Organic Insect Management in Sweet Corn

European Corn Borer

SARE Outreach
Ruth Hazzard, Pam Westgate | 2005 | 11 pages
Photo F. Caterpillar pests in sweet corn: European corn borer.
Caterpillar pests in sweet corn: European corn borer.

European Corn Borer (Ostrinia nubilalis)
European corn borer is an introduced pest that has spread across much of eastern and central North America. The number of generations of European corn borer ranges from one per year in the extreme north to four per year in the Southeastern U.S. Most of the range within the U.S. has two generations per year; sometimes these co-exist with a strain that has one generation per year. There are two strains of European corn borer: the Iowa (E) and New York (Z) strain, which are present in different ratios in different regions.

In northern areas, larvae over-winter in stalks of corn and other host plants, and pupate in the spring. In New England, adult moths emerge in late May or early June and mate in weedy or grassy areas. Fields that have been in field corn, sweet corn, or peppers for a long time will have higher pressure from European corn borer than other fields.

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, they lay eggs 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, and 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 ¾” to 1″ long and move fast when disturbed (Photo F).

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 generation. European corn borer flight also can be monitored on-farm using blacklight or pheromone net traps. Consult your state Extension program to determine what type of traps are recommended in your area. Trap captures will tell you exactly when flight begins, when it peaks, and how high the population is [1, 9].

If you are using pheromone traps, consult your state Extension program to determine whether one or both E and Z strains are present. If 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. Make sure the bottom of the trap stays close to the top of the weeds [1, 9].

Once flight is detected, blocks of sweet corn with newly emerging tassels should be scouted weekly by inspecting the tassels of 50 to 100 plants, in groups of 10, for the presence of European corn borer 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 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 European corn borer is not controlled.

Control
European corn borer can be controlled through releases of parasitic wasps that attack the egg stage, or with foliar sprays of Bt or spinosad that target larvae. During silking, oil applied to the silks for corn earworm control also will control European corn borer larvae that enter the ear through the silk channel, but may not give complete control of those entering from the side.

Trichogramma releases: Trichogramma are small parasitic wasps that lay their eggs in the egg masses of host insects. Trichogramma larvae feed and pupate inside the egg, killing the egg and preventing hatch. Trichogramma ostriniae lays its eggs in European corn borer egg masses, and is the species that has shown the highest level of European corn borer control in field trials, reducing larval infestations enough to avoid the need for sprays. A closely related species, Trichogramma brassicae, is commercially available and also will suppress European corn borer.

Since Trichogramma control European corn borer by parasitizing egg masses, they should be released just as the moths start to lay eggs, when the corn is in the four- to six-leaf stage. Knowing when European corn borer flight begins, reaches a peak, and ends in a given field is key to the proper timing of Trichogramma releases. You can use regional information about flight activity from your state or county Extension program and target your releases to that; however, to coordinate timing on your farm you should monitor flight activity in your own fields.

Release rates and number of release locations within the field vary depending upon the species. Consult a supplier of beneficial organisms for more details; orders should be placed in advance of the growing season. We recommend releasing the species Trichogramma ostriniae, which is available through IPM Laboratories (www.ipmlabs.com; 315-497-2063).

Foliar sprays: 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, spray once with Bt or spinosad. The ideal time is just before or during tassel emergence but before silking and before larvae move into the ear or stalk. Use a spreader-sticker for better control. Scout again in five to seven 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. With Bt use at least two-thirds the maximum label rate as low rates can result in lower levels of control.

If European corn borer moths are active (e.g., pheromone trap captures >7 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.

Sprayer design and needs depend on how much sweet corn acreage you grow and how important spraying is in your operation. For growers with small acreage, it may be practical to use a backpack mist blower, walk through the corn and cover two or three rows in each direction with a concentrated solution. Tractor-mounted boom sprayers that can be lifted as corn grows are well suited for moderate to large acreage and should be configured for over-the-top coverage of the tassel as well as coverage of the foliage and ear zones. Drop nozzles are recommended.9 Bt and spinosad products can be used whenever European corn borer or fall armyworm are the target pests, regardless of the time of season or stage of crop growth.