Mid-Season Insect Watch: Corn Rootworm Beetle

If you’ve ever dealt with significant pressure from corn rootworm beetles in your fields, you know how severely this insect pest can impact corn production if not properly controlled. In fact, you might see as much as 15 percent yield loss on average for each root node that is pruned due to larval feeding.1

High populations of rootworm beetles can scrape the chlorophyll from corn leaf surfaces. This results in less leaf area to manufacture sugars to be translocated to the corn kernels, limiting kernel fill and translating to significant economic damage. 

Adult rootworm beetles also feed on corn silks during pollination, leading to poor pollination and limiting the number of kernels to be produced. 

To make matters worse, adult beetle feeding can also provide entry points for secondary pests and disease. 

Planting Bt hybrids is one way to reduce the threat of rootworm pressure. But if you’ve planted non-Bt and/or conventional corn—or if you’re experiencing resistance issues after planting Bt corn—it’s important to know what to look for and how to control rootworm beetles in your fields.

And while control is most ideal in the larval stage, sometimes a mid-season rescue treatment is necessary and can help you protect your crop against rootworm beetles.

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What should you look for when scouting for rootworms?

Emerging from the soil between late June and the middle of August, adult beetles will mate and, approximately 14 days after emergence, the female rootworm beetles deposit their eggs into the soil near the corn plant’s root zone. 

There are three types of corn rootworms, and the way to identify which species is present in your fields is to find the pest in its adult beetle stage of the life cycle. Here is a bit more information about each species:

Western Corn Rootworm 

Western Corn Rootworm (WCR) is the most common rootworm beetle and most damaging species across the Corn Belt. The adult beetle is yellow with black stripes to almost completely black.

Northern Corn Rootworm

Northern Corn Rootworm (NCR) are usually found in smaller numbers than WCR in the northern portion of the Corn Belt. NCR have the unique ability to enter an extended diapause stage, meaning they can remain in the egg stage of its life cycle for an extra year. This allows NCR to survive in a one-year crop rotation system. NCR are lime green in color.

Southern Corn Rootworm

Southern Corn Rootworm (SCR) generally causes the least damage of the 3 types, but there can be situations where adult SCR beetles must be controlled to protect against silk clipping during corn pollination. SCR eggs do not survive the winter in most of the Corn Belt, but they are still of some concern in the southern Corn Belt. The adult beetles are yellow to light-green with black dots on the back. 

How do you control rootworm pressure in your corn crop?

Timely scouting and treatments are needed, but you can achieve acceptable corn rootworm control growing non-Bt and/or conventional corn hybrids. Here are a few methods:

Crop rotation practices

One method is to utilize a one-year crop rotation to another crop, such as soybeans or wheat. Crop rotation practices work quite well for Western Corn Rootworm (WCR) but not for Northern Corn Rootworms (NCR), which can enter an extended diapause and require a 2-year rotation.

Spraying for larval control

There is no reliable method to achieve total control over rootworm larvae once your corn is planted, but many insecticides are available for in-furrow application to provide larval control. Scouting beetle populations during the growing season in fields planned for corn next year will indicate if a soil insecticide will be required in the spring.

Using a Group 3A insecticide product such as Bifenthrin LFR—or, if not adding it to your starter fertilizer, an emulsifiable concentrate like Willowood Bifenthrin 2EC—will help you address rootworm pressure at planting.

You can achieve some degree of control by timing your insecticide application for when female beetles become gravid, or full of eggs, and before egg deposition takes place. This will sometimes require two treatments in highly populated fields. You may also want to consider spraying when silk clipping is evident in order to protect your corn during the pollination period.

Spraying for adult rootworm beetles

Several insecticides are labeled to control the adult beetles, as well, and can be applied by air, by ground or through a sprinkler system. You’ll want to again use a Group 3A insecticide such as Willowood Lambda-Cy 1EC or Willowood Bifenthrin 2EC to manage adult rootworm beetles in your fields.

Always read and follow the label directions for any insecticide to be used.

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Sources:
1. Corn Rootworm Management Update, Iowa State University Extension and Outreach
 
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