Western Bean Cutworm

Western bean cutworm (WBC), Striacosta albicosta, is native to the Great Plains region and was a common and significant pest of corn in Iowa in the early 2000’s. Recently, there has been a resurgence of reports of WBC injury across the state. Despite the name, WBC does not feed on soybeans; rather, they have a clear preference for dry beans and corn.

Identification

Adult: Adult WBC is a medium-sized moth, approximately 1.5 inches long, with grey-brown wings and a tan, fuzzy body. Patterns on the forewings can distinguish WBC moths from similar moths: WBC has a cream-colored stripe on the outer wing margin, cream-colored stripe with fringe hairs at the wing tip, a circular spot in the middle of the wing, and a “bean” or “comma” shape below the circular spot closest to the wing tip (Photo 1).

Egg: Clusters of WBC eggs can be found on the upper leaf surface of leaves in the upper canopy. Individual eggs are sort of square with ridges. They are initially white (Photo 2) but turn tan and then purple as they mature (Photo 3).

Larva: First instars have a purple hue and black heads (Photo 4). As they grow, their heads and bodies turn tan. Larvae are generally mottled grayish brown with alternating, wide stripes along the body. They are granular in appearance and mostly smooth, like other cutworms. As larvae mature, two black rectangles behind the head darken and become noticeable (Photo 5).

Pupa: Mature larvae burrow into the soil to create a pupation chamber for overwintering (Photo 6).

Be aware of look-alikes! Probably the caterpillar that is most often confused with western bean cutworm is corn earworm, largely because they feed on the ear in a similar way. Upon close investigation of the larvae, differences are noticeable. Both have tan heads but corn earworm has thinner stripes along the body, and is bumpy and hairy (Photo 7).

Biology

Western bean cutworm was historically found in the western U.S., and eastward range expansion was relatively slow between the 1940s and early 2000s. The first economic injury to corn in Iowa was recorded in 2000. After that, eastward expansion accelerated, and WBC can be found across the northern part of the eastern half of the U.S. and into Canada.

Corn and dry beans are the primary hosts for WBC. Soybean is not a known host in the field. Other hosts include teosinte, tomato, nightshades, and ground cherry, though larval survival is poor.

Western bean cutworms have one generation per year. Moths begin to emerge from the soil in early June. Mating occurs during July and August, and females lay eggs on a variety of cultivated and wild plants, though corn is the preferred egg-laying site in Iowa. Females seek out the uppermost leaves of plants that are still in the whorl stage (not yet reproductive) and lay clusters of eggs on the upper leaf surface (Photo 8). In corn that has already silked, females may lay eggs closer to the ear or directly on the husks. Once eggs hatch, larvae typically consume their eggs before moving either upward to the tassel or down the plant to the ear. Western bean cutworm larvae may be found feeding on pollen in leaf axils on their way to the ear. It seems that WBC larvae must feed on tassel tissue to complete development on corn; larvae fed only leaf tissue always died. Larvae develop through six larval instars before forming a pupation chamber in the soil to overwinter inside.

Plant Injury

Unlike other cutworms, WBC does not cut corn plants at any stage of development. In corn, we are most concerned about feeding on the ear (Photos 9 and 10). Although they also feed on tassels and pollen (Photo 11), this does not seem to interfere with pollination or become a source of economic injury. Larvae can feed on kernels anywhere on the ear, consuming the entire kernel or just scraping it, and frass or entrance holes may be present (Photo 12). Another concern is the development of molds that may be toxic to livestock or reduce quality of the grain (Photo 13).

Infestations may be patchy within fields, and larvae may move between plants which can lead to greater injury per larva. The yield reduction from one larva per plant is estimated to be up to 15.1 bushels/acre. In some fields, the greater concern with WBC is loss of grain quality due to ear molds and mycotoxin contamination, which can be detrimental to livestock.

Risk Factors

Western bean cutworm tends to be more common in fields with sandy soils, though we have heard reports of infestations in fields in Iowa that are not particularly sandy. Later-planted fields are at greatest risk of infestation since females are most attracted to corn that is close to pollination for laying eggs.

Scouting

Trapping moths: Green bucket traps baited with pheromone lures may be used beginning in June to capture adults. Traps should be checked weekly to determine the presence of moths in the area and relative abundance. These trap captures do not correlate to infestation in a field.

Scouting for egg masses and small larvae: Fields should be scouted 1-2 times per week before tasseling to determine the percentage of plants with egg masses. Scout 20 plants in five areas of the field (total of 100 plants) and look for egg masses on the upper surface of leaves in the upper canopy. If at least 5% have egg masses, an insecticide application is warranted. Scouting for egg masses can cease at R3 because females seek out later-developing fields for egg-laying.

Scouting for larvae in the ear: Although finding larvae in corn ears means it is likely too late for management, it is good practice to make note of any infestations and monitor those fields for mold development that may impact quality of the grain, especially if that field is used to feed livestock.

Management

Tracking development: Growing degree days (GDD) may be useful to help time scouting efforts for WBC. Scouting should begin when 25% of adult emergence has occurred, which is predicted to be around 2,577 GDD since March 1^st (base 38°F, max 75°F). Use the Pest Maps and Forecasting tool on the Iowa Environmental Mesonet website to see a map with estimated GDD accumulation for WBC.

Cultural: Cultural control options, such as adjusting planting date or using tillage, are unreliable given the biology of the pest.

Biological: Common predators, such as lady beetles, minute pirate bugs, lacewings, and ground beetles, are known to consume eggs and small larvae. Parasitoid wasps have been recovered from egg masses. It is unknown how much any of these natural enemies contribute to suppression of WBC, but when WBC is present in high enough numbers, biological control is insufficient.

Plant-incorporated protectants: Unlike some other caterpillar species in corn, there are few Bt proteins that target WBC. The Cry1F label was amended in 2003 as WBC moved eastward across the U.S. to include control of WBC, but laboratory studies showed that larvae had a high tolerance to the protein. Further field and laboratory studies documented rapid field-evolved resistance of WBC to Cry1F hybrids, and many fields planted with these hybrids had extensive feeding. Now, only hybrids containing the Vip3A protein are recommended to combat WBC. However, we caution that fields planted to a hybrid containing Vip3A should still be scouted for WBC larvae, because bioassays demonstrate that the level of Vip3A expression may not be sufficient to kill older larvae and they may only get a sub-lethal dose of the toxin if they moved from a refuge plant (a plant without the Bt toxin). Consult the Handy Bt Trait Table to learn more about which corn hybrids contain each Bt trait.

Chemical: Many insecticide products are labeled for management of WBC larvae in corn. Insecticide applications should be timed such that 90% of tassels have emerged. If tassels have emerged and egg hatch is occurring, applications should occur when 70-90% of eggs have hatched to ensure insecticides contact larvae as they travel toward the ear. Although field-level resistance to insecticides has not been documented for this pest, it is worth noting that larvae collected in Nebraska (where pyrethroids are commonly used) were less susceptible to bifenthrin than larvae from Ontario (where pyrethroids are used less often).

Additional Resources

Smith, J. L., C. D. DiFonzo, T. S. Baute, A. P. Michel, and C. H. Krupke. 2019. Ecology and Management of the Western Bean Cutworm (Lepidoptera: Noctuidae) in Corn and Dry Beans – Revision with Focus on the Great Lakes Region. Journal of Integrated Pest Management, 10(1): 27; 1-10. DOI: 10.1093/jipm/pmz025

C. DiFonzo. The Handy Bt Trait Table for U.S. Corn Production.