Latest helicoverpa thresholds for mungbeans

Revised thresholds for helicoverpa in flowering/podding mungbeans are based on a rate of damage of 35 kg/ha per larva per square metre in podding crops. The new thresholds are nearly double the old threshold of 1/m2, and make allowances for variations in control costs and crop value. For a typical scenario with pesticide control (including aerial application) costing $40/ha and an anticipated crop value of $600/t, the new threshold (see chart) is 1.9 larvae/m2.

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

New Helicoverpa thresholds in vegetative soybeans

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.

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 thresholds 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.

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.

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.

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

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.


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)

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

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.

Helicoverpa management in chickpea – a refresher

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.