The Online Gardener's Handbook
Chapter 4: Vegetables
Table of Contents
- Corn Earworms
- Corn Rootworms
- Corn Smut Galls
- European Corn Borers
- Sap Beetles
- Seed-corn Maggots
- 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.
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.
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.
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.
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.
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
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.
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.
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.
Avoid damaging developing ears. Use overripe melon halves as bait
in the garden. Control corn borers to reduce sap beetle populations.
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
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