Bring on NPV against grubs on grain sorghum

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.

Will Rutherglen bug damage sorghum post grain fill?

As many of the early sorghum crops reach physiological maturity, and approach harvest, questions are being asked as to whether it is necessary to control large populations of nymphs in these crops. More specifically, whether these RGB will cause any damage to the maturing grain between physiological maturity and harvest.

Photo: Dave Murray (BigBug) out looking at a sorghum crop at Glen Ogden's property on the Darling Downs.

In summary, the best available information suggests that sorghum is will not suffer yield loss as a result of RGB feeding from physiological maturity (hard dough) through to harvest.

DPI&F trials to examine the impact of RGB on grain as it matured, did not provide a conclusive answer to this question. In 2008 we are undertaking field trials to try and answer this question.

However, if we look at what is happening with the sorghum plant from physiological maturity on, we can draw some conclusions about the potential for RGB to cause damage to maturing grain. Importantly, once grain reaches physiological maturity it has reached its full potential weight, and from then on starts to lose moisture as it matures. This means that even if large numbers of RGB continue to feed on the sorghum plant (on stems and leaves) their feeding will not impact on the development of filling of the grain at this stage.

Photo (left): Physiological maturity is reached when a black layer appears at the base of the seed, near to where the seed is attached to the stem

Another question that we do not have a definative answer to is whether RGB continue to feed directly on the maturing seed, or if they feed only on the sorghum plant once the grain reaches hard dough. In trials where plants at hard dough were exposed to RGB we did not see the evidence of feeding damage to grain that we saw when heads were infested at earlier stages of grain development (see earlier posting on RGB damage).

Crops that have had early infestations of RGB, even some of those that have been treated, now have moderate to large populations of nymphs and adults in them. The adults are likely to be newly emerged, having developed from nymphs in the crop. The nymphs will have emerged from eggs that were laid by an earlier infestation.

Controlling RGB to prevent the problems associated with infestations at harvest remains an issue. The inclusion of an insecticide along with the herbicide when the crop is being sprayed out prior to harvest is a practical approach. Be alert to the witholding period of any insecticide used. Treatment with insecticide prior to harvest is not a guarantee that the crop will be free of RGB at harvest. There remains the possibility of reinvasion by adults at any time.

Watch out for midge this season

Adam Hardy

Senior Entomologist, Queensland Department of Primary Industries and Fisheries, Toowoomba.

Photo: Adam Hardy (right) and Bernie Franzmann inspecting midge rating trials.

Over the last decade sorghum midge have not caused many headaches for sorghum growers.

However, staggered sorghum plantings mean that sorghum midge are likely to be found building up in numbers in later planted sorghum crops with the potential to cause significant economic loss.

Even if midge are not in high numbers in a crop, the high grain price combined with the low cost of the insecticide of choice (synthetic pyrethroids) means that it is likely that spraying for midge will be a simple economic decision.

In the past the best way to avoid midge damage was to plant as high a midge rated variety as possible as early as possible in the season, and to plant crops well outside overlapping flowering windows of 2-3 weeks within districts. This has not been possible for many growers this season as rain came in a late and staggered fashion. This means that many crops planted over progressive flowering windows are likely to be at risk of midge population build up.


The only way to avoid economic damage is to monitor very closely for midge numbers EVERY DAY during head emergence and flowering and know what your spray threshold for midge is, based on crop value and cost of control, before the midge turn up. This insect pest requires very careful monitoring at exactly the right time of day. By the time you see the adult midge, they are already causing damage.

HOW TO COUNT FOR MIDGE

Generally, peak midge activity occurs between 9-11am, and this is the best time to look. However changes in weather can bring midge into a field from surrounding areas at any time of day. Midge numbers can differ widely both within a crop and between plants, even between those right next to each other.

Photo: Female midge laying into a sorghum floret.

Sorghum heads are most attractive to midge at mid flower. It is not uncommon to see double or triple the number of midge per panicle on panicles at early-mid flower vs pancles at the end of flowering. At lower midge densities, adult flies will move around and lay almost exclusively on the flowering portion of the panicle.

Midge flies are only 1-2 mm long, so if your eyesight is not what it used to be, then make sure you get someone with good eyes to carefully check each head. It is very easy to underestimate midge numbers if you are not careful.
The easiest way to 'get your eye in' is to look at the top half of mid flowering panicles and look for MOVEMENT of the small red flies against a still sorghum panicle looking from side on and slightly above side on one section of the sorghum panicle at a time. Keep your eyes focused over a couple of branches of florets for several seconds at a time to detect female midge walking around the branch or bobbing up and down probing their ovipositor into each floret. Female midge generally walk around quickly when they are ‘on the job’ spending as much time walking around as they do laying eggs.

