The latest published thresholds for helicoverpa in vegetative soybeans (Rogers and Brier, 2010) show that while soybeans can tolerate damage inflicted by moderate helicoverpa populations up to 7 larvae/m2 without yield loss, severe yield loss is inflicted by populations >7 larvae/m2 at a rate 4-5 times greater than during the pod-fill stage. The conundrum therefore is that while vegetative soybeans are far more tolerant of low to moderate helicoverpa populations (< 7/m2) than podding soybeans, they are markedly less tolerant of populations > 7/m2 than are pod-filling soybeans (see Figure 1).

The reason for this severe yield loss is that unlike most leaf-feeders such as loopers, helicoverpa also attack the plant’s auxiliary buds and vegetative terminals, completely destroying these structures.

Damage to auxiliary buds potentially reduces yield as these structures are the precursors to the plant’s flowers and (subsequently) pods. Damage to vegetative terminals is potentially bad for yield. The reason for this is that while plants may compensate by setting additional side branches, pods formed on these are often closer to the ground and are more difficult to harvest.

Where helicoverpa populations are not excessive, damage is spread over a number of plants and the crop is able to recover and compensate without yield loss.

However, once populations exceed critical level in vegetative crops (about 7/m2), damage per plant reaches a critical level beyond which the subsequent plant growth is severely affected.

 To reduce the risk of severe early helicoverpa damage, the sampling guidelines have been revised to “sample crops twice weekly from the seeding stage onwards”. This more-intense sampling regime maximises the chance of helicoverpa larvae being detected while they are still small (ideally <7 mm) and thus able to be controlled with a Helicoverpa virus biopesticide, eg VivusMax or Gemstar.

Remember that the use of biopesticides in pre-flowering soybeans is a key in the “Go Soft Early” strategy to minimise the risk of silverleaf whitefly (SLW) attack in soybeans. The “Go Soft Early” strategy also promotes the build up of beneficial insects attacking other pests such as helicoverpa and loopers.

Vegetative soybean crops can tolerate populations up to 7 larvae/m2 and it is not necessary to kill every helicoverpa larva in a crop. Assuming only 70% control, even populations as high as 20 larvae/m2 can be reduced to below the critical 7 larvae/m2 level in vegetative crops.

Very small plants have fewer nodes and hence fewer auxiliary buds and a given helicoverpa population will damage a greater proportion of auxiliary buds per plant. Severely damaged plants will also be more susceptible to subsequent helicoverpa attack.

Note that small larvae often feed in leaf terminals, so inspect these and look for the tell tale damage symptoms, small holes in the leaflets and frass. For the latest guidelines about applying helicoverpa virus, refer to the previous blog of 8th January 2010. In seedling and early vegetative crops, pesticide costs can be halved by banding the spray over the crop, and blocking off nozzles over the bare inter-row.

References:
Rogers D.J. and Brier H.B. (2010). Pest-damage relationships for Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) on vegetative soybean. Crop Protection 29(1): 39-46.
Rogers D.J. and Brier H.B. (2010). Pest-damage relationships for Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) on soybean (Glycine max) and dry bean (Phaseolus vulgaris) during podfill. Crop Protection 29 (1): 47-57.

Article by Hugh Brier. Images by Hugh Brier and Joe Wessels

Control of corn earworm, Helicoverpa armigera, in maize has generally not been practised because of the high cost associated with repeated insecticide application required during silking. In most years it is a case of forsaking the top of every cob to larval damage

However, in some years, very high pest activity results in more severe cob damage, with larvae often tunnelling into the sides of cobs. In such cases grain samples may contain fungus-affected grains and mycotoxins, causing a downgrade in the quality of harvested grain.

What can you do to prevent these losses?

Over recent years Helicoverpa nucleopolyhedrovirus (NPV) has demonstrated its versatility for corn earworm management. One of the major developments has been the effective application of NPV through overhead irrigation, sometimes referred to as ‘chemigation’.

ViVUS Max is currently the only insecticide registered in Australia for application in overhead irrigation water in a wide range of crops.

By adding NPV to irrigation water, growers can artificially inoculate their crop with NPV and achieve a high level of control of helicoverpa larvae.

The main guidelines for NPV use still apply. These include:

• Good coverage is essential as the product needs to be ingested
• Use in the temperature range 25-35°C when larvae are actively feeding
• NPV is more effective against smaller larvae
• Preferably target larvae less than 7 mm in length, but under ideal conditions larvae up to 13 mm in length will be controlled
• Larvae can take up to 8 days to die
• Spray water pH should be neutral (pH 7.0)

There are additional key points to the successful use of NPV via overhead irrigation.

• Application in overhead irrigation water provides the maximum coverage achievable
• Introduce NPV to the irrigation water at the appropriate rate using chemigation equipment
• If the NPV is diluted in water prior to injection into the irrigation water, ensure that the dilution water is clean and not silty with a pH of 7 or less
• Ensure constant agitation in the premix tank
• Ensure any diluted NPV is used within 10 hours of mixing
• Apply in no more than 10 mm of irrigation water

• For one-pass mobile irrigators such as centre pivots, laterals and travellers (guns), continuously introduce the required amount of ViVUS Max into the irrigation water over the course of irrigation.

