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.

Control considerations for Helicoverpa in chickpea

The recently released brochure outlining how to calculate an economic threshold for helicoverpa in chickpea is available on the DPI & F website at www.dpi.qld.gov.au/fieldcrops. Click on the link to Helicoverpa management in chickpea where you can view or download the brochure.

Control considerations – which product when?
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. Larvin (thiodicarb) is another option, particularly where efficacy of this product in the local area is known to be high.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 hot areas (with an extension to 30 Sep this season); 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.

A better way to make decisions about Helicoverpa control in chickpea

A new guide to calculating economic thresholds available now
Growers and agronomists are now able to calculate how much yield they will lose if they do not control an infestation of helicoverpa in chickpea. They can calculate the point at which it is economic to control an infestation based on the potential yield loss, their costs of control, the expected grain price and their own preference for benefit:cost.

Being able to calculate the economic thresholds based on an individual grower’s situation is a significant advance on the old recommendations which were fixed at 1-4 larvae per square metre, and did not allow for adjustment when there were changes in grain price or costs of control.

The calculation is now possible following several seasons of research by Melina Miles and Richard Lloyd in Toowoomba, and Paul Grundy and Sherree Short in Biloela. The DPI&F entomologists have worked out that one helicoverpa larva, surviving from hatchling to pupa, will consume 2 grams of chickpea grain. This information is the basis on which potential crop loss ($/ha) is calculated.

The research also examined the impact of spray timing on yield and found that no yield loss occurred when larvae were allowed to feed in vegetative and flowering crops. Yield loss only occurred when larvae fed on developing, filling and maturing Many hours observing larval feeding in the field supported this result. Observations found that larvae of all sizes fed readily on leaves, large larvae did the majority of feeding on pods, and that neither small nor large larvae fed on buds and flowers. However, there are seasons when larval pressure in vegetative and flowering crops may necessitate control.

Grain quality was assessed in both threshold and spray timing trials, and no loss in quality (increase in splits or damaged grain) was seen within the range of densities that it was economic to spray.
In summary, the recommendations for helicoverpa management in chickpea are:

• Use a beatsheet to sample helicoverpa in chickpea.
  
• Exclude very small larvae from threshold calculations – they are difficult to assess accurately in the field – they will be counted as smalls at the next count.
  

Adjust the number of small larvae for natural loss from disease, predation etc (30% loss)




• Calculate potential crop loss ($/ha) using the equation.
  
• Unless pressure is extreme, delay any control until crops are setting, filling or maturing pods. It may be necessary to apply control during flowering if targeting small larvae that will be medium-large by pod set.


• Details of the research, and calculations of economic thresholds are in the new brochure – get yourself a copy!
  
  This research was supported by GRDC and DPI&F.

Get a copy of new Helicoverpa Management in Chickpea brochure from the DPI&F Call Centre, phone 13 25 23. Will soon be downloadable from the web.