Armyworms organising coup in winter cereals

In summary

LOOK OUT! There have been increased reports of Armyworms in cropping regions throughout NSW and southern QLD.

Be sure to monitor for eggs and larvae in cereals crops, particularly as crops approach maturity, as this is when damage from armyworms is most severe.

Larvae are identifiable through three parallel white stripes along their bodies. Damage is often in the form of scalloped or tattered leaf tissue, or lopped grain heads in severe cases.

Identifying the life stage of the pest population can allow you to make informed decisions about the best way to achieve control.

Armyworm odentification

There are a number of species which are identified as armyworms. The most widely spread being the Common armyworm. This is the caterpillar of the native noctuid-moth (Leucania convecta, known previously as Mythimna convecta).

It should be noted that armyworms can be sometimes be confused for Helicoverpa caterpillars and Cutworms (Agrotis spp.).

Larvae of Common Armyworm (L. convecta). Image: QDAFF

Adult of Common Armyworm (L. convecta). Image: Mark Stevens

Distinguishing between the various armyworm species can be difficult but due to similar habits, damage and methods of control, identification of individual species is not always essential. Key characteristics for armyworms include:

Adults:

• Wingspan up to 40 mm
• Wings fold in inverted ‘V’ over abdomen

Larvae:

• Up to 40 mm in length
• Smooth bodies with a minimal fine hairs
• Body colour can vary but three longitudinal white stripes extend from the ‘collar’ (behind the head)

Eggs:

• 0.5 mm in diameter
• Light brown, turning darker as they develop further
• Often laid in masses

Damage

Armyworms feed on leaf tissue. They typically leave a scalloped / tattered appearance on leaf margins.

In drastic cases, entire leaves may be severed from the stem. During crop vegetative stages, large populations can cause severe defoliation, with heavy feeding leaving only the midrib.

As the season progresses into spring, further yield potential can be lost as armyworms move onto the flag leaf, flag -1 and flag -2. Trials from North Star have demonstrated that larvae will preferentially seek these leaves despite green leaf being available lower down.

Head-lopping, particularly of wheat and barley can be quite devastating to final yield. As crops begin to senesce and leaves dry up, larvae will move up the plant to feed on the last green remnants. These are the nodes that attach the heads to the main stem, which severs the connection, hence head-lopping.

Monitoring and Thresholds

Late in the day is the best time to monitor for armyworm.

Beat sheets have been found to be the most effective in indicating the population of small and medium sized larvae as it does a better job dislodging them from the canopy.

Large larvae can often be found near the base of plants. It is highly recommended to further examine soil near the crop row and underneath litter.

Armyworms tend to be more active at night, so identifying the proportion of large larvae, which are responsible for the majority of larval damage, close to the ground during the day can improve accuracy of population surveys.

Ideally, monitor crops at least fortnightly during vegetative stages and then increase to weekly if larvae are found as the crops begin to senesce. Look for signs of caterpillar droppings (frass) on the ground, damage to leaves and plant heads.

Economic thresholds are particularly important in barley since it is the most likely to experience head-lopping. Thresholds for wheat and oats are slightly higher than those below:

• Vegetative outbreaks – 8 to 10 larvae per m²
• Maturing crop outbreaks – 2 to 3 large larvae per m².1 head of barley/m² equals 10 kg/grain/ha. 1 larvae/m² can cause a loss of 70 kg/ha grain/day.

Control

Early identification of the population is vital to making an informed decision around whether control is required.

Consider the proportion of the population which are capable of causing damage (instar 4-6) and those that are smalls. Small larvae take 8-10 days to reach a size which is capable of head-lopping, so a large population of smalls in a near harvestable crop may mean control may not be required.

While the same population in a crop which is still green and filling grain will likely require control. Table 1 outlines potential options for treating common armyworm. It should be noted that large larvae may be difficult to control.

Table 1. List of potential actives to be used in winter cereals crops on common armyworm and their harvest withholding periods as of August 2019.

Be aware that labels can be reviewed and harvest withholding periods can be changed.

Also be mindful that grazing withholding periods can be different to harvest withholding periods. Always read the label.

When applying insecticide, spray towards the evening when armyworms are most active. High water rates will be required to ensure adequate coverage in the crop canopy where larvae are hiding. Be mindful of weather conditions and implement adequate spray drift management.

Keep in mind withholding periods when spraying close to harvest. Ensure to follow all label directions and use of relevant personal protective equipment, such as chemical filters in tractor cabins, as some modes of action can impact the nervous system.

For more information contact your Local Land Services Ag Advisor. You can call Local Land Services on 1300 795 299 from 8.30am to 4.30pm Monday to Friday.

References and further resources

Armyworm, 2018, Government of Western Australia

Armyworm, 2018, The State of Queensland

Armyworms, 2018, The State of Victoria

Armyworm in winter cereals, 2017, State of Queensland

Miles M, 2019, Armyworm outbreak

Pest Notes Southern - Armyworm, 2019, Cesar,

GRDC Grow Notes – Wheat: Section 7 Insect Control, 2016, Grains Research & Development Corporation,  pp. 8-11.