musings « Cutworm Central

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.

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

Assisting with community inquiries is one of my favorite parts about being an entomologist. It’s nice to be able to help someone, and inevitably, I end up learning something new! This week, thanks to a call from Toledo, OR – I learned about the Pandora Moth (Hemileucinae: Coloradia pandora). True, this doesn’t classify as a ‘cutworm’, but I needed a place to post about it, so here we are. These large beauties have an unusual lifecycle, can be massive defoliators of pine trees in the Western US, and are used as a food source by Indigenous Peoples. Click the “Continue reading” link below if your curiosity is as strong as mine!

Continue reading →

A few months ago, a paper was published, confirming that Bogong moths (Agrotis infusa) use geomagnetic fields as a navigational tool for their en-masse aestivation flight.

Say what, now?

Long-range migrations are common in the insect world, the most ‘notable’ (as in..the one in which most people take an interest) is that of the monarch butterfly. The migrations occur each year, are purposeful, and serve some ecological function; to reach breeding sites, etc..

In the case of the Bogong moth, millions of moths leave the plains of Australia and fly towards the high country of the Victorian alps to escape the heat and avoid desiccation. They congregate in such large numbers in the mountainous habitat that indigenous peoples would go there to collect the moths as a food source. They are also a preferred staple for mountain pygmy possums. In autumn, the moths return to the plains to lay eggs and the cycle begins again.

Aestivation is just a fancy word for: reduced state of biological activity that occurs in response to hot and dry conditions. It is a sort of ‘summer hibernation’, if you will.

Closer to home, we have Euxoa auxiliaris that exhibits the same pattern (replace pygmy possums with grizzly bears and Australian Alps with Rocky Mountains). The study found that successful orientation relies on a combination of visual cues and magnetic frequencies emitted by the earth, and that when either cue was missing or altered, their flights became uncoordinated and erratic.

Many other creatures exhibit magnetoreception (using geomagnetic signals to navigate): migratory songbirds, salmon, sea turtles, and others. 2021 update: leaf-cutter ants

Now we just someone to develop the superhero series for these Persistent, Amazing, Perceptive, and Strong …mutants?

No, wait – that name’s already taken:

Author note: not sure why the Dreyer et al. is being referred to as the ‘first’ discovery – 1. Baker, R.R. and J.G. Mather, Magnetic compass sense in the large yellow underwing moth, Noctua pronuba L. Animal Behaviour, 1982. 30(2): p. 543-548.

Other note: concerns of pesticide distribution, bioaccumulation and toxicity levels in this type of system have been raised since at least 2006, and warrant further review, IMO

*Apamea* moths are flying now. Larvae are cutworm pests of grasses.

Apamea cutworms can be serious pests of grasses. There are a few different species that are common in this region. Two of them have fun common names.

A. cogitata, the thoughtful Apamea

and its more unfortunate contrary:

A. dubitans, the doubtful Apamea

The current activity of this common group of moths is not of direct concern. Rather, it is the active, overwintering larvae that are considered pests because they feed on grass roots. A. dubitans and A. cogitata could be similar to the congeneric A. devastator, which only has 1 generation per year. I made a post about them in February.

In western Oregon, glassy cutworm (Apamea devastator) moths emerge in late June, peak in July, and larvae take 100-120 days to develop, depending on daylength¹. The species we caught in traps this week (thoughtful and doubtful) are less well-studied. But I wager a guess that we could begin to see larvae in fields by about mid-September.

Fun fact: “cogitare” translates from Latin as “to think”. One of Francis Bacon’s most famous works “Cogitata et visa” (“Thoughts and Conclusions”) was written in 1612. In it, he posed that philosophy and science have common interests.

¹ Kamm, 1990. Biological observations of glassy cutworm in western Oregon. Pan-Pac.Ent. vol. 66

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Tag Archives: musings

Fall Armyworm or Just…Armyworms in Fall?

Identification

References

Pandora Moth

MagNEAT-O Migrations

Grass pest moths detected

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