Corn earworm, Helicoverpa armigera, are frequently found in winter cereals but usually numbers are too low to warrant control. Occasionally, however, corn earworm numbers may be sufficient to cause economic damage. The high value of today’s grain is further reason to carefully check for grub infestations.
It is not unusual to find both corn earworm and armyworm in cereal crops. Correct identification of the species present is very important as it influences damage potential and control measures.
Virtually all of the helicoverpa in cereals (barley, wheat, triticale, oats, canary) are the corn earworm, H. armigera. This is very relevant, because this species has developed resistance to many of the older insecticide groups that have been used to control it.
Corn earworm
Armyworm larvae (such as common armyworm Leucania convecta) are distinguished from corn earworm larvae by the presence of three pale stripes just behind the head, and by their smooth skin, without any hairs or bumps. While corn earworm will be active on the crop day and night, most armyworm will shelter on the ground during the day and feed at night.
Common armyworm
Life history
Corn earworm development on winter cereals is very much like on sorghum, where moths lay eggs singly on the preflowering heads, soon after emergence from the boot leaf. This results in relatively synchronous development stages in the crop, depending on moth flights, weather conditions and the spread of flowering in the crop. Larvae hatch from the eggs in 3 to 5 days and develop through the grain fill stage over 14 to 28 days, depending on temperature. Larvae tend to graze on the exposed tips of developing grains. Rather than totally consuming a low number or whole grains, they graze on a larger number of grains, thus increasing the potential losses. Most of the feeding will be during the final two instars. When mature, larvae drop to the ground and pupate in earthen cells. Moths will emerge 2 to 3 weeks later and start the cycle again.
Corn earworm do not cause the typical head-cutting of armyworms as seen in ripening barley crops.
Damage
How much damage do larvae cause to cereals? There are currently no data from cereals on which to base this decision, but in the past extrapolation from the old sorghum damage value (1.5 g grain loss per larvae) has been used as a guide.
To put this corn earworm damage value into perspective, there are some comparative data for other crops. The damage value for sorghum was recently revised upwards from 1.5 to 2.4 g per larva. The value for chickpea is 2.0 g per larvae, and the latest mungbean value is 3.5 g per larva. Using the old sorghum value (1.5 g per larva) is not unreasonable and may be on the conservative (low) side, but it provides sound guidance for decision-making.
Based on the preceding information, we can make an estimate of what represents a problem. One larvae per m2 potentially causes 15 kg grain loss/ha (based on the figure of 1.5 g/larva eaten). At $350/tonne, this loss equals $5.25/ha. For this example, the break even point where cost of control ($28.50/ha) equals potential lost grain is 5.4 larvae/m2 (28.50÷5.25). This is based on the cost of control being $28.50/ha (methomyl at 1.5 L/ha ($16.50) plus aerial application ($12.00/ha)).
Any of these parameters can be varied to suit individual costs, and can incorporate a working benefit:cost ratio. A benefit:cost ratio of 1.5 is common and means that the projected economic benefit of the spray will be 1.5 times the cost of that spray. Spraying at the break even point (benefit:cost ratio of 1) is not recommended.
It should also be remembered that larval damage is irrespective of yield potential of the crop i.e. each larva will eat its fill whether it is 1 tonne/ha crop or a 3 tonne/ha crop.
Management
In many cases corn earworm larvae are not identified in cereals until they are medium or large in size i.e. >7 mm in length. This has implications for their management. Because corn earworm have historically had high resistance to pyrethroids, a pyrethroid is unlikely to provide satisfactory control, particularly if larvae are greater than 3 to 5 mm in length. While resistance levels to pyrethroids may have declined in recent years, control of medium-large corn earworm larvae using pyrethroids is not recommended.
Methomyl is another registered option at 1.5 to 2.0 L/ha, with the higher rate against larvae greater than 20 mm in length. It has contact action only (no residual), but that is not a problem because reinfestation is most unlikely. Resistance to carbamates (methomyl) has been a problem in the past, so any decision to use this product should be based on its recent performance against pest infestations.
