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.
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.
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.
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.
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.