European corn borer (ECB) is a common pest of corn. The impact of ECB on corn yield is often underestimated due to lack of scouting, large changes in ECB populations from year to year, and the ability of corn to withstand some feeding injury.

Life Cycle

In Canada there are two common biological types of corn borer, univoltine and bivoltine. Univoltine ECB produce one generation per year and bivoltine ECB produce two generations per year. Single generation, or univoltine, ECB are present throughout cooler/northern areas and bivoltine are present in areas of southwestern Ontario, Quebec and Western Canada.²

There is a direct correlation between number of generations of ECB and colder temperatures. Regions of single– and multi-generation ECB may overlap, and insect pressure may seem to be constant in these areas.

Univoltine ECB may develop slower and can extend emergence over a five week period, which can complicate management. Late June or early July is the typical time frame univoltine ECB emerges²; the second generation emerges from July to mid-August³. Consequently, if univoltine and bivoltine ECB are both present in a field it may appear that the insect pressure is never-ending.

Moth emergence may vary depending on conditions that season. Larvae damage is most often limited to leaf feeding and stalk tunneling. Female moths prefer to lay eggs in fields that are tasseling and in the green silk growth stage. In general, the heaviest egg lay and infestations will occur in later maturing fields or hybrids. Later instar larvae continue feeding on the corn stalk tassel, ear shank, and cob. ECB larvae can enter through different locations on the ear and may not leave entrance holes.

Identification

Larvae are usually pinkish in color, have a dark (almost black) head capsule, and five pairs of abdominal prolegs, including a pair of anal prolegs. Additionally, they have a dark gray mid-dorsal line across their body length (Figure 1). Full grown larvae are approximately 0.8” to 1.2” (20 to 30 mm) in length.

Management

Since univoltine ECB have an extended emergence window, using growing degree day accumulation to determine when to start scouting is recommended (Table 1). Scout for corn borer by examining at least 10 corn whorls at 10 locations in each field.² Look for egg masses and pinhole or shot-hole leaf damage. Record the total number of plants damaged. Pull up and unroll several whorls at each field locations and count the number of live worms present. Presence of eggs or feeding injury on 5 percent of examined plants may justify insecticide application.²

 

Table 1. Degree Day Accumulation* Model For Estimating Univoliate European Corn Borer Moth Emergence. Source: Knodel, J. European Cron Borer Management in North Dakoda. North Dakoda State University
Accumulated Degree Days	Estimated Percent of Emerged Moths
911	10%
986	25%
1078	50%
1177	75%
1274	90%
Growing Degree Days are calculated by taking the average daily temperature (*F) – 50 if the daily temperature is above 86F or below 50F, the calculation uses 86F and 50F, respectively for high and low temperatures

 

 

Use These Calculations to Estimate if it is Economic to Treat a Non-B.t. with an Insecticide:

Univolitine Strain (areas with only one generation of ECB per year)

1. % shot-holed plants = plants with shot-holes + total plants checked. Unfural one of the shot-holed plants from each location and look for larvae
2. Larvae per plant = number of live larvae per unfurled plant x (A)% shot-holed plants + 100. Example: 25 shot-holed plants and 1.5 larvae per unfurled plant. Larvae per plant 0.38 = 1.5 x 25 + 100. A yield loss of 5% per live larvae is estimated.
3. Potential % yield loss = (B) x 5+ 100
4. Potential $ loss = © potential % yield loss x expected yield t/ha (bu/acre) x value $/t ($/bu). A 75% effectiveness of a pesticide treatment is estimated.
5. $ preventable loss = (D) potential $ loss x % effectiveness of pesticide treatment
6. Treatment cost = pesticide cost + application cost
7. Gain (+) or loss (-) if treatment is applied = € - (F)

 

Bioltine Strain (areas where there are two generations of ECB per year)

1. Larvae per plant ________ = number of egg masses/plant x borer / egg mass (Cumulative counts taken 7 days apart)
2. % yield loss _________ = (A) larvae / plant x 4% yield loss per larvae/plant
3. Yeild loss t/ha (bu/acre) _________ = % yield loss x expected yield t/ha (Bu/acre)
4. $ loss/ha (acre) __________ = © yield loss t/ha (or bu/acre) x expected $ price per bu
5. Preventable loss per ha (acre) ___________ = (D) $ loss per ha (or acre) x75% control
6. Treatment cost _______ = pesticide cost + application cost
7. Gain (+) or loss (-) if treatment is applied = (E) - (F)

 

Scout corn fields for damage from late-emerging or second generation ECB on a weekly basis. Examine fields for stalk tunneling, ear shank damage, and damage to the ear tips (Figure 2). Ear tip feeding can cause kernel loss, but stalk and ear shank boring likely cause the most significant yield loss. Evaluate for the presence of frass (insect excrement) and tunneling activity where the stalk broke to distinguish lodging from greensnap. Determine if ear drop resulted from corn borer feeding or some other event. Thoroughly scout all corn fields because ECB populations vary among fields and the level of damage can vary by corn product. Economic thresholds vary, but consider treatment of larvae if 25% of the plants have fresh or hatched egg masses or young larvae in leaf axils.

Allowing an infested crop to remain in the field may result in higher yield losses due to lodging and ear drop. Harvest order should be determined based on stalk and ear shank tunneling, lodging, and dropped ears.

Management of ECB starts with corn product selection. Products with the B.t. corn borer trait have provided control of ECB. B.t. corn hybrids produce an insecticidal protein derived from the bacterium Bacillus thuringiensis. These hybrids provide protection against the European corn borer equal to, and usually far greater than, optimally timed insecticides.