Large Yellow Underwing: A Cutworm Pest in Idaho

Background

Noctua pronuba (pronounced knock-TOO-ah pro-NEW-bah) is an accidentally introduced cutworm known as the large or greater yellow underwing for its distinctively colored adult moth stage. Native to western Europe, the insect was first detected in North America in 1979, when several moths were collected around a porch light in Nova Scotia, Canada.

No one knows how it arrived in North America from Europe. N. pronuba moths are unusually strong fliers, so it is conceivable they arrived by flight. However, they were probably carried here via human-aided transport (such as shipboard moths or larval-infested horticultural plants).

Once present in North America, the insect steadily dispersed south and westward across Canada and the United States. In the West, moths were detected in Wyoming during 2000, in western Oregon and Alberta (Canada) during 2001, and in Washington State and British Columbia during 2004. First detection in Idaho occurred on May 1, 2005, when a single full-grown caterpillar was discovered in a home vegetable garden in Moscow. This initial detection was followed three weeks later by the capture of an adult moth 300 miles away in Twin Falls County. N. pronuba may have been present in Idaho for several years before those first reports in 2005. The insect has since been confirmed in Boundary and Nez Perce Counties.

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Identification

Eggs. Eggs occur as a single large mass on plants and nonplant surfaces. Each mass consists of dozens to hundreds of individual pinhead-sized eggs arranged side by side in neat rows. Masses often occur at the leaf tips of grassy plants.

Several other common moths also lay eggs as masses, so this feature is not diagnostic for Noctua. Eggs are initially cream colored but darken with age.

Larvae. Full-grown larvae are about 1.5 inches long and have smooth, hairless bodies. Most are olive brown but some (Figure 1) have a distinct reddish tinge. Green variants are known elsewhere but have yet to be reported in Idaho.

Each body segment is marked with a bold black and cream dash on either side of the midline (Figure 2). The overall appearance is a series of dark broken dashes that run the length of the body. Marks are strongest on the back half of the body and fade toward the head. The tan head is marked with two thick black lines that point inward at the middle (Figure 3).

All other commonly encountered cutworms in Idaho lack the series of broken black dashes that identify N. pronuba. The only other strongly marked cutworm that overwinters as a larva is the spotted cutworm, Xestia sp. Spotted cutworm larvae bear diagonal black triangular slashes along the sides of the last four or five body segments that increase in boldness at the end of the body (Figure 4).

Pupae. Pupae are dark brown and about an inch long; pupal body shape is typical of many Idaho moths and is not distinctive to N. pronuba (Figure 5). Pupae occur without a silken cocoon under crop residue on the soil surface or within the first inch of loose soil.

Moths. Moths are easily identified by color and size. The hindwings are brightly colored in yellow or orange with a broad black border (Figure 6). Forewings of male moths are usually mottled dark gray with irregular black spots, while the forewings of females are light reddish tan with scattered freckles. Ten different color variants occur in Europe. Outstretched wings measure about 2 inches from tip to tip.

Moths fly at night and rest by day in dense vegetation. If these moths are flushed from daytime hiding places, their bright hindwings make them startlingly visible. Moths fly with jerky, fluttery movements and quickly drop back into vegetation. As soon as they land, they fold their drab forewings over the brightly colored hindwings (Figure 7). The camouflage coloration of the forewings makes these moths difficult to see. The visual effect is as if the moths have vanished into thin air.

A closely related European species, Noctua comes (pronounced knock-TOO-ah COE-mees), was accidentally introduced into British Columbia in 1982. This second exotic cutworm moth is similar in appearance to pronuba but has an obvious black dash mark in the center of the yellow area on its hindwing. N. comes is noticeably smaller than pronuba and so is named the lesser yellow underwing; its outstretched wings measure 1.5 inches from tip to tip. Entomologists at Washington State University found N. comes larvae at commercial vineyards in south-central Washington during 2005. This species is not known in Idaho.

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Biology

Life cycle. Figure 8 shows our current understanding of the pest’s seasonal life cycle in Idaho. N. pronuba survives Idaho winters as partially grown to almost-mature larvae under plant residue, low-growing weeds, and similar protected places on the soil surface. Larvae sporadically feed through the winter months whenever temperatures are above the mid-40s. Entomologists in Michigan—where the first US cases of crop damage were observed during 2007—nicknamed N. pronuba the winter cutworm and the snow cutworm for its ability to feed actively when other cutworms are dormant for the winter. Larvae there occasionally were observed crawling on snow.

