Beginner

Scientific Name
Ostrinia nubilalis

Identification

• Full-grown larvae:
  • 2.5 cm (1 in.) in length,
  • greasy white colour,
  • dark head,
  • spots on each segment.
• Eggs:
  • shiny,
  • white,
  • laid in clusters of 30 that resemble fish scales.
• Larvae feed briefly on the leaves but soon tunnel into the protection of the stalk
• As the crop matures, borers move from the stalk into the ear

Often Confused With
Corn earworm
Fall armyworm

Period of Activity
Borers overwinter as pupae in corn stubble.  Southwestern Ontario experiences two generations per year.  Usually the first generation adults emerge in mid-June, and the population peaks in mid-July.  Second generation populations peak in August.

In the rest of the province, single-generation corn borers usually emerge in late June or early July.  Populations peak in late July to early August.  Many areas within southwestern Ontario (Lambton, Middlesex, Oxford, Brant, Norfolk and Niagara) have an overlap of corn borer generations.  These areas experience sustained peak flight for much of July and August.

Sweet corn is susceptible to corn borer from the late-whorl stage (12- 14 leaves) until harvest.

Scouting Notes
Begin monitoring at the mid-whorl stage.  Scout fields for corn borer activity at least once a week.  Inspect a minimum of 10 groups of 10 plants throughout the field.  Look for egg masses and pin-hole feeding.  Indications of boring such as holes in the stalk or broken tassels indicate a lack of control, often due to poor spray timing.

General ECB populations can be effectively tracked using either milk carton traps or Heliothis traps.  Heliothis traps are cheaper and more convenient to use.  However, while they work well for monitoring peaks in corn borer activity, they are poor at detecting low numbers of ECB, especially at the beginning of the flight period.  Milk carton traps provide better detection of low ECB activity.

Thresholds

5% feeding injury or the presence of eggs warrants an insecticide application.

Advanced

Scientific Name
Ostrinia nubilalis

Identification
European corn borer (ECB) egg masses are creamy white and flattened.  They are laid in clusters of approximately 30.  Each egg is slightly overlapped, giving the egg mass the appearance of fish scales.  As hatch approaches, the dark black heads of the larvae become visible at the centre of each egg.

ECB larvae are greasy white to pale grey with two small spots per abdominal segment and a dark brown head.  Newly hatched larvae are approximately 3 mm (1/8 in.) long.  Fifth instar larvae reach 2.5 cm (1 in.) in length.  The abdominal spots are not always present, especially on the younger instars.

Adults are light-brown moths approximately 2 cm (4/5 in.) long with dark wavy lines running across each forewing.  Male moths are darker and smaller than females.

The plant is susceptible from the late-whorl stage onwards.  Newly hatched larvae produce pin-sized holes on the leaves.  The larvae will often bore through the young whorl, creating a straight line of pin holes across the width of the leaf.  Occasionally the initial feeding damage will appear as elongated window panes on the leaf surface.

As they mature, the borer larvae move into the mid-vein of the leaf and subsequently into the stalk.  Frass is often present at the entry hole.  The borers feed on the internal stalk tissue, often causing the tassels to break.  Infected stalks are also more prone to lodging.

Fourth and fifth instar larvae exit the stalk and move to the ear zone, where they will feed upon the kernels prior to pupating.  An entry hole is often (but not always) present at the base of the cob.  Damaged ears will attract birds and sap beetles.

Second generation ECB will frequently lay eggs on the ear leaf.  The larvae enter the ear through the silk channel, leaving little indication of feeding outside the ear.

Often Confused With
Corn earworm
Fall armyworm

Biology
There are two distinct strains of European Corn Borer in Ontario, bivoltine and univoltine.  Bivoltine moths complete two full generations each summer.  In warm years, bivoltine areas will experience a partial third generation in late summer.  Bivoltine populations are found in Kent, Essex and parts of Elgin counties.  These areas experience two distinct peaks of borer activity each summer.

Univoltine moths complete only one full generation each year.  In warm years, these areas may experience a partial second generation in late summer.  Univoltine populations are commonly found in areas north of the 401 and in eastern Ontario.  These areas experience only one distinct peak of activity each summer.

