Avoiding Worms in Sweet Corn: How to Time Management Options

Three species cause ‘wormy’ corn: the European corn borer, corn earworm, and fall armyworm. Insect traps baited with sex pheromones help time management options.
Avoiding Worms in Sweet Corn: How to Time Management Options - Articles


European corn borer: mouthparts pointing forward, glassy or cream colored body, wriggles when flicked with a finger as an escape behavior. Invades ear from side, bottom, or tip. Photo credit: M. E. Rice.

When sweet corn tassels are just beginning to become visible it becomes very attractive to three species of night-flying moths that lay eggs in the corn. Hatching larvae rapidly burrow into the plant. Counts from pheromone traps help you see when moths are active in your area, and if numbers are increasing, which can help time management. The most accurate data come from traps on your farm. Also, through Penn State Extension, about 25 to 40 sites in Pennsylvania post trap catches to PestWatch, where they are mapped, and you can click on each mapped point to see a time-series of captures, and find links to fact sheets for each species.

European Corn Borer

European Corn Borer overwinters well in PA, so it can be present in damaging numbers during early season corn. Larva feed on leaves as well as ears. If they tunnel through rolled-up leaves, a characteristic 'shot-hole' pattern emerges as the leaves unfurl. European Corn Borer populations, however, have been dramatically decreasing recently due to the effectiveness of Bt-corn. Sprays timed at row-tassel tend to clean-up infestations in vegetative corn. Well-timed Bt-sprays can help, and release of the parasitic wasp Trichogramma ostrinae may be an option, especially as we see declining populations. Also, Bt-sweet corn cultivars are most effective against European Corn Borer. There are two races, called the 'New York' and the 'Iowa' race. If you are trapping make sure to have two pheromone traps, one for each race.

AdultEuropean corn borer female left, male right. Photo credit M.E. Rice.

Corn earworm

Corn earworm has mediocre overwintering capacity. Populations in PA initiate from migrants flying in from other areas. Corn earworm tends to become a problem in July, sometimes not until August. Counts from pheromone traps for this insect work the best for relating to field populations. Corn earworm lays eggs directly on silks - they strongly prefer fresh (yellow or pink) silks, but will lay eggs on drying silks, or other plant tissues or hosts, if not much else is available. It tunnels in through the tip, and is also called 'tipworm.' Feeding results in large amounts of frass (insect poop) in the tip area. Time your sprays to match flights and tighten spray intervals when traps are high to control corn earworm. Organic options include applying horticultural oils to silks, and a device to make this easier is called the 'Zealator'. Bt-sweet corn cultivars also reduce, but do not totally control, corn earworm.

Corn earworm. Light colored head, body with varying colors, mouthparts pointing down, curls up in a C-shape as an escape behavior. Almost always invades ear from the tip. Photo credit H. Fescemyer.

Corn earworm moth. Photo credit H. Fescemyer.

Fall armyworm

Fall armyworm cannot survive freezing. It migrates here from overwintering populations in Texas or Florida, and usually does not arrive until late in the season. Recent research is allowing us to determine the overwintering source of populations arriving in PA based in genomic patterns: so far, most of our populations are coming from Texas.

Fall armyworm. Dark color, inverted Y on head capsule, mouthparts pointing down, curls into a C shape as an escape behavior. Invacdes ear from side, bottom or tip.

Fall armyworm moth male.

We use Hartstack wire cone traps to monitor European corn borer and Corn earworm. They are cumbersome, but effective. Contact your Extension Educator if you want to trap on your farm. You can also use fabric traps, which are not as consistent, but used in much of the northeast. We use green/yellow/white bucket traps (called 'UniTraps'), which are commercially available, to track fall armyworm.