Intensive larval feeding begins again with warm weather in late February or March. Like other cutworms, N. pronuba larvae feed at night and hide during the day under crop residue or clods. When disturbed, they curl into a C shape typical of cutworms. Caterpillars pupate during May and emerge as night-active moths beginning in early June. Based on anecdotal evidence, it is likely that most seasonal flight and egg-laying activity occurs during August. Moths are active through October.

Females require at least 4–6 weeks to mature their eggs, after which they lay egg masses on leaves and stems as well as on sticks, fences, and vehicles. A single moth lays up to 2,000 eggs. Eggs hatch after 2 or 3 weeks. Caterpillars feed from mid-August through fall and winter. Larvae develop through six instars, each larger than the previous. The earliest-hatching larvae have enough time to develop into full-grown caterpillars during the fall, while the last-hatching larvae do not reach full size until the following spring. It is these partly mature larvae that pose the threat of economic damage when they begin feeding again the following March and April.

Host plants. Larvae eat a wide variety of leafy vegetables, root crops, ornamental flowers, and weeds in addition to alfalfa, wheat, and other agronomic crops (Table 1). In Britain, N. pronuba is a common but minor pest in home flower and vegetable gardens.

Damage. Damage combines the stem-girdling and crown-feeding habits typical of subterranean cutworms with the leaf-feeding behavior associated with climbing cutworms. On some plants the cutworms also feed in the canopy on flowers buds and open blooms. On root crops, larvae chew around the crown and gnaw on roots.

There are two main periods of larval feeding: late summer through midfall and early spring. Larvae also feed on mild days throughout the winter months. In Michigan, N. pronuba larvae became nuisance pests around homes when they crawled en masse like armyworms from nearby infested crops into garages, sheds, and barns. Roads in some places were slippery from their crushed bodies.

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Pest Status

United States. Even though N. pronuba had been present in North America since 1979, crop damage was not reported until fall 2007 when Michigan farmers reported severe infestations in alfalfa hay and winter wheat fields. Larval feeding began in October and continued sporadically through March. Michigan homeowners subsequently complained their lawns were invaded by countless thousands of larvae that remained active under the snow during January. Even more bizarre were reports of dogs vomiting blood and passing whole larvae in their stool after eating these caterpillars.

Idaho. Following first pest detection in Idaho during spring 2005, informal sightings of moths gradually increased, but larvae were rarely seen and never reported for any crop. The first damaging infestations of larvae in Idaho were observed in February 2009 in a winter wheat field near Lewiston. Leaf clipping was severe enough that the field was treated with insecticide, plowed down, and reseeded. A second winter wheat field near Lapwai likewise suffered enough damage during April 2009 that it too was sprayed with insecticide and no-till reseeded. Damage appeared as if plants had been clipped with scissors (Figure 9). During the day, larvae could only be seen by searching under crop residue or digging between the rows 4–5 inches deep; night inspections with flashlights detected larvae feeding on plants and crawling on crop residue. Industry field staff subsequently reported infestations on Austrian winter peas and lentils, but these reports remain unconfirmed.

The Nez Perce County infestations—though limited—represent the most westward reports of economic damage from N. pronuba in the United States. The only other states reporting field crop infestations are Michigan (winter 2007–8) and North Dakota (spring 2008).

Any “cutworms” or “armyworms” actively feeding on warm winter days or crawling on snow-covered yards should be considered suspect specimens of N. pronuba.

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Infestation Risk Factors

Although our direct experience with N. pronuba in Idaho is limited, we believe the following three factors increase the risk of damaging infestations:

Fields with green vegetation from early fall through spring. Larval infestations are most likely in any fall-seeded crop (but especially winter wheat or other grass cereals), fields with perennial crops continuously present (particularly alfalfa and grass forages), and fallow fields with abundant volunteer cereal plants or winter grassy weeds. All of these allow for fall egg laying and subsequent larval feeding through winter and the following spring.

Spring crops planted in fields where volunteer wheat and barley grew during September but were sprayed out in the fall. Volunteer wheat and barley plants are potential egg-laying sites when they are present during the August–September moth flight period. Even if volunteer cereal plants were subsequently eliminated with herbicides during October, any larvae already present might survive the winter and be ready to feed again in the spring. Early seeded spring crops would be especially susceptible to damage. Our limited experience suggests that winter wheat after winter wheat is the crop sequence most likely to develop damaging larval infestations.

This same principle extends to backyard vegetable and flower gardens. Early season vegetables seeded or transplanted into weedy garden plots during April are at highest risk of damage from overwintering larvae. Larval presence in home landscapes during early spring is associated with mats of chickweed in flowerbeds and gardens.

