrids. In 8+ rated hybrids, yield losses can be reduced by over 90% with this spray timing.

If the total cost of applying a synthetic pyrethroid by plane is around $20/ha, we can see that at a grain price of $150 per tonne, it is simply not economic to spray mid to high rated hybrids at 1 midge/head and 8+ hybrids at 3 midge/head. It should be emphasised that 8+ is the highest rating that can be assigned to midge resistant hybrids. There are some 8+ lines that would have a considerably higher rating if the scale was extended, and are practically immune to midge damage.

The above information shows the importance of calculating thresholds for the current situation, rather than relying on a fixed value from one year to the next.

Posted in Sorghum | Tagged | 2 Comments

Bring on NPV against grubs on grain sorghum

By admin | Published: November 26, 2008

Every year caterpillars of the corn earworm (helicoverpa), Helicoverpa armigera, cause losses to sorghum crops. Regular inspection during flowering is important to detect caterpillar infestations and properly time control measures.

Pre-flowering heads of grain sorghum are very attractive to egg-laying moths of the corn earworm. On any individual head, most eggs are laid prior to the start of flowering, as indicated by the presence of yellow anthers.

By the end of flowering, when brown anthers are present at the base of the head, eggs will have hatched and most larvae will be less than 7 mm in length.

A timely spray application of the naturally occurring biopesticide NPV (nucleopolyhedrovirus) remains the best control option for grain sorghum crops under attack from corn earworm.

NPV performs exceptionally well on grain sorghum, with well timed sprays usually achieving greater than 90 per cent control while leaving beneficial parasites and predators to mop up survivors.

If the spread of flowering in a crop is large, it may be better to spray earlier rather than wait until 50% of the crop is at the brown anther stage. This is because caterpillars on the earliest flowering heads may be larger than the ideal size to target with NPV, and they will cause some damage if not adequately controlled with NPV.

Research has shown that early application of NPV creates a disease outbreak and secondary NPV infection will control most caterpillars on late flowering heads.

Other issues to ensure good results with NPV

Water quality
Water used in spray mixes should have a pH of 7. Alkaline water will seriously reduce the performance of NPV, so buffer water with Li700 or equivalent to neutralise pH.

Water volumes
For high-volume, water-based sprays, a minimum of 30 L water/ha is recommended for aerial application, and 100 L water/ha for ground rig application.

Coverage
NPV must be ingested to be effective, so the challenge is to achieve good coverage of the target. This means paying particular attention to water volumes, nozzles, operating pressure, weather conditions, etc. You want to spread NPV over as much of the head as possible to ensure caterpillars have a high chance of picking up a lethal dose as they feed on the head.

Additives
Additives such as Amino Feed, etc. are not recommended when NPV is applied to grain sorghum.

Paying attention to the detail will ensure the best results from NPV.

Article by Dr. Dave Murray

New Beat Sheet contributors

The beat sheet blog team has been expanded and includes two new contributors, Kate Charleston and Zara Ludgate.

Kate is the development extension officer with the entomology team in Toowoomba. Her role is to provide information about IPM in field crops as well as training to growers and industry in managing insect pests according to IPM principles.

Kate joined entomology in June 2008 and has previously worked as a research scientist, agronomist, plant health inspector and extension officer. She started her career with the Department of Primary Industries in Tasmania and joined the Queensland Government in 1999 as an extension officer in a sugar project at South Johnstone in Far North Queensland. Kate has worked with sugarcane, cotton and pulse crops and has considerable training and extension experience.

 

Zara has just started her career in entomology research. She comes from a rural background and completed a degree in plant and soil science at St Lucia, Brisbane in 2007. For her honours year she investigated the effect of a plant defence compound on the fitness of diamondback moth.

While in Brisbane she provided technical support for research into bio-pesticide production for helicoverpa and green vegetable bug management. She is now based in Toowoomba with the crop protection systems – entomology unit. Her current research interests include insecticide resistance in whitefly and integrated pest management in grain crops.

Posted in NPV, Sorghum | Tagged | 2 Comments

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.

Posted in Chickpeas, helicoverpa | Tagged | Leave a comment

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 armyworm, Winter cereals | 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.
Posted in Chickpeas, helicoverpa | Tagged | Leave a comment

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

Posted in Winter cereals | Tagged | Leave a comment

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 aphids, barley, wheat, Winter cereals | Leave a comment

Bad Behavior has blocked 192 access attempts in the last 7 days.