Farmers have enjoyed the benefits of Bt corn since its introduction in 1996, particularly the “in the bag” transgenic protection from insect pests and the yield loss they inflict. European corn borer was particularly challenging for corn growers and was the target of the first corn hybrids that contained the Bt trait.
The adoption of Bt corn in the U.S. prompted a widespread suppression of ECB. Even so, ECB still shows up in conventional cornfields in Iowa and can be a devastating pest.
Iowa State University Extension entomologist Erin Hodgson, and ISU Extension program specialist Ashley Dean, provide the following update and recommendations for controlling ECB in 2020.
Non-Bt corn susceptible
In 2019, about 15% of the corn planted in Iowa and 17% of the corn in the U.S. did not contain a Bt trait and would be susceptible to ECB infestation. Non-Bt corn hybrids have been of interest recently due to lower seed costs of non-Bt corn amid low grain prices and crop values.
Percent of corn acres planted to hybrids with only Bt traits or stacked (Bt plus herbicide tolerance) traits in Iowa and the U.S. (Source: USDA NASS)
For farmers who opted out and didn’t plant Bt corn this year, scouting for ECB (and other corn pests) will be essential. A comprehensive guide to understanding ECB and its association with other pests is available from the ISU Extension Store and can serve as a management guide.
Scouting for ECB
If you’ve planted conventional corn — that is, a corn hybrid without the Bt trait — you need to scout those fields for corn borer. ECB larvae feed on almost any part of the corn plant except the roots. On leaves, feeding can appear as shot-holes or a windowpane effect. This pest can also tunnel into the stalk, midrib of the leaf and the ear shanks. The result of ECB feeding injury is poor ear development, broken stalks and dropped ears. Reduced grain quality from ear molds can occur due to ear feeding, which increases infection by disease pathogens.
There are typically two generations of ECB in Iowa, and each generation has unique scouting requirements to ensure effective management. Life stages and behaviors can be predicted based on degree day accumulation from the date when ECB adults are first captured in the spring, which is called the biofix. Therefore, monitoring for adults is essential for timely scouting and treatment decisions.
European corn borer overwinters in the larval stage, with pupation and emergence of adults in early spring. The adult moth lays eggs that hatch to produce the larvae or worm stage, which eats holes in the leaves of corn plants.
Corn should be scouted for first-generation larvae once susceptible plants reach V6 to determine the number of live larvae present. Look for feeding injury in the whorl and on the youngest leaves; plants without these symptoms are unlikely to contain larvae.
For every 40 to 50 acres, 20 consecutive plants should be sampled in five areas of that field to obtain a representative sample of 100 plants. If more than one hybrid is planted in the field, consider each hybrid as a separate field for scouting and treatment determination.
A cost-benefit analysis table can help determine if applying an insecticide is economically justified to control an infestation in vegetative corn (see Page 9 in the online publication, which you can download). Early-planted fields will likely have higher populations of first-generation ECB.
2nd-generation egg masses
Adults produced from the first generation begin laying eggs when the corn is around VT to R1 stage of growth, and egg-laying lasts for about 20 days. The accumulation of degree days can be used to determine when egg-laying begins (see Page 5 in the publication), and scouting should occur eight to 10 days after that date. Eggs are laid primarily on the underside of corn leaves.
Use the same sampling plan as before (check 20 consecutive corn plants in five areas of the field) and count egg masses on seven leaves: the ear leaf plus three leaves above and below the ear leaf. After R3, when silks are brown, scouting for new egg masses is unnecessary if the economic threshold was not reached.
To determine if an insecticide is economically justified, use the cost-benefit analysis table for reproductive stage corn (see Page 10 in the publication). Insecticide treatments for second-generation ECB must be timed accurately to be effective. Applications should be made soon after egg hatch but before larvae enter the leaf axil, sheath, collar, ear tip or before they bore into the stalk or ear shank.
Depending on the duration of egg-laying, a second application may be warranted. Fields at VT to R1 stage of corn growth and with green silks are most attractive to female moths. Late-planted corn will typically have the largest population of second-generation ECB.