Northern Plains Integrated Pest Management GuideIPM

Authors: Buyung Hadi, John Witkowski, Robert Wright, R. Jeff Whitworth, Holly Davis and J.P. Michaud

Identification

The European corn borer, Ostrinia nubilalis, is a pyralid moth. The early instar larva of European corn borer is dull white with a series of small brown spots and a dark head. The mature larva is about 1 inch long, whitish to grayish in color. The adult moth is triangular in shape with pale yellowish brown wings decorated with wavy dark colored lines.

A larva of European corn borer

A pupa of European corn borer

Female moth of European corn borer

Male moth of European corn borer

Life Cycle and Seasonal History

The European corn borer produces two generations per year in the southern half of the Northern plains. In the northern half of the region, the borer generally produces only one generation per year.

The European corn borer overwinters as a full-grown larva inside cornstalks and crop residue. In the spring, the overwintered larvae pupate. The adult moths emerge in mid-May, mate and lay eggs on the underside of corn leaves in the lower two-thirds of the plants. The hatching larvae initially feed on the leaf surface before migrating into the whorl, continuing to feed on leaf tissue within the whorl. As the larvae reach the third instar, they move out of the whorl and tunnel into stalk. The larvae feed within the corn stalk, typically moving towards the lower node and feeding until pupation. The mature larvae pupate within the corn stalk and after 8 days adults emerge. These newly emerging adult moths mate and lay eggs of the second generation sometime in July. The young second generation larvae begin feeding in the leaf axils, or ear tips; moving into the stalk, or burrowing into the ear tip, or ear shanks when they reach third instar. These second generation larvae burrow into the stalk where they enter diapause (dormant period) during the winter.

Plant Injury and Damage

Physiological loss occurs when larvae bore into the stalk and interfere with the transport of water and nutrients in the stalk, resulting in yield reduction; this is the most important yield loss due to European corn borer feeding. Feeding by small larvae on leaves produces ‘shot hole’ injury, or small, circular holes on the leaves.

Shot hole injuries on a leave

Corn lodging associated with stalk tunneling by European corn borer larva

Harvest loss may also occur from corn lodging or ear drop caused by stalk and ear shank tunneling usually associated with the second generation of European corn borer larvae. This loss varies considerably due to variety, environmental conditions, and the severity of borer populations. Potential damage varies with the time of infestation. Estimated yield loss per corn borer larva at specific corn growth stage is given in Table 1. Corn is most susceptible to European corn borer just before tasseling.

Table 1 Percentage yield loss per corn borer larva per plant as influenced by corn growth stage

Growth stage	% Yield loss/larva/plant
Early whorl	5.5%
Late whorl	4.4%
Pre-tassel	6.6%
Pollen-shed	4.4%
Blister	3.0%
Dough	2.0%

(from: North Central Regional Extension publication no. 327)

Corn and proso millet are the main hosts although corn borers can infest numerous other plants like soybean, sorghum, potato, and weeds.

Management Approaches

Scouting and Threshold

Begin scouting for first generation borers when corn reaches about 17 inches in extended leaf height. Check at least five locations across the field. Examine 20 consecutive plants at each location for a total of 100 plants per 40 acres. Record the percentage of plants with ‘shot hole’ injury. Dissect the whorl of at least five plants with shot hole injury per location, and record the number of live larvae in the whorl. It is important to determine infestation rate prior to the larvae reaching the third instar when they burrow into the plant and are protected from foliar spray applications. Estimate the expected yield according to the variety and field history and estimate the price of corn per yield unit. Economic thresholds for the first generation European corn borer are calculated based on the estimated prevented loss due to the borer as compared to the estimated cost of control. If the estimated prevented loss due to borer activities is higher than the estimated cost of control, pesticide treatment is recommended.

An example of 1st generation economic threshold calculation: A corn field was scouted for European corn borer and 50 % of the sampled plants were found to have shot hole injuries and an average of four larvae were found per damaged plant. The corn was in early whorl stage, so based on Table 1 the estimated yield loss is 5.5% per larva. Let us assume a final yield expectation of 160 bu/acre and that corn is worth $ 7.00/bu. Let us further assume that total application cost is $ 14.00/acre. An estimate of 80% control efficiency has been deemed attainable if insecticides were applied as spray over the whorls.

An economic threshold worksheet for this field would look like:

50% Damaged plants x 4 Larvae/plant = 2 Field level larvae/plant

2 Field level larvae/plant x 5.5% Yield loss/larva= 11% Yield loss proportion

11% Yield loss proportion x 120 Expected yield (bu/A)= 13.2 Loss (bu/A)

13.2 Loss (bu/A) x 7.00 Corn price ($/bu) = 92.4 Loss ($/A)

92.4 Loss ($/A) x 80% Control efficiency = 73.92 Preventable loss($/A)

73.92 Preventable loss/A - 14.00 Cost of control ($/A)= 59.92 Gain (+) or loss (-) per acre if control is applied

In this case, the preventable loss is higher than the cost of control, providing a gain of $84.56 per acre if control is applied. In this scenario, pesticide application is recommended.

Adults of second-generation corn borers prefer to lay eggs on corn in the silking and tasseling stages. Emergence of second-generation adults usually begins in late July and reaches a peak sometime the first or second week of August. For the second generation corn European borer, monitor the fields by examining plants for egg masses or larvae feeding the leaf axils. A nominal threshold of 50% of the plants with egg masses or larvae on an accumulative weekly basis has been used. Also, a worksheet similar to that shown above for first generation corn borer is available for second generation corn borer.

Cultural Methods

Stubble elimination by shredding, plowing or grazing may reduce the initial population of corn borer within a field. Unfortunately, adult moths migrating from other fields may still lay eggs on the plants, leading to some damage.. Planting date management is also important since egg laying behavior of adult moths is somewhat influenced by the maturity and height of the corn. First generation adult moths are most attracted to corn that is taller and more advanced in maturity than the surrounding crops although they will lay some eggs on shorter corn (2- to 4-leaf stage). Borer establishment is notably poorer in corn with less than six fully expanded leaves. Second generation adult moths are attracted to corn that is less mature than surrounding fields; at or near pollen-shedding stage.

Genetically modified crops

Genetically modified corn hybrids containing genes producing Cry1Ab or Cry1F Bt-proteins are highly resistant for European corn borer. Usage of these hybrids are most profitable in years when insect pressure is high or in years with late planting date. Overtime, European corn borer may develop resistance to Bt-Cry proteins. To manage European corn borer resistance, the use of non-Bt corn refuges is required as part of the EPA registration of Bt corn hybrids. The purpose of planting refuges is to lessen the selection pressure of a European corn borer population to develop resistance against Bt-proteins. Refuge requirements for different hybrids are available from the company selling the seed.