A larger-than-usual number of black cutworm moths are riding the winds of April storm systems sweeping up through the Corn Belt from their overwintering homes in the Gulf of Mexico.
Entomologists are cautioning growers to keep a wary eye and not to assume traits will take care of the pest. Fields with heavy infestations of winter annual weeds are most at risk.
Corn growers should be on alert to check their emerging corn stands regularly starting in mid-May and apply insecticidal rescue treatments if necessary, Purdue University’s Integrated Pest Management Specialist John Obermeyer told DTN. Some Bt-hybrids and seed insecticides do target the cutworm, but growers should be aware of the limitations of both technologies and scout regardless, he added.
MOTHS ON THE MOVE
Moth traps set by universities across the Midwestern and Southern states act as an early barometer of future crop damage from the cutworm larvae the moths will produce.
So far, University of Kentucky entomologist Doug Johnson has reported black cutworm moth trap numbers well above the state’s five-year average. If the moths continue to stream in at this rate, “the population will approach the outbreak levels seen in 2006 and 2008,” Johnson warned in a university newsletter. “These are years when known pest problems either caused loss or required extra insecticidal control.”
More northern Corn Belt states like Indiana and Illinois are seeing the first moth captures of the season, and some Indiana traps are already showing significant numbers, Obermeyer said.
Although moth numbers are lower so far in Illinois, the “distribution of captures suggests that black cutworm moth flights have likely taken place throughout Illinois and growers are encouraged to remain vigilant for early signs of leaf feeding when corn seedlings begin to emerge,” University of Illinois entomologist Mike Gray told growers in a university Pest Bulletin.
DON’T BET ON BT OR SEED TREATMENTS
The moths will lay eggs in winter annual weeds that are sprouting in fields across the Midwest. Once hatched, the weeds serve as a satisfactory breakfast for the cutworm larvae until farmers deliver corn plants for lunch and dinner in the coming weeks.
A late, wet fall prevented many growers from applying their usual fall burndown, and wet or windy conditions have kept many growers out of the field so far this spring, Obermeyer noted. As a result, many fields might be ripe for black cutworm infestations.
Some — but not all — Bt hybrids contain proteins that target the black cutworm. To see if your hybrids are among these, check out this handy Bt trait table from Michigan State University: http://bit.ly/….
Don’t assume you’re safe even if you do plant a Bt hybrid that targets cutworms, Obermeyer said. In order to produce the proteins that are toxic to the cutworm, Bt corn plants have to be healthy, with growing, green tissue, and actively taking up nitrogen.
“So what happens is when we get into cool seasons like the springtime, you’ll often see yellow-looking corn plants, and there is very little to any Bt production at that time,” Obermeyer explained. As a result, a cutworm feeding on a Bt hybrid in this situation might well survive.
Seed-applied insecticides can be very effective on young cutworm larvae, which are smaller and more vulnerable to the insecticide in the plant tissue, Obermeyer added. But often cutworm larvae have had time to feed and fatten on winter annual weeds before a young corn crop emerges. These older and larger cutworms can often “feed right through the technology,” Obermeyer explained.
LOW THRESHOLDS FOR SCOUTING
Once a corn plant is in the four-leaf growth stage, it is more likely to survive cutworm damage, but newly emerged seedlings and young plants are at risk before then, Obermeyer said.
Because cutworms can take out multiple plants, especially as they grow larger, the economic threshold for a rescue insecticide treatment is low.
If the field sections you scout reveal just 3% to 5% of plants with leaf feeding or stems cut off at or belowground, it’s time to act. Foliar applications of pyrethroids can be very effective, but early detection is key, Obermeyer said.
For more information on black cutworms and how to scout and treat for them, see this University of Nebraska publication.
Purdue University uses heat unit accumulation and trap data to monitor the spread and likely emergence of cutworms in the Pest & Crop Newsletter here.