Winter crops might be safe from at least one armyworm species

Have you seen an influx of moths in southern to central New South Wales of late?

In mid-late July, large numbers of moths have been seen by growers and advisors around Hillston and Griffith in the NSW Riverina, and also further north-east near Dubbo.

The sheer magnitude of these moth flights may seem alarming, and naturally it has us asking, what does it mean for winter crops in these regions?

Who is this mystery moth?

Based on images sent to the team at PestFacts south-eastern this mystery moth most closely resembles a type of armyworm called Spodoptera exigua. It has various common names including the small mottled willow moth, the lesser armyworm, and the beet armyworm.

S. exigua is thought native to south-east Asia, and like several other sub-tropical and temperate countries, it has been introduced to much of mainland Australia.

S. exigua is a pest at the larval stage and it has a very wide host range spanning broadleaf plants and grasses, vegetables, flowering ornamentals, and field crops (some susceptible field crops include cotton, maize, peas, lucerne, sunflower, sorghum and rice).

A S. exigua larva. Image credit: Russ Ottens, University of Georgia, Bugwood.org, CC by 3.0 US.

S. exigua has been seen in large numbers in Hillston in the NSW Riverina. Images: Aaron Hutchison.

S. exigua is regarded as a warm-season pest – so, how did these moths end up in southern and central NSW, smack-bang in the middle of winter?

We don’t believe they arose from local populations. S. exigua does not have a diapause stage in Australia, and it is likely too cold in southern NSW to support population development.

Rather, it is more likely that S. exigua has migrated from warmer regions where temperature allows them to persist year-round. While there are very few studies documenting its migration pattern in the Southern Hemisphere, S. exigua is a known long-distance migrator.

Are winter crops at risk?

Based on available studies, we don’t believe winter crops in southern NSW are at risk of damage from offspring of S. exigua moths. One study estimated a temperature requirement of 12°C for S. exigua egg development. Another study observed a disruption in female egg-lay and curtailing of life-span at 15.6°C (compared to 21.1°C and 26.7°C). Even if it has been unseasonably warm during July and moths did have the opportunity to mate, drops in overnight temperatures and frosts will likely make it too unfavorable for egg and larval development.

S. exigua has been documented flying as far south as Tasmania during spring and summer. Despite this, the presence of the moths in Tasmania and ample host crops available, S. exigua has never established as larvae or as a pest in general there, most likely due to unfavorable conditions. Moth presence does not always result in larval population or damage.

If you are concerned about these moths and want to be extra certain that they aren’t establishing, monitor plants for sign of egg-lay. S. exigua lay their eggs in rafts that are covered in white scales that give them a furry appearance!

Note that armyworm is a catch-all common name for several crop and pasture moth larvae spanning temperate, sub-tropical and tropical species in Australia. The information in this article relates to S. exigua in southern NSW. Several moth species have likely been flying about, as evidenced by other species of armyworm causing damage in winter cereals at the moment.

For more information on other armyworm species in southern NSW see this issue’s article, Sizing up armyworm.

A raft of S. exigua eggs. Note the furry, ‘cottony’ appearance. Image: Alton N. Sparks, Jr., University of Georgia, Bugwood.org, CC by 3.0 US.

Field observations

Thanks to:

Aaron Hutchison, Grower (Riverina NSW)

Maurie Street, Grain Orana Alliance (Central West Slopes and Plains NSW)