1) Excluding very small larvae from threshold calculations. Estimating the number of very small larvae (VS) is time consuming to do in the field, and the accuracy of the estimate can be affected by the age and condition of the crop (figure 1). Whilst VS larvae are indicative of the size of the potentially damaging population in 2-3 weeks time, they are not relevant to a decision about controlling the current population to prevent yield loss.

Figure 1. Comparison of visual, beat sheet and absolute sampling for helicoverpa larvae on (a) dryland chickpea at late flower – pod fill, and (b) irrigated chickpea at mid flower to early pod set. VS=very small, S=small, SM=small-medium, ML=medium-large, L=large larvae. (Source: Melina Miles, 2004)

2) Applying a 30% mortality factor to small larvae. Essentially, applying mortality is acknowledging that a proportion of the population will not cause any yield loss because they do not survive to become medium and large larvae. Natural mortality is likely a result of dislodgement from the plant, disease, cannibalism, and predation. Remember that 80-90% of crop damage is caused by medium and large larvae.

This recommendation is based on trial work, largely in CQ, in which we followed infestations of helicoverpa and determined what proportion of the starting population of small larvae survived to large. The level of survival was variable between fields, but on average we determined that 70% of small larvae died before they reached large. In making a recommendation to include

In practice, in calculating how many larvae per m2 are likely to contribute to yield loss in the crop if left untreated, the following equation be used:

Where S = small, M= medium and L= large larvae

3) One well timed spray should be enough.

A spray may need to be applied during late flowering if targeting small and medium larvae to prevent them causing damage to a podding crop when they are medium-large. However, treating a population to prevent damage to a vegetative or flowering crop offers no yield benefit.

Trial results show that, in chickpea, helicoverpa larvae cause very little damage to buds and flowers (Figure 2). This is quite different behaviour from that seen in mungbeans, for example, where larvae show a clear preference for buds and flowers. Chickpea leaves appear more palatable to helicoverpa larvae than that of mungbeans.

Figure 2. Feeding preferences of small and large helicoverpa larvae on chickpea.

These observations are supported by yield data from time of spraying trials which shows no significant benefit in yield from applying a spray at flowering. Yield loss starts to occur when larvae are present in podding and filling crops (Figure 3).

Figure 3. The impact of helicoverpa on chickpea yield when controlled at different stages of crop maturity. Bars with the same letter are not significantly different from each other.

Remember
Only one application of Steward® (indoxacarb) per crop, with a cut off for use of 15 September in CQ, and 15 October in cooler regions. Currently there is no evidence of resistance to indoxacarb in helicoverpa populations from cotton, but there has been no testing of larvae from CQ, or from chickpea. I would encourage you to make collections from chickpea to send to Dr Louise Rossiter, NSW DPI, Narrabri. Contact Louise on 02 6799 2428 or louise.rossiter@dpi.nsw.gov.au.

Revised ready-reckoner for calculating the economic threshold

Economic Thresholds (larvae/m2) for Conventional Pesticides

Cross-reference the cost of control versus the crop value to determine the economic threshold (ET).
If the cost of control = $25/ha and the crop value =$450/t, the ET = 2.8
If the cost of control = $10/ha and the crop value =$550/t, the ET = 0.9
The lower the cost of control, and the higher the crop value, the lower the threshold.
(Table compiled by Gordon Cumming, Pulse Australia)

Your opinions and experience are important, so leave a comment on this article or helicoverpa management issues.

Maybe you would like to respond to the following questions:

- Is the revised ready-reckoner any easier to use?

- Would a threshold calculator be a useful tool? i.e. where you put sampling and crop data in (larval number and size, grain price, cost of control) and it calculates the threshold.

Article by Melina Miles

There is a technical brochure available at the QPIF website, which provides a comprehensive overview of sampling and the use of economic thresholds to guide decision making. The brochure can be accessed via this link: (http://www.dpi.qld.gov.au/cps/rde/dpi/hs.xsl/26_6821_ENA_HTML.htm).

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.

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.

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.