The Online Gardener's Handbook 2010
Chapter 4: Vegetables
Corn (Sweet)

Table of Contents

  1. Introduction
  2. Corn Earworms
  3. Corn Rootworms
  4. Corn Smut Galls
  5. European Corn Borers
  6. Sap Beetles
  7. Seed-corn Maggots
  8. Learn More


In this chapter, a description of various corn pests will be provided along with suggested management options. These management options will not include the use of pesticides. Some biopesticides and certain reduced risk pesticides are still available to the homeowner for controlling weeds and pests in lawns and gardens. For more information, refer to Chapter 2 of this handbook and the Ministry of the Environment's website. For suggestions on managing specific weeds and pests, consult local horticulturalists, Master Gardeners or your local garden supply centre.

Corn Earworms

In some years, the tips of corn ears can be infested with robust caterpillars with brown, green and yellow striped bodies. It is very difficult to predict when or if infestations will occur. The moths generally migrate from the south and do not arrive before, arriving in southwestern Ontario in mid to late July and in eastern and northern areas in mid to late August, so early sweet corn will not be affected.

Management Options

Time planting to avoid late season damage. Varieties with long, tight husks may be less susceptible. Cut ear tips off corn before cooking. This insect is susceptible to bacterial and viral disease as well as parasitoids.

Corn Rootworms

Two types of beetles, one pale green, the other yellow with black stripes, each about 6 mm long, feed on corn silks and pollen. There is only one generation per year. The beetles emerge about the first week of August and the females lay clusters of eggs up to 5 cm deep in the soil at the base of corn plants. The larvae tunnel into corn roots and feed heavily, causing plants to break as corn matures; they then emerge and burrow into the crown of the corn stalk and remain in the residue until next spring. If silk is eaten before pollination takes place, the ear will not fill properly. This is a particular problem on late-maturing corn.

Management Options

Rotate crop so that it is planted at a distance from last year's to prevent damage from larvae. Adult beetles are difficult to control on corn silks, except by covering the silks with screen caps.

Corn Smut Galls

Corn smut whose symptoms are large, irregularly shaped galls on any visible part of the corn plant. A thin silvery-grey membrane appears, forming a gall that breaks open to release black spores. Spores are carried by the wind and may survive in soil or plant remains for several years, reproducing and carrying infection from cornfields in the vicinity.

Management Options

Plant resistant varieties. Destroy smut galls before they burst open. Very infested plants should be removed and burned or destroyed before galls burst and release spores which could affect subsequent crops. Do not till plants into the soil and remove old stalks at the end of the season. When the disease has appeared, do not grow corn.

European Corn Borers

European corn borer larvae are a dirty-white or pinkish colour with brown dots and about 2.5 cm long when full grown. Masses of eggs, resembling fish scales, are laid on underside of leaves near midrib. Larvae feed on leaves, making small holes, then bore into the stalks causing breakage, and feed on cobs and kernels. There are two strains of this insect in Ontario, one having two generations per year, the other only one.

The two-generation strain occurs in the area south of a line between Simcoe, London, and Sarnia. Its eggs are laid on corn between early June and early July, and again from late July to early September. The one generation strain occurs throughout Ontario, laying its eggs from late June to mid-August. Both strains may be present in the same area at the same time.

Management Options

Some cob damage may be tolerable. Cut out infested areas on cobs before consumption. Plant tolerant varieties. It may be possible to split stalks at borer entrance holes and pull out and destroy the worms. This insect is also susceptible to bacterial disease, several parasites and predators.

Sap Beetles

Small black beetles with four cream-coloured markings sometimes infest corn ears, especially those with previous insect or bird injury, or which have loose fitting or previously opened husks.

Management Options

Avoid damaging developing ears. Use overripe melon halves as bait in the garden. Control corn borers to reduce sap beetle populations.

Seed-corn Maggots

Seed-corn maggots are a dirty-coloured yellowish-white larvae and about 8-10 mm long with long, sharply pointed heads. They burrow into the seed where they can be found under the seed coat. The seed usually fails to germinate or sprout, or growth is weak and spindly. The adult flies lay their eggs in May, favoured by wet soil conditions or a cold spring, in recently manured soil and rotting vegetation. Corn, squash, cucumber, radish, beans, peas, and beets are affected. For more information on maggots, see Chapter 1.

Management Options

Spread manure and till soil in fall. Avoid applying too much manure. Seed shallowly to promote quick germination. Sow crops after June 1 in southern Ontario and June 10 in northern areas. In areas with a history of damage, grow transplants in containers indoors or in a frame tightly covered with screening or cheesecloth, if possible. Transplant when environmental conditions are suitable. This insect is not a problem everywhere; be guided by local experience in previous years.

Learn More


For more information:
Toll Free: 1-877-424-1300
Author: OMAFRA Staff
Creation Date: 21 July 2005
Last Reviewed: 25 June 2010