Fall Armyworm or Just…Armyworms in Fall?

The common name ‘armyworm’ is added as an identifier based mostly on behavior. Larvae move in large groups, sometimes over quite impressive distances. In severe cases, a feeding wave can be seen, like the front line of an advancing Army.

Taxonomy (the science of grouping and naming organisms) can be equally important though. Of the 11 species of Spodoptera that are present in the U.S., 10 of them have armyworm as a common name. That is to say… Spodoptera = armyworm. There are of course armyworms from other genera, but we’ll have to explore that another time, or browse the blog here. Oddly, “true” armyworm is one example.

For many years since I’ve lived in Oregon, something has been bugging me. I’ve heard reports about “fall armyworm”, but all of my go-to websites list Spodoptera frugiperda as not occurring here: PNW moths (a huge NSF-funded, collaborative regional effort); nor MPG (moth photographers group which started as a hobby project and is now a great resource ran by Mississippi State); nor GBIF (an international network of biodiversity data). If you still don’t believe me and want to consult iNaturalist instead…we can’t be friends…but you’ll find the same thing – over 8,000 observations…none from Oregon.

Sparks (1979)

GBIF (1979 to 2022)

iNaturalist (2022)

Identification

Adult moths in the genus Spodoptera vary in size and wing patterning. Eggs are laid in large clusters and covered with a dense layer of scales from the female’s abdomen, giving the clusters a ‘fuzzy’ appearance (see examples).

Larvae have a black spot on either the mesothoracic segment (e.g. beet armyworm, S. exigua) or the first abdominal segment (e.g. fall armyworm, S. frugiperda)(see diagram). Patterning, stripes, color of the larvae all vary widely, even within populations. What S. frugiperda DOES have is four large, evenly colored tubercles (spots) arranged in a square on the dorsal side of abdominal segment A8. For GREAT pictures of them, please see reference #4. If it doesn’t have those four spots, it’s not fall armyworm. But caution here too… black cutworm has spots on A8 but they’re not in a square. Corn earworm has them in a square, but two of them are less prominent. Ok, I’m rambling and losing some of you, sorry :). My point is that…

I suspect that some of the reports of “fall armyworm” in the PNW have been from improper identification of larvae. If I’m wrong, please leave a comment so we can chat! There are MANY species of Noctuidae that can actively feed during the fall and winter. The photo below shows adult moths detected in pheromone traps during the same 2 days in late September 2022. All of them have larvae that feed in fall…but none of them are fall armyworm.

References

Early R, González-Moreno P, Murphy ST, and Day R. 2018. Forecasting the global extent of invasion of the cereal pest Spodoptera frugiperda, the fall armyworm. NeoBiota 40: 25-50. https://doi.org/10.3897/neobiota.40.28165

Sparks, Alton N. 1979. “A Review of the Biology of the Fall Armyworm.” The Florida Entomologist, vol. 62, no. 2, pp. 82–87. JSTOR, https://doi.org/10.2307/3494083

Passoa, S. 1991. “Color identification of economically important Spodoptera larvae in Honduras (Lepidoptera:Noctuidae)”. https://digitalcommons.unl.edu/insectamundi/414/

Zuefle, M. 2018. “Sweet Corn Larval Pest Identification”. NY State IPM Program. https://hdl.handle.net/1813/57328