What rates to use? How often?

For ViVUS Max in maize, the registered rate for normal foliar application is 150 mL/ha. When applied in overhead irrigation water, reduced rate repeat applications have been used successfully. An effective prophylactic strategy would be to make the first application at full tassel emergence (50 to 150 mL/ha depending on larval numbers and size) and then low rate (50 to 75 mL/ha) applications every 5 days or so until late blister/early milk stage.  A total of 4 applications would use about 250 mL/ha (perhaps more under high pressure). This product cost is around $30/ha and will keep things very clean.  

What are the economics of losses to larvae?

A back of the matchbox calculation can be used to give some insight to the damage caused by larvae. An average plant population is 70,000 plants/ha with one cob per plant and one larva per cob. Assume one larva consumes 15 kernels in its lifetime (Note: this value has no validated scientific basis). With an average kernel weight of 4,000 kernels/kg, one larva consumes about 3.8 g. If maize is valued at $300/t, this loss amounts to 262 kg/ha or $79/ha. Based on these rough figures, and assuming a high level of control, there is likely to be an economic benefit from using NPV. Larvae damaging early silks can also reduce pollination, which can result in even greater yield reductions.

Other benefits

As NPV is safe to natural enemies, parasites and predators remain in the crop and keeps working in your favour. This is particularly relevant for the egg parasite Trichogramma which is sometimes very abundant in maize crops. Untreated maize crops can also generate large numbers of helicoverpa moths, so control of larvae in maize can reduce subsequent pressure in nearby crops.

Article by David Murray and Anthony Hawes.  Image of lateral move irrigator by Graham Harris.

Helicoverpa threshold table for mungbeans 2008
Based on data from 2006/07 threshold trial
Assumes yield loss of 35kg/ha for every larva/m2. No allowance for larval mortality, but this most likely cancelled out by sampling inefficiency with a beat sheet. Yield loss is probably at the upper end of that likely as the trial showed no yield loss for up to 8 larvae/m2 at flowering. Very high control costs included in table reflect extremely high application costs in coastal crops.
 

Cross-reference the cost of control versus the crop value to determine the economic threshold (ET).

If the cost of control = $35/ha and the crop value =$450/t, the ET = 2.2

If the cost of control = $25/ha and the crop value =$650/t, the ET = 1.1

The lower the cost of control, and the higher the crop value, the lower the threshold.

Note that the thresholds are at the break even point, where the cost of control = the value of the likely damage, i.e. where the benefit: cost ratio is 1:1, or in other words where there is not net gain if you spray and no net loss if you don’t spray. Hence control is only recommended if the population exceeds the economic threshold, in other words if the benefit:cost (B:C) ratio is greater than 1.

While IPM guidelines traditionally recommended a B:C ratio of 2:1, most growers using the control cost scenario (above $40/ha) are unlikely to tolerate another $40 of damage/ha before taking action. Therefore use the above table as follows: Decide how much extra potential damage (in $/ha) you are willing to accept before taking action. For example if you are only willing to accept another $10 of damage/ha before taking action, and control costs and likely crop values are $40/ha and $600/t respectively, then adjust your control costs up to $50/t, and cross reference with the above crop value to give an action threshold of 2.4 larvae/m2.

While early reproductive damage at flowering may be totally compensated for, significant early damage can delay harvest maturity, and may reduce ‘commercial harvest yield’, i.e. the yield in crops where desiccants are used to dry out green pods lagging behind the main crop of black pods. For this reason, the threshold is conservatively set from flowering to podfill.

Recent data suggest early moderate damage can be totally compensated for with no delay in harvest, in well growing crops with plentiful moisture. In such crops, growers might consider using a helicoverpa NPV product such as VivusMax for low-moderate populations (eg 2/m2) provided they are able to guarantee thorough coverage, include an Aminofeed adjuvant and are targeting small larvae (ideally not greater than 5 mm long).

In view of the recent changes to the Helicoverpa threshold in vegetative soybeans, a provisional threshold of 4-5 larvae/m2 has been set for vegetative mungbeans, in lieu of the old 33% defoliation threshold (which still holds for loopers). This is because helicoverpa are also likely to target the mungbean’s auxiliary buds which are the precursors to floral buds.

The threshold is set lower than the vegetative soybean threshold because mungbean plants are smaller than soybeans. Note that this vegetative mungbean threshold is provisional and has to be verified in replicated field trials.

Helicoverpa and mirids
Recently we received a number of reports of flowering mungbean crops with above threshold mirid populations and low numbers of Helicoverpa. In such instances, dimethoate (250 mL/ha) plus NPV can be mixed with no risk of incompatibility. However it is critical to add a buffer such as LI700 to tank mix water to keep the pH below 7, as both dimethoate and NPV are deactivated in alkaline water (pH >7).

Note that dimethoate is recommended at the lower 250mL/ha rate as this has proven efficacy in DPI&F’s trials and has far less impact on beneficials than the full registered rate of 500mL/ha. Preserving as many beneficials as possible will complement NPV’s impact on helicoverpa larvae and will reduce the risk of subsequent sprays to control this pest..

Article by Hugh Brier

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.

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.

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.

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

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.

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.