On windy days you may have to hold each head still and shelter the panicle with your body or a sheet of cardboard (or a large jacket or umbrella?) for 10-20 seconds before scanning each panicle to allow you to more accurately see midge movement. If you can avoid the wind by getting out into the paddock slightly earlier in the morning then your job will be much easier.

Growers should monitor for midge over 10 metres of row in at least 4 different locations in your crop. It may be necessary to spray only one section of crop at a time, or the whole crop accordingly.

A common experience is that the first midge seen are caught in spiders webs in the field - a sign that midge are active in that particuar field.

As the season progresses, you may also start to see the midge parasitoid, Eupelmus spp (Photo: C. Freebairn). Whilst it can be present in reasonably large numbers, this parasitoid does not occur early enough to prevent midge from causing damage.

MANAGING MIDGE IN SORGHUM

Adult female midge lay their eggs inside sorghum florets where chemicals cannot reach. Insecticides only target the adult midge as they move about the crop and do not kill the eggs or hatched larvae that are already present inside the sorghum florets. While these midge adults only live for one day, they do most of their egg laying (and subsequent damage to the crop) in the morning.

It is possible to calculate theoretical yield loss estimates for a particular crop senario (see table). These yield loss estimates are based on extensive field trials by DPI&F that determined the average yield losses per midge per day on different rated midge hybrids

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 you will receive the same average midge pressures 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 $250-300/t, it is economic to spray low to mid rated hybrids at even 1 midge per panicle and 8+ hybrids at 3 midge per panicle.

Many growers may choose to include a spray for midge with a synthetic pyrethriod in with a virus application for helicoverpa, or may look to clean up midge and rutherglen bug at the same time with one or two well timed pyrethriod applications. In all cases spraying pyrethoids will devastate beneficial insect populations and the implications of this should be included in the decision to spray. Pyrethroid applications on their own are likely to flare helicoverpa and aphid populations.

Rutherglen Bug in sorghum

Rutherglen bug infestations of grain sorghum
This season we are seeing large infestations of Rutherglen bug (Nysius vinitor) (RGB) in sorghum crops from Central Queensland to the Downs, and further south. The large infestations are most likely a result of storm activity and the growth of weed hosts on which they breed up. RGB are very mobile, with large numbers of bugs appearing in (or disappearing from) crops before or after storms. There doesn’t seem to be particular stage of crop development at which RGB start to infest sorghum. The timing of infestation is determined by storm activity, or by hot dry weather that causes weed hosts to dry off, forcing bugs into nearby crop.

What we are seeing in the field this season
Where RGB are active, early sorghum crops, or early heads in a crop, are most affected. Damaged grain is red where bugs have fed on the portion of the seed not covered by the glumes. Dark spots are also visible on the grain. Grain is small and shrivelled, and has not continued to fill beyond the point at which it was damage. Cutting open the damaged grains, there is fungal and bacterial infection of the seed, making it black and mushy. This grain will not continue to fill, and is likely to be lost at harvest.

Sampling for RGB
Because RGB populations can start to build up at any stage of crop maturity, and our data show that RGB can significantly impact on seed set during flowering and early seed development, monitoring for RGB should start at mid flowering. Practically, RGB can be sampled for along with Helicoverpa; beating individual heads into a bucket. The distribution of RGB is typically patch across the field. A few heads with high numbers, lots of heads with lower numbers. This means that more, rather than fewer, heads need to be sampled to get an idea of the overall level of infestation in a field.

RGB and their damage potential
….. in sunflower
There has been little research on RGB in any crops other than sunflower. In sunflower they are known to be a major pest, feeding on the seed as it sets and matures, resulting in reduced yields and oil quality.
…….in sorghum
In the USA there is a related species, the false chinch bug (Nysius raphanus), which causes yield loss and reduced seed viability in sorghum. Yield loss is caused not only by the direct feeding of the bugs, but also through allowing a fungus to infect the seed through the feeding wounds, causing further deterioration and discolouration of the grain. Threshold trials showed that chinch bug caused a reduction in seed set when present on grain from flowering through soft dough stages. Where seed set was reduced, overall grain weight increased. In other words, the plant compensated with fewer, heavier grain in damaged heads.

DPI&F research on RGB in sorghum - what we know

RGB feeding post flowering will reduce grain set
Preliminary research on RGB has been done in sorghum and has shown that adult RGB will reduce seed set by around 20% at densities of 50-100 bugs/head. If bugs infest after grain is set, adult RGB will feed on the seed. Affected seed looks spotty and red externally (see photo), and hollowed out internally. Fungal infection also occurs resulting in further deterioration of the developing grain and ultimately seed that is small, shrivelled and light – and likely to be lost at harvest.

