Blast from the Past: European Corn Borer is Back on the Radar

European corn borer (ECB) is a pest that most farmers haven’t had to think about since the late 1990s. ECB was the main target of the first Bt corn hybrids introduced to the market in 1996. Since then, Bt hybrids have effectively managed ECB populations and even provided economic benefits to farmers that don’t use Bt through areawide suppression. Before this, ECB was estimated to cost U.S. farmers over a billion dollars annually in yield losses and control costs, earning it the title of “the billion dollar bug.”

Although Bt hybrids targeting ECB have been successful for more than two decades, field-evolved resistance is occurring in several Canadian provinces, and most recently, Connecticut. The first case of resistance to the Cry1F protein was documented in 2018 in Nova Scotia, and Cry1F resistance has since been documented in New Brunswick, Manitoba, and Quebec. In 2022, the same field with Cry1F resistance in Nova Scotia was found to also have resistance to the Cry1Ab protein. In 2023, researchers documented unexpected injury to corn hybrids expressing the Cry1A.105 + Cry2Ab2 proteins in Nova Scotia and Connecticut. These four proteins constitute the list of effective Bt proteins for ECB; Vip3A is another protein for caterpillars, but it is not effective on ECB.

Although resistance has not yet been documented in Iowa or the Corn Belt, these developments are concerning, and early detection of field failures or increased populations of ECB will be essential to mitigate resistance development in Iowa. Understanding how to scout for ECB and what management strategies to implement will help us with detection and mitigation.

Scouting for ECB

Although corn is the preferred host of ECB in Iowa, this pest has a wide host range. ECB overwinters in Iowa and can have 1-3 generations per year, although we typically see two generations. The larvae, or caterpillars, are the damaging life stage of ECB, and they can feed on any above-ground part of the corn plant throughout the growing season. Larvae are gray to creamy white in color and have a black head and many dark spots along the body. They are ¾ to 1 inch long when fully grown. Organic fields or fields with non-Bt hybrids are at greater risk for infestations and should be scouted first.

Early-season injury to corn (Photo 1) begins as small circular feeding areas on the leaves or shot holes from where larger larvae chewed through a leaf in the whorl. ECB is not the only insect that might leave shot holes in leaves, so check within the whorl for ECB larvae to confirm. Larvae can tunnel through the whorl and enter the plant stem, or they may enter through the stem, leaf axil, or midrib. It is easy to see the entry points, and typically there will be frass around them. The first generation is often attracted to early-planted fields.

Late-season injury to corn (Photo 1) is the most damaging. Larvae may continue to feed on leaves, but tunneling into tassels, stalks, ears, and ear shanks is the major concern. This could lead to tassel breakage, ear drop, reduced kernels, lodging, stalk rots, and ear rots. Like the first generation, entry holes are apparent and usually coated in frass. The second generation is often attracted to late-planted fields.

Aside from looking for larvae, scouting for egg masses may be useful for in-season management options. However, there is a limited window from finding egg masses to when larvae will tunnel into the plant, rendering them protected from insecticide applications. Egg masses usually contain 15-30 eggs, are found on the undersides of leaves, and look like fish scales (Photo 2).

Moth (Photo 3) traps may also be useful for determining the biofix (start date) for accumulating degree days to track the development of ECB. Reach out to us if you would like more information on trapping for ECB.

Resistance Mitigation

Adult ECB disperse across the landscape, making tracking of suspected resistant individuals challenging. Scouting for ECB and reporting larval feeding, even in non-Bt corn fields, will be essential to early detection of ECB issues. Please reach out to us if you find any ECB larvae in your Bt cornfields. Currently, ECB presence and severity around the state are largely unknown due to the areawide suppression effect of widespread Bt corn planting.

Since most corn hybrids now contain pyramids of Bt traits and come with refuge-in-the-bag (RIB), farmers likely will not have to change their current production practices. However, here are a few things to keep in mind as resistance continues to develop:

• Choose pyramided Bt hybrids to ensure multiple toxins are contributing to ECB control. Use the Handy Bt Trait Table when making decisions to ensure a suitable hybrid is selected.
• Make sure to plant at least 5% refuge (non-Bt plants) in each field. Hybrids with RIB already have 5% refuge, but a non-Bt hybrid should be planted within the field if a hybrid is chosen that does not have RIB. Studies show that higher proportions of refuge contribute to reductions in resistance. Different refuge requirements may exist outside of the Corn Belt.

Additional integrated pest management practices apply, such as the conservation of beneficial insects in the landscape and shredding or burying corn residue to reduce the survival of overwintering larvae within fields. Tillage activities should fit in to the overall goals of the operation.

The Iowa State University Extension Store has two publications that might be of interest to those hoping to learn more about European corn borer biology, scouting, and management.

1. European Corn Borer – Ecology and Management and Association with Other Corn Pests; $8.00 for a hard copy or PDF. https://store.extension.iastate.edu/product/3067

2. Ecology and Management of European corn borer in Iowa field corn; free PDF. https://store.extension.iastate.edu/product/15141