Native species of armyworm (Persectania spp. or Mythimna convecta) have been seen in very high numbers in some (but not all) cereal paddocks and pastures in south-eastern cropping regions.
Here’s what to know about armyworm in crops this spring.
Why are numbers so high in some paddocks?
According to senior entomologist Dr Garry McDonald, whenever there is a drought, there is almost always an insect outbreak, and armyworms are an example of plant pests that can exploit post-drought conditions.
During post-drought conditions armyworm can undergo intense breeding and lay thousands of eggs. Natural enemies of armyworm are much slower to recover from droughts, giving armyworms a strategic head start.
While it is too difficult to say or predict where the breeding likely occurred, it’s important to recognise that armyworm are highly migratory and could have flown in from distant locations throughout the cropping season.
On the march
The name ‘armyworm’ comes from their behaviour as larvae when they sometimes ‘march’ out of crops and pastures in en masse in search of food.
This behavioural change usually occurs under extreme food depletion and crowding.
When armyworm larvae are in this gregarious state, they become darker and can be seen during the day (they are usually nocturnal!).
While marching is uncommon, it has been seen by some agronomists and growers this spring.
Like a moth to a…high stubble load!
Armyworm moths show a particular preference for laying their eggs in very tight cracks and crevices.
One known egg-laying site is the remnant crevice between the flag leaf and dead tillers in dried grasses and cereal stubble.
This preference helps to explain why larger larval populations are often found in stubble-retained paddocks and why cereal crops sown into cereal stubble can be at a higher risk of armyworm feeding damage.
Cereals and grasses are on the menu
The species of armyworm that occur during winter and spring in south-eastern Australia are quite specific feeders of cereals and grasses.
They generally do not damage oilseed and legume crops, although there have been some unusual exceptions to this rule.
For example, native species of armyworm have been caught lopping and defoliating the occasional lentil branch during previous springs.
If armyworm are in a gregarious marching state, it can also lead to unusual feeding behaviour.
Recently vetch plants were reported to have shown armyworm feeding damage in two paddocks. One paddock was a mixed barley and vetch crop, and the other contained barley volunteers. It’s likely that the barley was the primary and preferred target and the vetch experienced feeding damage as a result of the exceptionally large population that appeared to be ‘marching’ and depleting their primary food source.
If you are seeing significant chewing damage in a broadleaf crop by moth larvae and suspect armyworm, do share your observations with us.
Large grubs are the hungriest
It is large armyworm larvae, those that are in their final instars that are capable of the most damage.
Larger larvae eat greater volumes of green foliage and they are also capable of head-lopping when crops are ripening.
So, we can use a model that simulates the development times of the common armyworm and the southern armyworm using 15-year averages of temperature data to estimate when current populations will reach their ‘hungriest’ stage – the sixth instar (> 25 mm long) – in key cereal growing regions of southern NSW and Victoria.
We don’t have models for all armyworm species, and we don’t expect that you will distinguish between the common and southern armyworm in the field; we can provide both development times to provide a guide and date range.
Contact us to access the model.
Search down low
There is no doubt that a sweep net is a handy tool for monitoring crop canopies during spring. But it’s not the best tool for accurately assessing armyworm larvae numbers or sizes as they are generally night feeders and tend to shelter during the day in the soil or amongst stubble.
For example, recently an agronomist was finding armyworm larvae sheltering in 2-year-old canola stubble during the day.
We recommend incorporating a direct ground search, along with sweep net or beat sheet monitoring. Watch a quick demo on ground searching with QDAF entomologist Dr Melina Miles in the video below.
While armyworm have been reported in several cropping regions, not all cereal paddocks will need intervention and the decision to spray should be made on a case-by-case basis. Here’s what to consider:
There are other species of moth larvae around in cereals at the moment, including herringbone caterpillars (Proteuxoa spp.), brown pasture looper (Ciampa arietaria) and pasture day moth (Apina callisto).
To identify armyworm larvae, lookout for the white stripes on the collar behind their head, which can sometimes extend down the length of their body.
Rule of thumb thresholds
While we don’t currently have dynamic economic thresholds for armyworm, we do have nominal or ‘rule of thumb’ thresholds, which have been estimated based on observations and judgements of experienced consultants and researchers.
A threshold of 8 – 10 armyworm larvae per m2 in cereals provides a guide for spray decisions during winter and early spring.
But when crops are approaching ripening and are rapidly drying off, this threshold drops to 1 – 3 larvae per m2 due to the risk of head lopping. This lower threshold is more relevant to barley. 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.
Friendly forces are growing
There is a suite of beneficial natural enemies including parasitoid wasps and flies and predatory shield bugs, which build up in spring and help to supress armyworm populations.
Signs of parasitoid wasp activity includes the presence of furry or woolly pupae near dead larvae. These are the wasp larvae that have hatched inside the armyworm and have ruptured out in order to pupate and become adult wasps. You may even catch the wasp larvae wriggling out of the armyworm larvae before they pupate!
Keep an eye out for the eggs of the spined predatory shield bug as an indicator of their presence in crops. These irregular clusters of black eggs have circle of long spines around their rim.
What’s more if we do have a wet spring (fingers crossed!), this will likely help to create a humid canopy. Humid environments are conducive to fungal (and bacterial) pathogens which can also suppress armyworm populations in somecircumstances. Keep an eye out for bent over and limp caterpillars as an indication that they are affected by disease!
Thanks to the following for providing field observations: Greg Toomey, Chris Dunn, Hayden Lunn, Matt Tubb, Heath Griffiths (Nutrien Ag Solutions), Tom Batters (Agrivision), Rob Fox (Fox and Miles), Darcy Bullen (Western Ag), James Challis (Nutrien Ag), Claudia Higgens (Western Ag), Sheree Hamson (Elders), Andrew James (Dogshun Medlin).
Thanks to Dr Garry McDonald for providing expert advice.