An overlap band exists between Sarnia and Niagara where both strains are present.  These areas will experience a prolonged peak in activity lasting from early July through mid-August.  Affected areas include Lambton, Middlesex, Oxford, Brant, Haldimand, Norfolk, Hamilton-Wentworth and Niagara.  Univoltine and bivoltine moths emerge at different times.

European corn borer overwinter as larvae in corn stalks and other residue left on the surface from the previous growing season.  As day length increases and average day temperatures exceed 10°C (50°F), the larvae pupate.  Pupae are found within larval feeding tunnels and require 2 weeks to develop before adults emerge.

Adult moths mate in the grassy vegetation along the field edges, such as fencerows, ditches and hedgerows along fields.  Moths may fly up to ½ km (¼ mile) to find a mate or host crop.  Once mated, females enter the field to lay eggs on the host crop.  Eggs are generally laid on the underside of leaves, close to the midrib.  The eggs mature in three to nine days.  There are five larval instars.  Depending on the temperature, larvae feed for 20- 30 days before pupating.

Period of Activity
Borer development and population peaks can be predicted using accumulated growing degree-days(GDD) with a base temperature of 10°C.

To calculate growing degree days:

1. Determine the mean daily temperature.
2. Subtract the base temperature from the mean temperature to give a daily GDD. If the resultant GDD is negative, count it as zero.
3. Each daily GDD is then added up (accumulated) over the growing season.

   Daily Max Temp	+	Daily Min Temp	-	Base	=	GDD
   	2

GDD Accumulations
Event	Univoltine areas	Bivoltine areas
		1st generation	2nd generation
First catch	300	150	700
Peak catch	650-700	300-350	1050-1100

In the bivoltine area, the first generation adults emerge in mid-June, and the population peaks in mid-July.  Second generation populations peak in mid-August.  In the univoltine areas corn borers usually emerge in late June or early July.  Populations peak in late July to early August.  Overlap areas experience a sustained peak flight for much of July and August.

Sweet corn is susceptible to corn borer from the late-whorl stage (12– 14 leaves) until harvest.

Scouting Notes
Begin monitoring at the mid-whorl stage.  Scout fields for corn borer activity at least once a week.  Inspect a minimum of 10 groups of 10 plants throughout the field.  Look for egg masses and pin-hole feeding.  Indications of boring such as holes in the stalk or broken tassels indicate a lack of control, often due to poor spray timing.

General ECB populations can be effectively tracked using either milk carton traps or Heliothis traps.  Heliothis traps are cheaper and more convenient to use.  However, while they work well for monitoring peaks in corn borer activity, they are poor at detecting low numbers of ECB, especially at the beginning of the flight period.  Milk carton traps provide better detection of low ECB activity.

In Ontario, European Corn Borer moths respond to the “Iowa strain” pheromone lure.  Use 3 milk carton traps or one Heliothis trap per location.  Place traps in the grassy (preferably unmowed) area at the edge of a corn field.  If possible place traps near a field that was in corn the previous year.  Place milk carton traps at least 15 m (50 ft.) apart and mount cartons on wooden stakes roughly 1 m (3 ft.) above ground level.  Secure the trap to the stake with rubber bands or twist tie material.  When using Heliothis traps, the bottom of the trap should be at least 1 m (3 ft.) above ground level.

Note: Pheromone lures for the milk carton and the Heliothis traps must be replaced weekly.  Do not place traps within 200 m (650 ft.) of any other type of pheromone trap.

Thresholds
5% feeding injury or the presence of eggs warrants an insecticide application.  Use the following chart to determine if a spray is necessary:

Sequential Sampling Chart
Plants Examined	Spray if > than
>5- 25	3
30- 50	4
55- 105	5>

Management Notes

• Effective control must target small (early instar) larvae.  The first spray application should coincide with the first sign of feeding damage or early tassel emergence.  Control failures are often linked to poor spray timing or poor spray coverage.
• Direct the sprays down into the whorl and at the developing ears. For good corn borer control, all sprays must target the small larvae. Second generation larvae often move quickly towards the ear. Maintain good spray coverage of the leaves, tassels and ears.
• Applications at 5– 7 day intervals usually give satisfactory control.
• Shredding sweet corn residue after harvest is an effective way to destroy borers overwintering in stalks and stubble.