Reduced tillage. Any cropping system that reduces soil disturbance potentially increases survival of N. pronuba larvae and pupae already present on the surface. Similarly, any management system that creates mats of crop residue on the soil surface potentially increases survival of larvae already present by serving as protective daytime hiding places.

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Managing Larval Infestations in Commercial Crops

No research has been conducted on management of N. pronuba in any agronomic crop in the United States or elsewhere. Until specific tactics can be devised, we recommend the following general integrated pest management practices:

Visually examine fall-seeded crops monthly after plant emergence through winter months when fields are not covered in snow and continue scouting through early May. Especially check winter wheat and alfalfa during February and March on mild days after the first snow melt.

Focus on bare patches in fields with thinned plant stands and look for plants showing cutworm leaf feeding and stem clipping. Confirm that living larvae are present by searching under crop residue; in no-till fields where soil is loose, dig 4 or 5 inches deep between the rows.

Economic thresholds are not available. You can judge the potential for continued feeding injury from larval size. If most larvae are at least 1.5 inches long, they already have done most of their feeding for the year and will pupate during May. In contrast, if most of the larvae are less than an inch long, they will continue feeding until they reach their full-grown size in early May.

The night-flying moths are attracted to lights and can be monitored with insect light traps. Captures anticipate the timing of egg-laying, but no research is available to predict the severity of larval infestations from moth captures. Limited studies in Europe suggest that N. pronuba females produce a male-attracting pheromone, but no commercial lures have been developed.

Biological control. Several species of naturally occurring predatory ground beetles could potentially prey on Noctua larvae and pupae, but there are no practical ways of manipulating these beneficial agents to increase their impact, other than minimizing the use of broad-spectrum insecticides. Parasitic wasps and flies that kill larvae are known in Europe but are unstudied in North America.

Cultural practices. Avoid planting winter wheat after winter wheat. Manage crop residue after harvest so that thick mats do not accumulate on the surface where larvae can hide.

Insecticides. Observed crop damage in Idaho has been limited to winter wheat. Table 2 lists commercial insecticides labeled for cutworm control in wheat. All of these products should be highly effective against N. pronuba larvae, especially if field scouting detects larvae while they are still small. Table 2 omits products that list armyworms or climbing cutworms without naming cutworms; these unlisted products may not be as effective in controlling soil-dwelling caterpillars.

Wheat seed treated with imidacloprid (Gaucho and others) or thiamethoxam (Cruiser) at high label rates for grasshopper control might also suppress early season Noctua infestations. However, because product labels specifically state that these seed treatments only provide early season protection from grasshoppers and because neither seed treatment lists cutworms or armyworms on the label, it seems likely that any Noctua suppression would be temporary. Seed treatments are most effective against sucking insects or insects that directly feed on seed. Insects with chewing mouth parts that feed on leaves—such as grasshoppers and cutworms—are more difficult to kill with seed treatments.

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References

Pacific Northwest Pest Management Handbooks—https://pnwhandbooks.org/.

About the Authors

Edward John Bechinski—Professor of Entomology and Extension IPM Coordinator with the Division of Entomology (retired), University of Idaho

Larry J. Smith—Extension Professor and Nez Perce County Extension Educator (retired), University of Idaho

Frank W. Merickel—Manager (retired), Barr Entomological Museum, University of Idaho

Reed Findlay—Extension Professor and Bingham/Bannock County Extension Educator, University of Idaho

Disclaimer

ALWAYS read and follow the instructions printed on the pesticide label. The pesticide recommendations in this UI publication do not substitute for instructions on the label. Pesticide laws and labels change frequently and may have changed since this publication was written. Some pesticides may have been withdrawn or had certain uses prohibited. Use pesticides with care. Do not use a pesticide unless the specific plant, animal, or other application site is specifically listed on the label. Store pesticides in their original containers and keep them out of the reach of children, pets, and livestock.

Trade Names—To simplify information, trade names have been used. No endorsement of named products is intended nor is criticism implied of similar products not mentioned.

Groundwater—To protect groundwater, when there is a choice of pesticides, the applicator should use the product least likely to leach.

CIS 1172 | Published January 2022 | © 2022 by the University of Idaho

Issued in furtherance of cooperative extension work in agriculture and home economics, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Barbara Petty, Director of University of Idaho Extension, University of Idaho, Moscow, Idaho 83844. The University of Idaho has a policy of nondiscrimination on the basis of race, color, religion, national origin, sex, sexual orientation, gender identity/expression, age, disability or status as a Vietnam-era veteran.