A3856

estern bean cutworm: WA pest of field and sweet corn EILEEN CULLEN & JIGYASA JYOTIKA Appearance The adult female moth is most Eggs. Adult females (moths) attracted to corn just before tas- lay round, white eggs on the upper seling and lays eggs primarily on surface of corn leaves. Each egg the upper surface of the flag leaf mass contains between 20 and 200 (top-most leaf on the plant). eggs (average 50 eggs per mass) Larvae hatch 5–7 days later. clustered close together. Eggs Newly hatched larvae remain clus- change from creamy white to tan, tered together for several hours, and then turn purple 12–24 hours feeding on the shells of their egg mass before dispersing. If the tassel beforeLarvae. larvae emerge. First instar larvae are has not yet emerged, larvae crawl Marlin E. Rice dull orange with a black head. Full- into the whorl and feed on pollen. estern bean cutworm grown larvae are approximately As the tassel emerges, larvae switch W 1½-inches long and are tan colored to feeding on green silks, entering Striacosta albicosta ( ) is native with a broad, faint tan stripe along the developing ear through silk to North America and has tradi- the back and no markings on the channels. tionally infested crops in the central sides. The most distinctive feature By early September, mature High Plains and western Corn Belt of the larvae is that the pronotum larvae leave the ear and drop to the states. Since 2000, however, western (“neck” area behind the head) has ground. They burrow into the soil bean cutworm (WBC) has where they create a chamber for twoAdults. broad brown stripes on it. expanded its range eastward into Adult WBC are brown overwintering. the Midwest; it was first detected in moths ¾-inch long with an Wisconsin in 2005. extended wingspan of 1½ inches. A late-season pest of field and There are three distinctive mark- sweet corn, WBC larvae feed on ings on each forewing: a white bar corn ears, damaging and con- along the front leading edge, a cir- suming kernels. Unlike corn borers, cular spot in the center, and a they do not tunnel into stalks. boomerang-shaped spot toward the Heavy infestations have caused outer edge. yield losses of up to 40% in Marlin E. Rice Colorado. WBC damage to corn in Life cycle Wisconsin has been sporadic, with WBC has one generation per low to moderate levels of damage year, with six or seven larval stages, since first detected in the state. and overwinters as a full-grown As its name suggests, western larva, inside a soil chamber. Spring bean cutworm is also a pest of dry development begins when temper- beans. Larvae feed on leaves and atures exceed 50°F. Larvae pupate blossoms of dry beans, and chew in May and moths begin to emerge holes in pod walls and developing Marlin E. Rice from soil chambers in late June. Creamy white eggs turn purple seeds. WBC is not considered a pest Peak emergence in the Upper shortly before western bean of soybeans. Midwest is typically between the cutworm larvae emerge. second and third weeks of July. WESTERN BEAN CUTWORM: A PEST OF FIELD AND SWEET CORN

Damage to field and Using degree days to predict sweet corn The primary damage caused to moth emergence corn is direct—WBC larvae feed on WBC larvae overwinter in the One behavioral characteristic of kernels at the corn ear tip, middle soil, pupate in spring, and moths WBC larvae is that multiple larvae and near the shank. WBC feeding emerge in response to accumulation can feed on the same corn ear, on corn ears may also create entry of heat units or degree days (DD) resulting in severe damage. This routes for secondary damage from rather than the calendar. Depending feeding behavior is due to the fact fungal pathogens. on degree-day accumulations in a that, unlike corn earworms, WBC Corn scouting given year, WBC moth emergence larvae are not cannibalistic. If silks begins in late June to early July and at the tip of the corn ear are already Scouting field and processing most eggs are laid during the period being fed upon by one of its “sib- sweet corn for WBC eggs and small of peak moth emergence, typically lings,” WBC larvae will chew a hole larvae is critical to determine field mid-July in the Upper Midwest. through the husk on the side of the infestation levels and prevent popu- Begin counting degree days on same ear. lations from reaching the economic May 1 using a base temperature of Damage caused by WBC injury level. Foliar insecticides effec- 50°F. To calculate the daily total heat becomes evident between early tively suppress WBC larval popula- units accumulated (degree days) for August and early September. tions, but only if applied before this insect, use the formula below Damage will be more apparent larvae enter the ear. and keep a running total for each toward the end of this period since There are two methods for day beyond May 1. larvae grow as they feed and larger determining when to begin There are two important degree larvae cause more damage. Damage scouting. One is to predict the day milestones for WBC: by WBC will be easiest to detect in period of moth emergence using 1,320 DD. Approximately a fourth August when most of the larvae are degree days. The other involves of the moths have emerged at still on the corn ears. Once mature using pheromone traps to see when this point; field scouting should larvae drop to the soil, it may be dif- moths actually begin flying. begin now. ficult to determine which corn ear 1,422 DD. pest (WBC, corn earworm, or Half the season’s WBC second-generation European corn moth population has emerged; borer) has caused the damage. known as peak flight period.

