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.


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.


  1. 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.
  2. Sparks, Alton N. 1979. “A Review of the Biology of the Fall Armyworm.” The Florida Entomologist, vol. 62, no. 2, pp. 82–87. JSTOR,
  3. Passoa, S. 1991. “Color identification of economically important Spodoptera larvae in Honduras (Lepidoptera:Noctuidae)”.
  4. Zuefle, M. 2018. “Sweet Corn Larval Pest Identification”. NY State IPM Program.

Other regional agencies will be initiating a trapping program for this species and other armyworms. The presence of moths reported here are considered ‘non-target by-catch’ = the large yellow underwing moths (Noctua pronuba) were caught in pheromone traps deployed for other species in another program (OSU VegNet). Nevertheless, it has proven to be at least an indicator of timing over the past few years.

N. pronuba moths are easily recognized by their large size (2-3″ wingspan) and bright yellow-orange hindwing bordered by a thick black margin (FIG. 1). Review similar posts for more information about this species. Larvae behave as armyworms and can be active throughout the fall and winter.

moths in hand showing wing pattern
moths were detected in 2 different cropping systems this week
wing and ruler - 1 inch
‘large’ is a key aspect of this moth

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

…Could be the name of a Halloween party cover band, don’t you think? But in fact, these are the common names for Pyraloidea adults and larvae, respectively. Pyraloidea is the third largest superfamily of the Lepidoptera, and is comprised of two families – Pyralidae and Crambidae. Together, there are about 16K species worldwide.

Within the group, there is incredible diversity of feeding habits. Larvae feed on corn, rice, and other grains and grasses, ferns, fruit and vegetable crops, potted tree seedlings, and even excretions of sloths and bats. Some species have larvae that are fully aquatic – very rare indeed for a baby moth or butterfly! Common names “CRANBERRY GIRDLER” and “GRASS-VENEER MOTH” also reflect feeding behavior.

Adults are called ‘snout-moths’ because of the long, upturned labial palps on the mouth region of the head (FIGs. 1 and 2).

anatomical sketch
FIGURE 1. Head anatomy of Pyraloidea. ILLUSTRATION BASED ON: GF Hampson 1892. Fauna of British India. Public domain {{PD-US-expired}}
FIGURE 2 – Adults cranberry girdler with eggs. PHOTO CREDIT: CD Barrentine,

Adult snout moths are slender and hold their wings folded over their back when at rest. Wing patterns vary by species. These moths fly low and usually in a zig-zag pattern; this habit can be useful when scouting.

Larvae are slender, white to cream-colored caterpillars with tan heads. They have 3 thoracic (true) prolegs and 5 abdominal prolegs and can therefore be confused for cutworms (earning them a spot on my blog here!). Figure 3 shows identifying characters of webworms. The comparative ID guide for cutworms can be viewed here.

Larvae vary in color from black to green to tan depending on food source, species, and age. Raised, sclerotized areas of the integument look like brown ‘spots’ along the body. The spots become more prominent with age but be careful: cranberry girdler’s spots are the same color as the integument and therefore not obvious. Mature larvae range from 5/8 to 3/4 inch long.

FIGURE 3 – Webworms can look very much like cutworms. Both have 2 setae (hairs) on the lateral side of the prothorax, but one must examine the orientation of the crochets to tell the difference. In Noctuidae (cutworms and armyworms), the crochets are arranged in a row or half-circle.

Cranberry girdler is more difficult to detect and control than other, related sod webworms because it typically feeds at or below the thatch layer. This species is sometimes called ‘subterranean webworm’. Digging and sifting through soil is one way to detect the presence of larval webworms, but if an infestation is suspected, the most effective way to confirm it is to pour 2 gallons of a soap solution over an area of about 1 square yard. This irritates the larvae and causes them them to come to the surface. 5 webworms per square yard is considered to be an action threshold in most grasses.


Sod webworm seems to thrive in drought conditions, so regular watering can help reduce the risk. Various products are registered, consult the PNW Insect Management Handbook for suggestions. Mid-summer is the best time to apply treatments, because larvae overwinter deep in the soil column and therefore are harder to reach in fall and spring.

Please take note that the 2nd flight of Mamestra configurata is much higher than normal. Larvae of this species exhibit armyworm behavior – larvae move and feed in groups. Visually scan fields for areas of defoliation or seedpod feeding, and search surrounding soil – or better yet flag the location and return at night.

