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Battling European corn borer, corn earworm

corn earworm

May 15, 2008 By Dan Woolley

European corn borer and corn
earworm are two of the most economically significant pests for sweet
corn growers says Bernie Solymar of Earth Tramper Consulting.

Corn earworm is one of the most economically significant pests for sweet corn growers. Ontario crop consultant Bernie Solymar is currently examining a reduced-risk integrated pest management program for the pests.
Photo by Jack Dykinga, USDA-ARS

European corn borer and corn earworm are two of the most economically significant pests for sweet corn growers says Bernie Solymar of Earth Tramper Consulting.

Solymar is currently working on establishing a reduced-risk integrated pest management program for the pests. He has four trials underway in eastern Ontario and Quebec utilizing biological controls to manage them.

Consumers in the fresh produce market “have zero tolerance for insect-damaged corn,” he remarks, adding few growers have their own consultants or crop scouts to do crop monitoring for diseases and pests.

Corn borer overwinters in stalks and cobs, pupating in the spring and feeding on the leaves of the new corn crop. Corn borer adults appear in late May and early June and the later corn borer adults tunnel into sweet corn stalks and cobs.

The corn earworm, however, cannot survive Canada’s cold winters. Instead, it blows north on winds originating in the southern U.S. and feeds on corn silks and ear tips. It is resistant to pyrethroid insecticides, although Solymar says there are other chemical controls that are either very expensive or have a high environmental impact. The corn earworm reproduces through three to four generations in just one growing season in the U.S.

In his research plots, Solymar is examining reduced-risk materials – such as Dipel or Success – as well as bio-controls, including Trichogramma wasps, plus Bt corn varieties, which “very few growers are growing,” he says.

The two wasp species he is working with – T. brassicae and T. ostriniae – are parasites that feed on the eggs of the European corn borer.

While 2007 was a low-pressure year for European corn borer and corn earworm was not a pest management issue, Solymar reports that Dipel performed as well as the conventional controls, either in consecutive applications or alternated with a conventional insecticide, such as Matador.

On 50 per cent of the test sites, T. ostriniae held European corn borer below a damage rate of 2.5 per cent, while T. ostriniae kept corn borer damage at below 25 per cent damage on 100 per cent of the sites.

Solymar estimates that four applications of T. brassicae at 12-day intervals will cost just over $280 per hectare, remarking that just one application of T. ostriniae is worth examining.

According to Solymar, a cost comparison of conventional and alternative controls per hectare includes:

T. brassicae = $71
Lannate = $38 to $60
Dipel = $44 to $48
Sevin = $41 to $46
Furadan = $45

T. ostriniae = $ 38
Other pyrethroids = $25
Matador = $15

For the 2008 growing season, Solymar is continuing to examine the use of Dipel. He is also testing a new reduced-risk product from Dupont that he feels “has a lot of promise” as a corn borer control, as well as continuing his work on earworm controls. His Trichogramma trials will also continue and he hopes to locate more sites for organic trials.

European corn borer
The European corn borer has four stages in its life cycle – adult
(moth), egg, larva (caterpillar), and pupa. The winter is spent as a
full-grown caterpillar in or near last year’s host plant. While most
corn borers probably overwinter in field corn, they can also be found
in other host plants such as large-stemmed grasses and various
vegetables.

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

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