(daily high temp + daily low temp) Degree days = –50 2

Keep a running total of degree days beginning May 1. At 1,320 degree days, begin scouting fields for eggs. Marlin E. Rice Larval feeding damage may also lead to fungal rots.

Marlin E. Rice Using pheromone traps to monitor flights different stages of development sep- arately. In field corn planted to a Bt Pheromone traps use a WBC corn hybrid with WBC resistance, pheromone lure to attract male WBC remember to scout the non-Bt moths. The pheromone lure mimics refuge for WBC. Continue scouting the chemical structure naturally fields for 7–10 days after the peak secreted by WBC females to attract a flight to detect eggs and larvae. mate. You can make your own trap Threshold levels for field and sweet corn (see directions at right) or rely on regional trap catch information. For field corn, foliar insecticide Traps should be set by July 1 treatment should be considered and monitored weekly or more fre- when 8% of 100 corn plants quently until trap captures decline sampled have egg masses and/or Marlin E. Rice in August. When checking traps, small larvae. For processing sweet empty liquid into a jar, keep a corn, the threshold is lowered to 4% Making a running count of WBC moths, and infestation. pheromone trap replenish the trap with the water- Control antifreeze mixture. Biological and natural controls What you’ll need: a 1-gallon Pheromone lures should be plastic milk jug with lid, a large replaced after 4 weeks. Lures can Natural control plays a key role paper clip, a western bean cut- be ordered from Gemplers in regulating WBC populations. worm pheromone lure, propylene (gemplers.com), Great Lakes IPM Heavy rains can cause extensive (not ethylene) glycol antifreeze, Inc. (greatlakesipm.com), or Trece mortality of early instar larvae as dish soap, water, a 4-foot high Inc. (trece.com). can cold weather and high winds. post, and wire. If you’d prefer to monitor Colder winter temperatures will kill 1. Starting 2 inches from the regional trap data instead, visit WBC larvae overwintering near the bottom of the milk jug, cut www.ent.iastate.edu/trap/ soil surface. 4-inch by 4-inch square open- westernbeancutworm. This site, Several predatory insects feed ings on all four sides. coordinated by Iowa State on WBC larvae including larval and 2. Bend the paper clip into an University, has regional WBC trap adult lady beetle stages, as well as “S” shaped curve. Puncture catch numbers posted from various damsel bugs and spiders. the rubber lure with the end locations throughout Wisconsin and WBC larvae are susceptible to a of the paper clip and slide the the North Central region. Timing and technique for naturally occurring disease caused lure onto one end. scouting fields by the microsporidian Nosema sp., 3. Puncture a small hole in the but the impact of this disease is milk jug lid and attach the Once WBC degree-day accumu- unknown. lations reach 1,320 DD (25% moth Cultural control paper clip so the pheromone emergence), or when the first moths lure hangs inside the jug. Cultural controls such as tillage are detected in a pheromone trap in are not proven to be effective 4. Mount the trap on a 4-foot your area, examine 20 consecutive against WBC. Although plowing or high post at the edge of a corn plants at five locations in the disking soil may reduce survival of corn field. field to obtain a representative field overwintering larvae in soil cham- 5. Fill the bottom of the trap sample. Check the upper three or bers, the effectiveness of tillage as a with a 4:1 mixture of water to four leaves of each plant for pres- management tool requires further antifreeze, plus a few drops ence of WBC egg masses and small evaluation before it can be recom- of dish soap. larvae. Scout field corn hybrids or mended as a cultural control. processing sweet corn varieties in WESTERN BEAN CUTWORM: A PEST OF FIELD AND SWEET CORN