Early instar nymphs do not damage grain
No evidence of grain feeding was found for first, second and third instar nymphs. Older nymphs did feed on developing grain as do adults.

There are no soft-options for RGB control
Fipronil, indoxacarb and dimethoate were compared with alphacypermethrin in a replicated field trial on the Downs. None of the products provided control of nymphs statistically equivalent to the commercial comparison, alpha-cypermethrin. Compared to the unsprayed, alpha-cypermethrin was highly disruptive to predatory invertebrates.

What we don’t know

Can RGB damage grain from hard dough through to harvest?
The data for the impact of RGB on maturing sorghum does not provide a clear picture of whether grain continues to be damaged. We know that large populations of nymphs can be present in crops right up to harvest, but whether they are feeding on the plant, or on the grain are unknown.

RGB economic threshold for sorghum
Further work is needed to determine the density-damage relationship. This includes work to understand compensation when seed set is reduced early, the direct damage to grain during filling and maturation, and potentially indirect damage caused by RGB feeding on the sorghum plant which may impact on grain fill.

Soft options for RGB
Biopesticides, in this case the fungal disease Metarhizium is an option for RGB. It is currently being evaluated by DPI&F, and if effective will be an important addition to sorghum IPM.


Based on what we know, the following strategies can be suggested for minimising RGB damage to sorghum:

Start checking for RGB from flowering
Developing and filling seed are susceptible to damage from RGB adults, so protect the crop during these stages.

A threshold of 20-50 bugs/head is suggested. Treating populations of adults does not guarantee there will not be further infestation. RGB can reinfest a field overnight if the conditions are right.

RGB will breed in sorghum, so infestations of adults will result in increasing numbers of nymphs. Large populations of RGB can cause problems with machinery at harvest. Continue to monitor populations through to harvest as a decision to treat will need to take into account withholding periods for harvest.

Getting the most from NPV sprays on grain sorghum

Some issues have recently been raised on the Darling Downs regarding the use of Helicoverpa nucleopolyhedrovirus (NPV) against corn earworm on grain sorghum. These issues involve the delay in time to kill and the level of control not necessarily meeting growers’ expectations.

Seasonal conditions
The last couple of weeks on the Downs have been cooler than normal. The average daily screen temperature for Dalby for the week ending 11 December was 25.5°C, while for the week ending 4 December it was just 22°C. Daily minimums were as low as 14°C. These lower temperatures will influence the time to death of NPV-infected caterpillars.

Temperature affects the development rates of caterpillars, their feeding, and the rate at which they die from NPV once they are infected. The lower the temperature, the slower caterpillars develop and the longer it takes for NPV-infected caterpillars to firstly stop feeding, and then to die.

Studies by Chris Monsour (formerly DPI&F) investigated these relationships. At 30°C, an NPV-infected 6-day old caterpillar (late second instar) will feed normally for 2 days before feeding is greatly reduced. At 20°C, caterpillars feed normally for 5 days before feeding is reduced. Importantly, the total amount of food consumed by these NPV-infected caterpillars is similar, whether at 30°C or 20°C (see Figure). Also remember that most caterpillars feed on anthers until late fourth instar (21 mm in length) and this feeding will not affect yield.

Time to death from NPV

At 30°C, it takes on average 4.5 days after infection for a 6-day old caterpillar to die from NPV. This compares with 6.2 days at 25°C and 7.5 days at 20°C. Under recent field conditions, many larvae are taking more than 8 days to die from NPV after spray application.

Figure. Consumption of artificial diet by healthy and NPV-infected 6-day old caterpillars and time to death from NPV at three temperatures (Data from Chris Monsour).

High grub infestations
If we accept that NPV will kill 90% of caterpillars, a starting infestation of 10 caterpillars per head results in 1 caterpillar per head surviving. This would not normally be an issue, but with the current high value of grain sorghum, this number of survivors may be above the economic threshold. You need to ask, is any other product going to do a better job? The answer is ‘No’.

Beneficial safety
NPV is safe on beneficials (parasites and predators) and these are important in helping mop up any survivors, as well as ensuring aphids are not flared. The end result is generally better than a 90% job.

What do we need to do to make sure of good results with NPV?

Timing
Consider applying NPV sprays earlier i.e. before 50% brown anthers, particularly if the spread of flowering is large. In this way most early flowering heads will be fully protected and secondary infection will control most caterpillars on the late flowering heads. It is best to target caterpillars less than 7 mm in length when using NPV, and this is the size of caterpillars on heads that have just finished flowering.

