Western bean cutworm: Is it time to scout?

The pheromone trap data website shows that western bean cutworm flights are now occurring across Iowa. As mentioned in the previous two ICM Newsletters, scouting is the key to knowing if and when to apply insecticide treatments for control.

To scout corn, check 20 consecutive plants at five locations. The University of Nebraska recommends that if 8 percent of the plants have an egg mass or if young larvae are found in the tassel, consider applying an insecticide. Timing of the application is critical. If the tassel has not emerged when the eggs hatch, they will move into the whorl and feed on the developing pollen grains in the tassel. As the tassel emerges, the larvae will move down the plant to the green silks and then into the silk channel to feed on the developing ear.

If an insecticide is needed, apply it when 90 to 95 percent of the tassels have emerged. If the tassels have already emerged, the application should be timed for when 70--90 percent of the eggs have hatched. Once the larvae reach the ear tip, control is nearly impossible. If an insecticide application is needed, corn fields should be checked for the presence of spider mite colonies. If mites are found, select a product that does not stimulate mite flare-ups (increased population growth).

Continue scouting until the peak flight has occurred. To determine if a peak flight has occurred, look at the daily trap catches in your area. If you see a marked increase followed by decrease (1--3 days of declining numbers) in the moths captured, this may indicate a peak flight. If you see a decrease, consider other factors that may cause the drop, (e.g., rainfall, high winds, and cold temperatures). These environmental factors may decrease the number of moths in flight and thus you would not be seeing a true peak flight. If you are not sure if you have a peak flight, continue monitoring for at least 7 days to see if you see a rebound in the numbers. Secondary peak flights are not uncommon.

Once we have passed peak flight, egg laying will decrease with the decrease in moth numbers. Scouting should continue for 7--10 days after the peak flight.

The map shows cumulative captures by county. Where there are multiple traps from a county, the trap with the highest moth captures is used. The key is to know when peak flights have occurred near you, but we have arbitrarily shown the counties with more than 100 cumulative moth captures. Check the Web site to see when and where traps were placed near you, and how those trap captures have accumulated through time. The data is updated continuously.

2005-July-19 Trap Captures
Cumulative captures of western bean cutworm moths by county.

This article originally appeared on page 149 of the IC-494(19) -- July 18, 2005 issue.

Updated 11/08/2006 - 4:55pm