Chemical control modified by inserting a gene from acres do not contain the Bt insect Application timing is critical for an unrelated organism, the soil bac- trait used in the Bt planting. The foliar insecticides to be effective. If terium Bacillus thuringiensis (Bt). refuge must be planted to 20% of eggs have hatched in fields that Spores produced by the bacterium the corn acres on each farm, and have reached or exceeded the eco- contain a protein which, when there are specific configuration and nomic threshold, insecticide should ingested by a susceptible insect, distance requirements for the refuge be applied after 95% tassel emer- ruptures the insect’s midgut mem- in relation to the Bt corn planting. gence but before larvae enter the brane, prevents further feeding, and The aim of IRM is to maintain ears. If egg hatch has not yet kills the insect. WBC larvae ingest Bt susceptible insect populations by occurred and plants have tasseled, the Bt toxin when feeding on Bt way of a refuge. The refuge pro- treat as close to expected egg hatch corn plants. One group of Bt corn vides corn crop habitat that allows as possible, when egg masses have hybrids offers protection against target pest insects to feed, mate and reached the purple color stage indi- “caterpillar” pests such as European reproduce without being exposed to cating 12–24 hours to hatch. corn borer and WBC. Note that not the Bt trait. Without a refuge, target Foliar insecticides can be all Bt corn hybrids have WBC on the insect populations that are exposed applied by air or high-clearance label. Before purchasing, check the to Bt corn each growing season over ground equipment. Select an insecti- Bt corn hybrid label and check with multiple generations will eventually cide labeled for WBC larvae on field your UW-Extension county agent if become resistant to Bt. Mating or sweet corn with residual activity you have questions. between Bt susceptible insects from Bt corn and insect resistance of a few days. Follow label direc- management (IRM) the refuge and potential resistant tions to ensure sufficient water insects ensures that susceptibility to volume and coverage. Please Widespread adoption of Bt corn the Bt toxin is passed on to the next consult Extension publications Pest has increased the number of acres generation. Managementin Wisconsin Field Crops where target insect pests are Planting a refuge is required by (A3646) and Commercial Vegetable exposed to Bt active ingredients law through the U.S. Environmental Production in Wisconsin (A3422), or each growing season. This creates Protection Agency (EPA) as a condi- your UW-Extension county agent or potential for WBC and other target tion of Bt corn hybrid registration certified crop advisor for product pests to develop resistance to Bt. and market availability. Contact suggestions and label updates for Therefore, an insect resistance man- your county Extension office or see WBC larvae on field and processing agement (IRM) plan becomes more Extension publication Insect sweet corn. Transgenic Bt corn hybrids important in preserving the useful- Resistance Management and Refuge ness of Bt traits. The IRM plan is Requirements for Bt Corn (A3857) for Transgenic corn is another implemented by planting refuge more information. option for managing WBC in field corn acres on each farm where a Bt corn. Bt corn hybrids are genetically corn hybrid is planted. Refuge corn

Copyright © 2008 University of Wisconsin-System Board of Regents and University of Wisconsin-Extension, Cooperative Extension Author: Eileen Cullen is assistant professor and Jigyasa Jyotika is a project assistant of entomology, College of Agricultural and Life Sciences, University of Wisconsin-Madison and University of Wisconsin-Extension. Photos courtesy Marlin E. Rice, Iowa State University. Produced by Cooperative Extension Publications, University of Wisconsin-Extension. University of Wisconsin-Extension, Cooperative Extension , in cooperation with the U.S. Department of Agriculture and Wisconsin counties, publishes this information to further the purpose of the May 8 and June 30, 1914 Acts of Congress; and provides equal opportunities and affirmative action in employment and programming. If you need this material in an alternative format, contact the Office of Equal Opportunity and Diversity Programs or call Cooperative Extension Publications at 608-262-8076. This publication is available from your Wisconsin county Extension office or from Cooperative Extension Publications. To order, call toll-free 877-WIS-PUBS (947-7827) or visit our web site at learningstore.uwex.edu. Western Bean Cutworm: A Pest of Field and Sweet Corn (A3856) I-04-2008