Damage can be extensive on a number of economically important Oregon crops including: MINT, ALFALFA, BELL PEPPERS, CORN, VEGETABLE BRASSICAS, and CANOLA.

Pay special attention to weedy fields – lambsquarters and pigweed are often used as egg-laying hosts.

Outbreaks of this species are rare, but can occur every 6-10 years. The 2010 Willamette Valley outbreak had 2 distinct peaks of very high activity compared to the long term average(grey shaded). Damage from this 2nd generation can occur into early autumn.

Adult moths have prominent white reniform spots (‘kidney bean-shaped’) and an irregular thick, white band near the terminal edge of the wing. There are 2 overlapping generations per year, and adults can fly up to 50 miles from where they originated!

Identifying larvae is difficult because there are different color forms that can vary substantially. Usually present is a yellow-orange line separating the ventral (pale) and dorsal (darker) areas, and the head is tan or light brown. If you find many larvae and need help identifying them, contact me or the OSU Insect ID clinic

  • Scouting recommendations and more info: related blog post, 2018
  • New publication on dispersal patterns and source populations in W. Canada: Erlandson, M., et al. (2019). “Examining population structure of a bertha armyworm, Mamestra configurata (Lepidoptera: Noctuidae), outbreak in western North America: Implications for gene flow and dispersal.” PloS one 14: e0218993.
  • PNW Handbook – Section: Vegetable crop pests – Armyworm

Original content is great, but sometimes it is unnecessary to reinvent the wheel, or in this case, the YouTube video about armyworm damage in wheat.

We are starting to monitor true armyworm (Mythimna unipuncta) in the PNW, but efforts from Midwest states are much more deliberate because the species can be so damaging to large acreage of grasses and grains.

Today I found this excellent video from Chris DiFonzo at Michigan State University. Chris explains damage to flag leaves, thresholds and how to scout for larvae, and predicted impact on yield. Grab a cup of coffee and watch the 8-minute video here:

Cover image of YouTube video

Soil type and the parameters associated with it (water holding capacity, %OM, etc.) could be important as we continue to investigate armyworm population dynamics. The Web Soil Survey is a useful, interactive tool that is freely available to all.

Investigating soil properties at field level may reveal trends important for pest management.
  • We are trying to discern why certain sites in our Tillamook trapping effort have 10-20X the amount of armyworm moths detected.
  • We know that when there is abundant soil moisture, more eggs are laid, and more eggs hatch, which increases the risk for damage by larvae.
  • Sites less than 5 miles apart have similar pasture management but very different trap counts.
  • In low-lying areas like coastal Oregon, the distance to the water table might vary between fields

I encourage you to check out the Web Soil Survey tool (link above) and do some “digging” around – there’s a wealth of information available!

Predicting cutworm and armyworm pressure is difficult. But without predictive tools (monitoring of adult moths, crop scouting, and detection of larvae), cutworm and armyworm attacks are only noticed too late, when the most severe damage has already been done.

Here are some factors that affect activity levels:

  • Cyclic booms and busts of pest population levels affect the balance of natural enemy regulation, which is one reason why we seem to have ‘bad’ cutworm and armyworm (and looper!) years followed by years where populations seem more normalized.
  • Some of the cutworm and armyworms we have in this region arrive each year as long-range migrants from southern latitudes. Weather patterns such as el Niño and la Niña can affect migration patterns.
  • Another predictor of risk is winter moisture level, both locally and in areas where moths overwinter. These, and other abiotic factors contribute to cutworm and armyworm survival and potential damage each year.

Wet Winter = Worms

Okay, so the equation isn’t quite that simple, but we do know that armyworm outbreaks tend to be worse after years with excessive rainfall from Sept-March. Due to recent trends, we will be researching WHY outbreaks happen and if we can better predict them. Stay tuned… (and tell your friends to subscribe for updates!).

Armyworm species, in particular, exhibit moisture dependency. This simply means that the percentage of egg hatch is greater if there is abundant soil moisture. Our regional precipitation was relatively normal from January-March (below, left), but 200% wetter than normal in April (right), and the flooding in May could affect egg hatch as well.

2019 Data and Maps from: the PRISM program, Oregon State University white=normal; yellow=slightly less; green=approx. 2X increase.