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

Silverleaf whitefly in cotton – an update

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

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)