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

NPV historically performs very well on grain sorghum, usually achieving greater than 90% control when used alone. For this reason, additives such as AminoFeed, etc. are not recommended when NPV is applied to grain sorghum.

However, for all crops other than sorghum, the use of a molasses-based additive containing the reducing sugars glucose and fructose (such as AminoFeed) is recommended when applying NPV.

Friendly fighter conquers foe

Microplitis demolitor is just one of many friendly fighters that battle to contain numbers of one of our most important pests, the corn earworm, Helicoverpa armigera.

Corn earworm on grain sorghum is making its presence felt and many crops are being sprayed with Helicoverpa nucleopolyhedrovirus (NPV) to control above-threshold infestations of caterpillars.

The current high value of grain sorghum (over $250 per tonne) means that it is economic to control caterpillars at lower numbers (density) than growers may have sprayed previously when grain value was lower.

Low caterpillar numbers is a perfect situation for Microplitis to chip in a helping hand. It is not uncommon to find 30-40% of small caterpillars on grain sorghum parasitised by Microplitis. In many cases, this level of parasitism may be sufficient to sway a decision to not spray.

What is Microplitis?
Microplitis is a small native wasp that lays it eggs in (parasitises) small helicoverpa caterpillars. The life cycle from egg to adult takes about 12 days. This is made up of 7 days from egg laying to forming a pupa, and then 5 days for pupal development.

Adult Microplitis are small black-brown wasps. They are often seen flying slowly above the crop canopy in search of caterpillars (hosts). A female wasp will parasitise as many as 70 helicoverpa caterpillars. The parasite develops inside the host caterpillar. When fully developed, the Microplitis larva chews a hole in the side of the caterpillar and spins a fawn-coloured cocoon around itself and pupates. The caterpillar that was parasitised may still be alive, but it will soon die.

Clues to identify Microplitis activity include

• Adult wasps foraging on sorghum heads
  
• Split test of caterpillars to reveal internal parasites
  
• Distinctive fawn cocoons next to dead or dying caterpillars
  

Identifying parasitised caterpillars
In the field, parasitised caterpillars can be identified by performing a simple split test. Parasitised caterpillars will only grow to about 15 mm in length, so caterpillars smaller than this are potentially Microplitis hosts. Hold a caterpillar across a forefinger with one thumb and forefinger on the rear end of the caterpillar, and with the other thumb on the head. Gently stretch the caterpillar until the skin ruptures. A Microplitis larva developing within the caterpillar looks like a white maggot up to 4 mm long.

Interactions between Microplitis and NPV
Caterpillars infected with NPV within 3 days of parasitisation by Microplitis will die from NPV. The immature Microplitis will also die because of the death of its host.

When NPV is applied to control corn earworm, it is not unusual for some parasitised caterpillars to survive the treatment. Caterpillars parasitised more than 3 days prior to the NPV treatment will produce healthy Microplitis. Parasitised caterpillars feed less and may not ingest NPV.

In shaking sorghum heads to make post-treatment assessments, parasitised larvae may be dislodged free of the pupal cocoon attached to them. Careful inspection of these caterpillars may reveal a hole in the side of some of these caterpillars, indicating prior parasitisation. These larvae will eventually die.

Microplitis is an important natural enemy of the corn earworm and they need to be considered when making decisions about when to manage corn earworm.

For more information on Microplitis, follow the link to the brochure ‘Microplitis demolitor and ascovirus: Important natural enemies of helicoverpa’ http://www2.dpi.qld.gov.au/fieldcrops/17576.html

Corn earworm chews into sorghum profits

Sorghum growers across the Darling Downs can expect to see an influx of the corn earworm, Helicoverpa armigera, in their flowering sorghum crops over the next few weeks. Growers are well equipped to deal with the problem in an environmentally friendly way.

Moths are active and wanting to lay eggs on susceptible crops, and sorghum crops putting up heads are highly attractive - just what this insect pest loves. The majority of eggs are laid in a narrow window, between emergence of the head from the boot leaf and the commencement of flowering (yellow anthers). This results in highly synchronous development of larvae in a crop – larvae of uniform age in the crop.

Yellow anthers
during flowering
(too early to spray).

Eggs and newly hatched
larvae on sorghum.
Photo: D. Ironside

It is important that growers check their crops because in many cases feeding by corn earworm is likely to cause economic loss. One larva is estimated to consume 2.4 g of sorghum. Larvae up to 13 mm in length feed mostly on anthers and do not affect yield.

The table below provides examples of crop loss for different larval densities.
Table: 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.
*Based on estimated consumption of 2.4 g per larva.

