To scout for WBC, select 5 areas of a field and randomly choose 20 plants within each area. Look for egg masses, worms or adults (see images below) on the upper surface of the corn leaves within the upper ½ to ⅓ of the plant. The egg masses start off white in colour and turn purple as they near hatching.
Determine the Risk
Among the 100 plants examined (20 plants within each of the 5 areas of a field), total the number of plants on which either egg masses or worms were found. If egg masses or larvae were found on 5 or more plants of the 100 plants examined, the plants are at or above the risk threshold.
Here is a link to a video on WBC scouting procedures: http://bit.ly/P0Tzru
Course of Action
If the plants are at the threshold risk with 5 plants out of 100 examined showing either egg masses or worms, there are several factors to consider.
First, the genetics of the corn varieties planted can play a role in the control of Western Bean Cutworm. The ‘Herculex’, ‘Smart Stax’ and ‘Viptera’ genetics usually do an acceptable job of controlling WBC feeding but results can vary. Check with the seed supplier if you are unsure of the genetics of your varieties.
Second, stage your crop. When WBC larvae hatch, they migrate up to the tassel to feed on pollen. As they get bigger, they then move down to the cob to feed on the silks and then the cob. If there are no tassels emerged when they migrate up, they will have no food supply and will starve. If spray control is being considered, only spray once the tassels are out or emerging, and eggs are hatching or about to hatch.
If you have susceptible varieties, spraying with a registered insecticide is an option. Registered products on the market provide excellent control if the worms are sprayed when small (1 week after hatching).
If you have questions, please contact your Veritas agronomy team.