In situations where both corn earworm and armyworm are present, carefully assess the relative abundance of each. Head-cutting activities of large armyworm larvae in ripening barley crops can be very serious and require prompt action. While registered pyrethroids may be a preferred option for armyworm, they are unlikely to have much impact on corn earworm larvae. Methomyl is also registered for both pests in wheat and barley. As armyworm mostly feed at night, spraying at dusk is recommended for best results.
Always read the label and abide by relevant withholding periods and export grazing/slaughter intervals where feeding to stock may be involved.
Many natural enemies (predators, parasites and pathogens) attack corn earworm larvae. Where winter cereals have been previously treated with broad spectrum insecticides to control aphids, fewer natural enemies may be present and survival of caterpillar pests could be greater than normal.
Further information
Helicoverpa (QDAF).
Last week Glen Milne asked on the blog “Do you spray (wheat/barley) on numbers of heliothis or wait until there is grain damage?”
My initial response to this question is that if you wait until the grains start getting attacked, then the helis (all armigera in wheat) will also be larger by then and thus more difficult to kill. Once they get above 13 mm in size you only have 2 options left:
1. Disregard the larger (>13 mm) larvae and focus on controlling the smaller larvae in the crop (thus giving you some more options like an SP, NPV or combination of the 2 depending on the situation with mixed heli/armyworm populations).
2. Or if you are going to target these large heli larvae, the only viable control option left is a stand-alone and more expensive methomyl spray .
We haven’t heard any reports to date on how methomyl is working in the field against these larger grubs this season, though we understand some methomyl may have gone out. We have heard of mainly SP alone or SP+low rate NPV – I heard one report of only 70% control of helis from an SP spray – not certain what rate, but assume there must have been some larger larvae in there. We’d be interested to hear any news from readers on recent spray performance…
The other key issue is the stage of the crop:
i.e. Some judgements will need to be made about how quickly the larvae will reach damaging size and when this will occur in relation to the crop’s development.
For example, if the crop is nearing full maturity/harvest, and the grubs are still small, then there is most likely no need to spray.
The other extreme would be a late crop that is still very green and at early seed fill. In this case, any small larvae present will most likely reach their most damaging size in time to significantly reduce crop yield, and so a spray is likely to be required at some point.
Given the unlikelihood of reinfestation by the helis, an early intervention could be better than a later one (i.e. more options and potentially cheaper).
So that is another factor to weigh up.
Finally, just a reminder to watch the withholding periods if you are contemplating a spray, especially one late in the crop’s life.
Very timely. There are heliothis in the wheat crops but I find it hard to see much damage. They seem to eat the awns and a bit of leaf, but little grain damage as yet. Talking to Graham Spackman from Emerald he was saying they did not spray till they saw the damage in the wheat head. DO you spray on numbers of heliothis or wait until there is grain damage? Cheers Glenn
An indirect answer addresses larval development on the crop. H. armigera larvae on wheat and barley usually complete development before ripening of the grain. This is because most eggs are laid on the newly emerged heads (pre-flowering). Larval development will take 30 days at an average temperature of 18.5°C, 20 days at 21.3°C and 16 days at 24.6°C. Add 3 to 5 days for egg hatch, and this gives the approximate time after flowering when larvae will be mature and drop off the plants. This timing should see mature larvae leaving the plants to pupate during the early hard dough stage, and most feeding having occurred during the soft dough stage. The only complicating factor would be if there was uneven flowering in the crop, with larvae on late flowering heads hanging about for longer, and possibly moving onto other heads. Mature grains are not particularly appetising, nor easy to feed on because of the hard seed coat.
Remember that large larvae on the crop have probably completed most of their feeding, so killing them at this late stage may not be of much benefit. Hope this helps.
Very timely. There have been enquiries from irrigated wheat growers near Emerald re this over the last few days. In some cases, large Helicoverpa grubs that have survived an insecticide application targeted at armyworm.
One question – crops are approaching maturity (starting to ‘turn’) – how mature/dry do wheat grains need to be before they are unpalatable to Helicoverpa?
Richard Routley
Marvellous work DPI&F Ento team!