Features Chemicals Insects
Battling European corn borer, corn earworm

Adult – In the spring, the corn borer caterpillar changes to a pupa in
its overwintering site. A few weeks later, it emerges as an adult moth.
Males usually emerge a few days before females. While emergence begins
around the third week of May in the southern-most parts of Canada,
moths do not usually appear until mid-June in cooler parts of the
country. Corn borer moths are 1.5 to 2 cm long and about 1 cm wide when
the wings are folded at rest. Their colour varies from pale
yellowish-brown to medium grey. The forewings have wavy dark lines
running across them. Males have darker wings and are a little smaller
than females.

Egg – Corn borer eggs are laid in a creamy white mass that resembles
overlapping fish scales. In hot weather eggs may hatch in as little as
three days, but in cooler weather they may take up to nine days to
hatch. Each viable egg develops a black centre (blackhead stage) about
one day before hatching.

Larva – The larvae (caterpillars) that hatch from eggs are about 3 mm
(1/8 inches) long with a dark head and a spotted, dirty white body.
They go through five instars (growth stages) and reach a total length
of about 2.5 cm (1-inch) when fully grown.

Larval colour may vary from pale greyish-brown to dirty white or pale
pink; head capsules range in colour from medium to dark brown.

Corn earworm
The corn earworm cannot overwinter in Canada – the population is killed
by the winter’s low temperatures. Each spring, the moths must
re-establish themselves from overwintering populations in the southern
United States and Mexico. They begin their northward migration around
May and usually reach Canada sometime in August, carried by high-level
winds.

There are four stages in the corn earworm’s life cycle. Earworms arrive
in Canada as moths. The moths lay eggs, which then hatch into larvae
(caterpillars). After feeding for two to four weeks, the larvae become
pupae. Larvae are typically killed by frost before pupation.  

Adult – The corn earworm adult is a buff- or tan-coloured moth with a
wingspread of 3.5 to 4 cm (1.25 to 1.5 inches). The forewing may have
several darker markings and always has a central brown dot, clearly
visible on the underside of the wing and faintly visible from the top.
The hind wings are very pale in colour, with a darker brown border.

Egg – Although earworms may lay eggs anywhere on the corn plant, almost
all will be laid on fresh silks if these are available. Although eggs
are laid individually, one female can lay more than 100 each night, and
more than 1,000 during her lifetime. Each egg is nearly spherical, and
about the same colour and diameter as a corn silk.

Larva – Upon hatching, young earworms crawl down the silks toward the
ear. After feeding on the silks inside the husk for a few days, they
begin feeding on the kernels at the ear tip. The worms will grow up to
3.7 cm long (1.5 inches), with prominent stripes running the length of
their bodies.

Information courtesy of the Ontario Ministry of Agriculture, Food and Rural Affairs