Gaining Control Over Rootworm in Conventional Corn Without Bt

Steve Sick

Oct. 10, 2018

Corn rootworm can limit corn production severely if not controlled properly—corn rootworm has historically caused some of the greatest corn yield losses among any insect. But you can achieve acceptable control of this insect in conventional corn—it starts with knowing and understand the types of corn rootworms and their life cycles. This is true for managing any pest pressure on conventional corn.

Each stage of the corn rootworm life cycle can cause plant damage

Corn rootworms can do damage to corn yields in both larva and adult beetle stages of their life cycle, which reduces nutrient and water uptake and limits yield potential.
  • There is only one generation of corn rootworm per year. Life cycle stages are egg, pupa, larva and adult beetle.

  • Rootworm eggs hatch. The rootworm eggs hatch in the spring and the larva move to the roots of seedling corn plants from mid-May to mid-June.

  • The larva feed and enter the roots. Larvae hollow out the roots and end up pruning the roots limiting the roots ability to absorb nutrients and water. Severely pruned roots also reduce the standability of the corn plant, and can cause corn plants to lodge, making harvest difficult resulting in possible corn ear loss.

  • Larvae exit roots and pupate in the soil. When corn rootworm larvae reach full size (sixth instar stage), they exit the roots and pupate in the soil near the roots. Both larvae and adult beetle feeding can provide entry points for secondary pests and disease.

  • Adults leave the soil and mate. Adult rootworm beetles emerge from the soil in late June to mid-August and then mate. Adult rootworm beetles also feed on corn silks during the pollination period causing poor pollination of the ear and limiting the number of kernels to to be produced.

  • Females deposit eggs. Approximately 14 days after emergence, the female rootworm beetles deposit their eggs into the soil near the corn plant’s root zone. 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.

Black Western Corn Rootworm beetle feeding on silks of the corn plant ear during pollination

There are three species of corn rootworms

The adult beetle stage of the life cycle is only way to identify which rootworm species is present.

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

2. Northern Corn Rootworm (NCR) are lime green in color and are usually found in fewer numbers than WCR in the northern portion of the Corn Belt. NCR have the unique ability to enter a 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.

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

There are several ways to control corn rootworms

Specific crop planning, timely scouting and insecticide treatments can help control corn rootworm when growing conventional corn
  • One-year crop rotation to another crop, such as soybeans or wheat, works well for Western Corn Rootworm (WCR), but not for Northern Corn Rootworms (NCR), which can enter an extended diapause. A two-year rotation is required for NCR control.

  • Scout for any beetle populations this season in the fields you have planned for corn in 2019. This will help you to indicate if a soil insecticide may be required next spring, because once corn is planted, there is no reliable rescue method to control corn rootworm larvae specifically.

  • Soil-applied insecticides at planting time, either in-furrow or T-band.

  • Seed treatments can help to suppress rootworm populations.

  • Adult rootworm beetles can be controlled by spraying when female beetles become gravid (full of eggs) and before they deposit their eggs. This can require two treatments in fields that are especially populated with beetles; protect the pollination period by spraying when silk clipping is evident.

  • Place rootworm traps. Corn rootworm beetles can leave corn fields and lay their eggs in soybean fields. Placing rootworm traps in adjacent soybean fields is a good way to monitor corn rootworm activity and populations. Treating soybean fields with a high trap count is an effective way of helping control next year's rootworm populations.

  • There are a number of insecticides available to control rootworm larvae at planting, and to be used as an in-furrow application on conventional corn. Several insecticides are labeled to control rootworm adult beetles. Some adult beetle insecticides can be applied by air, ground or through a sprinkler system.

Always read and follow the label use instructions for any insecticide to be used—it is the law.

Sources: 1.

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Steve Sick

Oct. 10, 2018