The current high value of grain sorghum (over $300 per tonne) means that it is economic to control larvae at lower numbers (density) than growers may have sprayed previously when grain value was lower.

The economic threshold (i.e. the number of larvae per 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 per larva.

How to sample sorghum heads for corn earworm
Count the number of larvae dislodged from 30 heads to arrive at a control decision. Obtain 5 consecutive heads at the brown anther stage from at least 6 locations in a field, each location preferably more than 50 m apart. Use the palms of your hands to spin each of the heads into the bucket. Pour the contents of the bucket onto a beat sheet or tray and count the number of larvae in each size class
very small (VS=less than 3 mm in length)
small (S=3–7 mm)
small-medium (SM=7-13 mm)
medium-large (ML=13-21 mm)
large (L=greater than 21 mm).

Control
Effective larval control can be achieved with the use of commercially available nucleopolyhedrovirus or NPV sprays, sold as either Vivus Max® (succeeding Vivus Gold®) or Gemstar®.

NPV is dynamite against corn earworm larvae in sorghum and has the bonus of being a natural disease of the pest, so that spraying only kills the pest and not other beneficial insects and spiders in the crop.

Gemstar® and Vivus Gold® have both been registered for use on sorghum at 375 mL/ha. Lower rates (250-300 mL/ha) have been used successfully by many growers.

Please be aware that Vivus Max® now replaces Vivus Gold®. It is a more concentrated product (2.5 x) and has a registered rate of 150 mL/ha in sorghum (equivalent to 375 mL/ha of Vivus Gold®).

Research into the use of NPV sprays has shown several key points that growers and consultants should remember when using NPV.

First, checking is easy and important – it not only tells you whether you have the pest in enough numbers to justify spraying, but it also gives you information on when to time an NPV spray, since it works best when targeted against the very youngest larvae.

At the end of flowering (heads with brown anthers to base), most larvae will be first or second instar (less than 7 mm in length), and ideal to target with NPV. The best spray timing is when 50% of heads in the field have brown anthers to their base. A further delay of 3 days will help conserve the important larval parasite, Microplitis demolitor.

In crops where there is a large spread of flowering, it is better to spray before 50% of the heads are at the brown anther stage. In these cases, experience has shown secondary infection by NPV can kill a high proportion of the caterpillars that hatch after the NPV application.

NPV should not be used against larvae greater than 13 mm in length.

Good coverage over the plant and especially the sorghum head is critical, since a larva has to actually feed on an NPV particle to become infected with the virus. Sprays should be put on at the time of day that is best suited to getting good coverage, and this will often be in the morning.

Ultra low volume (ULV) sprays of NPV applied by a plane are highly effective in sorghum and allow for large areas to be treated in a relatively short time – this is good news when the pressure is on to treat large areas.

For ULV application, NPV is combined with approved spray oils such as D-C-Tron, Canopy or Biopest oil, to make a minimum spray volume of 3 L/ha.

Corn earworm larvae killed by NPV.

Further information on the use of NPV can be found in the brochure 'Using NPV to manage helicoverpa in field crops' by following this link

http://www2.dpi.qld.gov.au/fieldcrops/17677.html

No concern for tell-tale holes

Corn earworm larvae on vegetative sorghum crops produce characteristic holes in the leaves after feeding in the throat of the plant. These tell-tale signs are of no great concern as this type of feeding will not affect crop yield.



Caption: BEB alias Austin McLennan showing a characteristic holey sorghum leaf.


The recent presence of high numbers of corn earworm, Helicoverpa armigera, on winter cereals and chickpea could herald the beginning of a busy time ahead for grain sorghum in southern Queensland.

As larvae on cereal crops mature, they climb down the plant and burrow into the soil to pupate. Moths emerge from these pupae 2 to 3 weeks later, and start the next generation by laying eggs on suitable host plants.

Vegetative sorghum is attractive to egglaying moths, and larvae hatch from newly laid eggs in 3 to 4 days. Survival of larvae on vegetative crops may not be high, but vegetative sorghum can be an important intermediate host that bridges the gap between winter and summer.

Armyworm larvae may also be present in vegetative sorghum. Armyworm larvae cause sorghum plants to look ‘ragged’, but again this leaf feeding does not result in any yield loss in advanced and actively growing seedling crops.

Control of larvae on vegetative sorghum is generally not recommended as the damage is cosmetic and unlikely to affect yield.

While corn earworm larvae are advertising their presence in southern Queensland grain sorghum crops, of greatest importance are larval infestations after flowering and during grain fill.

Egglaying by corn earworm moths and larval management on sorghum heads will be the subject of a future posting.