Managing European corn borer in sweet corn
About one week after flight begins, females start to lay flat, white egg masses on the underside of leaves in early corn. Eggs hatch in about one week, depending on temperature. Larvae feed in the whorl and in the succulent emerging tassel. As the tassel opens up, these larvae move downward, bore into the stalk and tunnel into ears through the side, base or tip. When moths are active during silking, eggs are laid on leaves near the ear and the larvae move directly into the ear after hatching. These larvae may tunnel through the husk or move directly down the silk channel at the tip of the ear.
The moths are about ¾” long and light tan or brown in color with yellow bands, the male has darker coloring than the female. Larvae are either light colored or brown, with dark spots on each segment. The head capsule is dark brown and flattened in shape. Full-grown larvae are 3/4 to 1 inch long.
Monitoring and thresholds
Monitoring networks for European corn borer are maintained by Extension systems in many states and can be used to determine when flight begins in your region for each ECB generation. In Massachusetts, capture data are reported weekly in Vegetable Notes
ECB flight can also be monitored on-farm using Scentry™ Heliothis net traps baited with Trécé lures. This will tell you exactly when flight begins, when it peaks, and how high the population is. Determine whether one or both E and Z strains are present in your area. In the northeastern US, where both strains are present, use two traps one baited with an E (II) lure, the other with Z (I), placed at least 50 feet apart in weedy borders of corn fields, but not under the tree canopy. Make sure the bottom of the trap stays close to the top of the weeds - you will have to raise the trap through the season.
Once flight is detected, blocks of sweet corn with newly emerging tassels should be scouted weekly by inspecting the tassels of 50-100 plants, in groups of five or ten plants, for the presence of ECB larvae and fresh feeding damage. This can be done in whorl stage corn by pulling the developing tassel out of the plant and inspecting it for frass and small ECB larvae. Corn with emerged green tassels corn can be inspected for damage without removing the tassel. If more than 15% of the plants have one or more larvae present, then a spray should be applied because ear damage will be greater than 5% at harvest if ECB is not controlled.
ECB can be controlled through releases of parasitic wasps that attack the egg stage (see Biological Control, below), or with foliar sprays that target larvae. Insecticides that are labeled for European corn borer include broad-spectrum materials such as synthetic pyrethroids and carbamates as well as more selective materials. For specific products, consult the New England Vegetable Management Guide (www.nevegetable.org).
European corn borers can be adequately controlled with one to three sprays per block of corn. If, when you scout, you find that 15% (or more) of the ears have live larvae or fresh feeding damage, controls will be need to prevent injury to the ears. The ideal time is just before or during tassel emergence but before silking and before larvae move into the ear or stalk Scout again in 4-7 days, looking for live larvae, and use a second spray if the infestation is still over 15%. Shorter spray intervals should not be necessary, but be sure to scout again within a week after the first spray
If ECB moths are active (e.g., pheromone trap captures >12 per week) and laying eggs during the period when ears are forming an additional spray during silking can help reduce the number of small borers that move directly into the ear after hatching. If possible, this spray should be applied at ear height.
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