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Corn Rootworm Creates Challenges For Continuous Corn

TAGS: Soybeans USDA
Corn Rootworm Creates Challenges For Continuous Corn
Western corn rootworm is an adaptable and formidable foe for farmers who grow corn-on-corn.

One of the hottest coffee shop conversations all winter was about the coldest weather Iowa has had in three decades. Some farmers wonder if such cold weather kills the corn rootworm eggs that overwinter in the soil. However, entomologists say the cold weather isn't likely to have much effect on rootworm survival.

PRUNED ROOTS: Some fields planted to continuous corn have rootworm larvae that can now survive in large numbers and cause severe root injury to Bt corn. It's an increasing problem for many Iowa farmers.

"As much as we'd all like to find a silver lining in this situation, chances are corn rootworm populations won't be deterred by the cold winter," says Nick Benson, corn product specialist with Latham Hi-Tech Seeds at Alexander, Iowa. "Instead of throwing caution to the wind, farmers need to take an aggressive approach to managing corn rootworm larvae during the upcoming growing season."

Why is corn rootworm an increasing problem?
Some farmers blame the continuous planting of corn hybrids that have the same Bt trait. That is, planting the same trait year after year in the same field. However, no one factor should be solely blamed for the increasing corn rootworm problem. Just focusing on one factor can limit your management options and your ability to properly combat this threat, says Benson. Growers need to address their corn hybrid trait rotation, but don't ignore other contributing factors like scouting and crop rotation.

Every decision that you make influences the size and scope for corn yields. From the corn hybrid you select to the seeding rate and row width you choose. Download our FREE report over Maximizing Your Corn Yield.

"Rotating traits is certainly important, but it isn't the only solution," he says. Traits help protect plants, but rootworm larvae may still cause damage. Larval feeding on corn roots hinders a corn plant's ability to absorb water and nutrients, and negatively affects its ability to develop and remain upright.

Western corn rootworm adapts to Bt traits
So, it's back to the basics in terms of rootworm management as the western species of corn rootworm continues to adapt to the Bt tech traits in corn hybrids. Benson says you need to protect your 2014 crop from these yield-robbing pests by: (1) scouting your fields, (2) understanding root worm cycles and (3) implementing best management practices including respecting the refuge. He suggests the following steps as you develop a plan to protect your corn plants, yield and profit.

Scout your fields. Be sure to check what's happening both below and above ground. Notice what's going on below the ground by scouting for larvae. Come late May, at the same time the lighting bugs begin to light up the night, corn rootworm larvae will begin to hatch.

Once silking of corn plants begins or is about to begin, observe what is happening above ground. Adult corn rootworm beetles feed on the corn ear, silk or tassel. You can also identify corn rootworm damage by the "window paning" that may be taking place on the corn plants. This symptom occurs on corn leaves after the beetles emerge and begin eating leaves on the corn plant. This can be extremely damaging to the plants because of the role the leaves play in photosynthesis. Late-planted corn is especially at risk for window paning injury by the adult beetles. If this damage is identified in a field, a foliar insecticide should be applied.


Dig some roots and inspect them. Root digs during the growing season are crucial to help you see if damage is present and to help you make management decisions. The data you collect will tell you more than you could imagine compared to past field data. Also, take last year's crop performance into consideration. If your field is high risk for a corn rootworm infestation, consider alternative traits, and RIB (refuge in a bag) options and possibly even rotating the field to another crop such as soybeans or alfalfa—instead of growing continuous corn.

Understand the corn rootworm cycle. The western and northern corn rootworm species deposited their eggs last autumn, but it won't be until late May or early June when newly hatched larvae begin their feeding frenzy on corn roots. The larvae then pupate and become adult beetles. The male rootworm adult beetles will emerge first in the late June to early July time period, and the females emerge about a week later than the males. Peak egg-laying by the female beetles will begin in early August.

In recent years, controlling corn rootworm has become continuously challenging for a number of reasons. More farmers are planting continuous corn and more farmers are relying solely on genetic trait protection, which is often not providing the desired results. The western corn rootworm species is adapting to this management practice.

The northern species uses a different tactic to adapt, called extended diapause, which allows its eggs to lay dormant during the time fields are in soybeans. The corn rootworm eggs will then hatch after the field is planted to corn again, and the young worms will feed on corn roots. Here's how this happens: The adult corn rootworm females of the northern species have learned to lay their eggs in soybean fields, thereby allowing them to hatch the following spring when the field is planted to corn.

Every decision that you make influences the size and scope for corn yields. From the corn hybrid you select to the seeding rate and row width you choose. Download our FREE report over Maximizing Your Corn Yield.

Thus, with these changes in what used to be considered the "normal" life cycles of rootworms, it may become necessary to rotate the field out of corn for two seasons in a row instead of just one year. In other words, if you are having a diapause problem, plant two years of a non-corn crop between the years when you plant corn in that field.

Respect the refuge. Managing corn rootworm requires increased diligence including planting refuge acres. "Not only is planting a refuge each grower's responsibility, a refuge is also our best chance to keep the Bt rootworm control traits viable for the future," says Benson. "Refuge-In-the-Bag or RIB options are easy to manage, as they combine refuge seed with both the corn borer and rootworm insect protected corn seed. With RIB, corn growers have everything they need for the grower to be refuge compliant—and they have it in just one bag."

Control volunteer corn, as well as weeds. Corn rootworms essentially need corn to survive, so volunteer corn counts. You need to control volunteer corn when it appears in soybean fields. Because volunteer corn and pollen-producing weeds attract these rootworm pests, both the volunteer corn and the weeds must be controlled. It's advised that you apply a tank mix treatment of herbicide and clear any volunteer corn out of your fields. That will help avoid the harboring of rootworm beetles in that field. With herbicide use and careful crop planning, generally volunteer corn can be controlled in both corn and soybeans.

"Management of corn rootworm is a complex issue and the solution is more than just rotating the Bt traits," sums up Benson. "There are many factors and rootworm management options you must consider including scouting fields, understanding rootworm cycles and using best management practices. By being informed and prepared, you can be successful in keeping corn rootworm from devastating your crop in 2014." For more information visit

Every decision that you make influences the size and scope for corn yields. From the corn hybrid you select to the seeding rate and row width you choose. Download our FREE report over Maximizing Your Corn Yield.

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