Last year we received reports of armyworm being found in cereal crops as early as April, and in some (although not all) cases populations grew large enough to cause significant damage. Here we outline what is known about armyworm migration and how they might respond to conditions this year.
Types of armyworm
Armyworm is the common name for several crop and pasture moth larvae spanning temperate, sub-tropical and tropical species.
In south-eastern Australian autumn sown cereals, armyworm populations are likely to be native species; either the common armyworm (Mythimna convecta), the southern armyworm (Persectania ewingii) or the inland armyworm (Persectania dyscrita).
At the larval stage, these species are difficult to tell apart and correct species identification in the field is generally not critical because their habits, type of damage and control are similar.
For more information on armyworm including identification, control and behaviour visit our PestNote.
Migration and breeding
Armyworm species are migratory, which means it’s possible for them to migrate into paddocks and seem to appear overnight.
Native species of armyworm are examples of migratory moth species that are well adapted to breed on native and introduced grasses in production and more arid inland regions.
When common, southern and inland armyworm migrate, they can move quite short distances (10s, 100’s Km within or between crops) or fly up high into the geotrophic layer and take advantage of low pressure systems that can transport them 1000’s of Km from their starting point.
Therefore, migrating moths tend to be concentrated in areas where rain is likely, which then supports armyworm population growth. This is a useful migratory strategy, particularly in a year following a drought when natural enemy populations have not recovered full strength.
Under the right conditions in autumn armyworm populations can quickly build in numbers as winter grasses and crop host plants enter a vegetative growth phase.
Inland and southern armyworm are the species most likely to be found in Victoria during autumn and winter, as they are relatively cold tolerant, whereas the common armyworm is normally found in NSW over the winter with flights into Victoria occurring during spring.
The development of southern armyworm slows down over the colder months in Victoria helping them survive, but if populations immigrate into crops in autumn, and if conditions are mild, they can undergo two generations from autumn to spring.
Oviposition behaviour during autumn is central to armyworm populations having a ‘head start’ in crops. Armyworm moths lay their egg batches (in their hundreds) in very tight cracks and crevices in Gramineae species. One known egg-laying site is the remnant crevice between the flag leaf and dead tillers in dried grasses and cereal stubble.
Studies have showed that green grass pasture with a dried grass component attract more egg laying than a pasture with just green grass. This egg laying preference helps to explain why larval populations are often found in stubble-retained paddocks.
Will armyworm be a problem this year?
Outbreak events require adequate populations in source areas (i.e. supported by autumn rainfall in arid inland regions), suitable atmospheric conditions to facilitate moth migration to production regions, as well as rainfall and subsequent vegetative growth of plant hosts within a short time after egg lay.
Last year large populations in the Lodden Mallee and Eastern Wimmera caused noticeable crop defoliation, although there were also many cases where no damage was evident.
During 2020, it is possible that a mild winter in south eastern Australia favoured armyworm species. Across Australia, the January – November 2020 mean temperature was the third warmest on record. This combination of higher than average rainfall, and higher than average temperatures from autumn – spring over much of the country is likely to have supported the breeding of native species of armyworm in arid inland regions, as well as survival in cropping regions over winter.
The above average reports of armyworm in 2020 were also likely a result of post-drought conditions which allowed them to recover quickly by laying lots of eggs, while natural enemies were slower to recover.
When we consider 2021, it is likely to be mild in the leadup to June – similar to last year – but we are likely to see less rainfall compared to 2020, which will be less favourable to armyworm.
Natural enemies such as parasitoid wasps and flies, and predatory shield bugs should also be showing some recovery, and this will hopefully help to keep armyworm populations under control.
Warm wet conditions are also conducive to fungal and bacterial pathogens which have been shown to suppress armyworm numbers when populations are high. Keep an eye out for bent over and limp caterpillars as an indication that they are affected by disease.
When do armyworm cause damage to crops?
Armyworm aren’t generally a problem early in the season when they are in low numbers. If the crop is young, feeding occasionally sets back the crop, but generally there is minimal economic impact unless there’s heavy defoliation. The risk of head lopping occurs when the crop is drying off and late stage larvae move up the stem to feed on the last of the green material (such as the nodes).
Large larvae are most likely responsible for head lopping because armyworm feed more as they reach maturity, and they have bigger jaws for chewing than smaller grubs.
Wheat and triticale have thicker stems and are less susceptible to head loss than barley, while oat is likely buffered from significant damage as they have multiple panicles.
From a management perspective, it’s not necessary to distinguish between southern, inland and common armyworm as their developmental rates and damage are similar.
Armyworm larvae are most active at night, seeking shelter under debris and vegetation on the ground during the day. This means that the best way to monitor is to do a direct ground search along with a sweep net in the evening.
Control is rarely needed for any native species of armyworm although larval numbers should be monitored in relation to crop growth stage. A rule of thumb threshold of 8 – 10 armyworm larvae per m2 in cereals provides a guide for spray decisions during winter and early spring.
When crops approach ripening and are drying off, the threshold drops to 1 – 3 larvae per m2 due to the risk of head lopping.
If you find armyworm in your crops please let us know so we can keep up to date with their numbers in 2021.
Thank you to Dr Jess Lye for assisting with the development of this article.