Refresher on managing helicoverpa in chickpea

With the approach of spring, helicoverpa start to become active. In CQ, chickpea crops are attractive to moths, and it is timely to revisit some of the key points related to making decisions about control of this pest in crops.

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).

In this article, I wanted to discuss a couple of the recommendations which some growers and agronomists have found a bit challenging. The first is adjusting larval density estimates to focus on those larvae that will cause damage.

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 natural larval mortality we have been conservative, using only 30% mortality (the highest level of survival we found at any of the sites we monitored).

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. The timing of a spray should control larvae before they cause damage, but not simply be applied if larvae in the crop exceed threshold. Infestations of helicoverpa in vegetative and flowering crops have been shown to cause no yield loss in chickpea.

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:

- How are you calculating your economic thresholds? Are you using the equation suggested by QPIF, or is it close enough to 2 or 3 that you are just using that?

- 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

Winter cereal aphids – background to the potential impact of infestations

The most critical issues we face in managing cereal aphids currently is the lack of local knowledge about the likely impact of infestations on yield and quality (the damage thresholds).

In this article, I will not go over aphid basics i.e. identification and sampling. You can follow the links to previous articles to read about these (http://thebeatsheet-ipmnews.blogspot.com/2008/09/cereal-aphids-in-wheat-and-barley.html).

In this posting, I want to discuss what is known, from overseas research, and what we might draw on from this work to help us make decisions about aphid management and control. This review may provide some useful information, in the absence of any locally generated data on aphid impacts. Surprisingly, there has been very little work done on cereal aphids in Australia.

General points
The literature, largely from North America and Europe, indicates that there can be significant differences in the way different cultivars respond to the impact of aphids. For this reason, it is important to use this information as general information that may assist in understanding how your crop may be responding to an aphid infestation. In the absence of local data, it is a useful starting point.

Aphids have a requirement for nitrogen (N) to complete development and reproduce. Honeydew is a by-product of their feeding. Essentially aphids compete with the plant for available N, which can impact on the plant in at different stages of crop development.

Early aphid infestations (from seedling)
Root and shoot growth may be impaired as a result of aphids competing for N. Inadequate N for the crop may make the crop more vulnerable to the impact of an aphid infestation.

Prolonged infestation can reduce tillering and result in earlier leaf senescence. Controlling aphids generally results in a recovery of the rate of root and shoot development, but there can be a delay.

Late aphid infestations
There is no impact on yield after grain has filled and is maturing (soft-hard dough).

Infestations that occur during booting to milky dough, particularly where aphids are colonising the flag leaf, stem and ear, result in yield loss. Generally, the distal grains in the head fail to fill. Infestations at this stage in which aphids colonise the leaves, particularly lower in the canopy, tend to result in grain with reduced N (protein) rather than a loss in yield. Aphids are intercepting the N being relocated from leaves to the filling grain.

The relative impact of timing and location of infestation makes sense if you review it along with what is known about the contribution of different parts of the crop to yield. The figure below illustrates the contribution of the upper leaves, stem and ear to the yield of wheat and barley (GRDC Winter Cereal Crop Growth Guide 2005 http://www.grdc.com.au/director/events/grdcpublications?item_id=8D607A46EDDFD98A822CFAEC7FCC4EC2&pageNumber=1).

Ongoing research
There is currently research being conducted on cereal aphids, by QPIF and the Northern Grower Alliance (NGA).

In 2008, initial trial work by QPIF and NGA showed different results (see the GRDC Update, Goondiwindi, 2009 papers for NGA results. Briefly, NGA trial work showed an overall yield benefit of around 10% from using seed dressings containing imidacloprid. QPIF results showed no difference from seed treatment, but a yield benefit where a foliar treatment (pirimicarb) was applied at head emergence.

Article by Melina Miles