May 28, 2009
The time has arrived for black cutworm scouting in Iowa. Corn fields in southern and central Iowa should be watched closely starting May 24 to 26, and in northern Iowa around the first of June.
Those are the 2009 dates for first cutting that are currently being predicted by Iowa State University Extension entomologists Erin Hodgson and Jon Tollefson. ISU pest management specialist Rich Pope prepared the following map of Iowa with the predicted dates for various areas drawn on it. Following are their recommendations for black cutworm scouting and control.
The black cutworm does not overwinter in Iowa; however adult moths migrate into Iowa, riding up from the Gulf of Mexico on southwesterly winds in early spring. Potentially significant numbers of adults arrived in Iowa and were first documented on April 27 and 28 this year across most of the southern two-thirds of Iowa. A second notable flight occurred on May 5 and 6; that flight was recorded throughout the state, according to the ISU specialists.
Pay particular attention to certain areas of fields
The female moths mate and will deposit about 1300 eggs singly or in masses in field low spots or overflow ground; areas overgrown with grasses and winter annual weeds are particularly attractive locations. Although the black cutworm is a pest that can infest corn fields in any of these areas, the female moths often prefer to lay their eggs near soybean stubble. Newly-hatched larvae (worms) will feed on weeds until corn emerges. The cutworm larvae feed on the tiny weeds and small corn plants and pass through six instars or growth stages in about 35 days, depending on temperatures.
Based on trap capture of the moths in ISU's monitoring program across the state, and the adult black cutworm moth flights and temperatures this spring, cutworm larvae could be cutting corn beginning around May 25 and 26 in the southern two thirds of Iowa, and the first week of June in the northern three tiers of counties, says the ISU specialists. Black cutworm arrival in Iowa varies each year, and 2009 is about a week later than in recent years.
Here's what you should be looking for
* Appearance. Black cutworm larvae vary from light grey to black with an overall greasy and shiny appearance. Fully-grown larvae are about 1.5 inches long and curl up when disturbed. Distinguishing black cutworm from dingy cutworm larvae in the field is important; both species will feed on corn but dingy cutworms rarely cut leaves or cut the plants off.
Find out how to separate the two species by reading a previous ICM News article . The adults are night-flying moths with thick, grey bodies. Black cutworm adults have a wing span of 1.5 inches, and the forewings have a black dagger-shaped mark near the edge.
* Damage. Factors that favor black cutworm outbreaks include late/reduced tillage, late planting, the presence of weeds, and fields next to permanent vegetation. The larvae—which is the worm stage--will move from weeds as the weeds are destroyed or consumed and the larvae start feeding on emerging corn leaves. Young larvae make small, irregular holes in the corn leaves and feed aboveground on the plants.
Older larvae (fourth to sixth instar stage) can cut the stems of corn plants or clip leaves and they usually feed underground at night. Black cutworm larvae can consume four to six leaves before pupating. If soils are dry or crusted, larvae can burrow down to moist soil and move to new plants. Black cutworms have difficulty cutting plants past the V5 (five true leaves) growth stage of the corn plant, and therefore corn less than 15" tall is most susceptible to black cutworm.
* Sampling. Start looking for black cutworms in your fields as soon as corn emerges, and pay special attention to late-planted or weedy fields. IPM recommendations would be to examine 250 plants (50 plants in five locations) weekly until corn gets to V5 growth stage.
Check for wilted, discolored or damaged corn leaves, and missing plants. Sometimes a cut plant may look like it's coming out of the ground at an odd angle. If damaged leaves are found, dig around the base of the corn plant for the presence of larvae. In addition, you should flag suspected "hot spots" in the field and monitor larval feeding (or lack of) over a period of a few days.
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