Are you seeing chewing damage in establishing crops – and no critter immediately obvious?
Scratch the soil or stubble surface near plants during the day, and you might find some night-feeding larvae hiding.
Armyworms (multiple species) are one insect group that primarily feed at night. Armyworm was recently identified in a wheat paddock in the NSW Riverina, where a grower observed lopped leaves and holes in leaves. The damage, found across 1000 acres, was described as low.
Cutworms (Agrotis spp.) are another night-feeding insect group that have been identified damaging establishing crops this season. Not far from Grenfell in the NSW Central West Slopes and Plains, hollowed out seeds and stripped leaves were seen in a paddock of barley. The highest population was found on lighter, sandier soils, and where millet was grown over summer. North of Cowra, cutworm were reported coming in from paddock verges, where they destroyed 30 hectares of canola. The cutworm were visble under soil clumps, and close to emerging seedlings.
While armyworms are more likely to stick to cereals and grasses in Australia, cutworms are not as fussy, and can attack cereals and broadleaf crops such as canola. This can complicate identification in cereals.
How to tell an armyworm from a cutworm
Armyworm and cutworm have unique features that set them apart, but these features are more subtle in younger larvae. Here’s what to pay close attention to:
Stripes (or lack thereof)
The tell-tale feature of an armyworm larva is the three white stripes on a ‘collar’ just behind their head. These three stripes often continue all the way down the length of their body.
Cutworm do not have these three stripes, rather they sometimes have a single faint longitudinal midline on their ‘collar’ and body.
While armyworms generally have contrasting shades giving the larvae a stripy appearance, cutworms are more uniform in colour and are relatively drab or dull larvae.
Body shape and appearance
Cutworm are generally chubbier looking than armyworm (which are comparatively slimmer), and they are often described as greasy looking.
Armyworm and cutworm in establishing crops – what to expect
Armyworm can cause economic damage to cereals during the vegetative stage in autumn/early winter – but they are more of a concern in maturing cereals as they graze heads, and (at their worst) lop barley tops right off. For this reason, the rule of thumb spray threshold is 8-10 larvae/m² for winter outbreaks in cereals – much higher than the 1-3 larvae m² rule of thumb in barley crops during spring.
However, current larval populations in south-eastern cereal crops will likely pupate before crop maturation in spring, and as many armyworm species are highly migratory, most moths will likely leave the paddock. Hence, if you are considering spraying for armyworm now, this should be guided by level of current damage, rather than an attempt to protect crops from spring damage.
Unlike armyworms, cutworms are solely an establishment pest in south-eastern winter crops. And while they are a sporadic pest, they can destroy entire patches of young crops. The rule of the thumb threshold for cutworm is 2 larvae per 0.5 m row in cereals and canola.
There are several insecticides registered for armyworm and cutworm control. If spraying is warranted, we encourage the use of ‘softer’ chemistries that have fewer impacts on beneficial predators and parasitoids than broadspectrum alternatives (where available).
If spraying broadspectrum chemistries is unavoidable, consider spot spraying if possible, to minimise the impact on beneficials (cutworms are often confined to specific regions within paddocks).
Spraying late in the day or in the evening may be necessary given the night-feeding behavior of most armyworm and cutworm.
The status of beneficials should be factored in when making the decision to spray. Armyworm and cutworm may be suppressed by natural enemies including spiders and parasitoid wasps, as well as naturally occurring fungal diseases.
Send us your grubs!
Collectively, there are thousands of moths and butterfly larvae that are foliage feeders in Australia. While only a handful are notorious in broadacre crops and pastures, sometimes species that are not recognised as typical pests can overflow into paddocks. In such cases, they may have a nibble on crops, and then quietly move on without leaving behind any serious damage.
But if you have seen an unrecognisable species causing economic damage, we highly recommend that you send in a sample to us*.
Even if we cannot identify the larva, we can try to rear it to adulthood (making identification is much easier!). This will allow us to record observations to better prepare growers and advisors should they have the same critter in their crops in the future.
*Please don’t send us suspected exotic pests – call the Exotic Pest Hotline on 1800 084 881!
Phil Bowden – Bowden Rural Services (South West Slopes NSW)
Aaron Hutchison – Grower (Riverina NSW)
Peter Watt – Elders (Central West Slopes & Plains NSW)
Bill Webb – AGnVET Services (Central West Slopes & Plains NSW