Agronomy Blog

Corn Rootworm – Life Cycle and Management

by Ryan Dunsbergen | Jul 14, 2017

This past weekend we had a major rain event go through Northern Iowa.  This event brought high winds and some very damaging hail.  The high winds led to some greensnap in corn as well as root lodged corn.  The root lodged corn has revealed underground corn rootworm pressure that was previously unknown.  Travis Dollarhide (LG Seeds Agronomy and Product Manager) recently posted an article on greensnap in corn that you can find here.  Steve Crafton (LG Seeds Sales Agronomist) recently posted an article on the process of digging roots and scouting for corn rootworm larvae found here.  The remainder of this article will focus on the life cycle and management of rootworms in corn.

Life Cycle of Corn Rootworm
The two major rootworm species that cause extensive damage across the Corn Belt are the western corn rootworm (WCR) and the northern corn rootworm (NCR).  These two species have four developmental stages.  Those stages are egg, larva, pupa, and adult. The NCR and WCR both overwinter as eggs in the soil.  Egg hatch is dependent on temperature.  On average, it takes 725 heat units for 50% of the egg population to hatch and they immediately begin feeding on corn roots.  This usually takes place toward the end of May or beginning of June.  The larvae feed on corn roots for nearly one month until they are about ½ inch in length.  At this developmental stage, the NCR and WCR are indistinguishable from each other as there are no marked differences between the two.  They then pupate, turn into adults and emerge from the soil throughout the month of July and into early August.  The male beetles will emerge before the female beetles.  WCR beetles will usually emerge a few days before the NCR beetles.  The adult beetles are distinguishable from each other (See photos below).  The adult female beetles will lay their eggs in the soil after mating, beginning the cycle again.  Each female has the potential of laying over 1,000 eggs during her lifetime.
ryan corn rootworm 1 cited  
ryan corn rootworm 2 cited

Rootworm Adaptability
Rootworms are an extremely adaptable pest.  First of all, they have been documented as showing some level of resistance to every rootworm trait that is currently on the market.  Second of all, they have demonstrated the ability to adapt their life cycles in order to survive and thrive.  The variant WCR beetle will fly out of corn fields and lay eggs in soybean fields where corn will be planted the following year.  The NCR has adapted by having a two year life cycle.  This is called extended diapause.  The NCR beetles will lay their eggs in the corn field, but instead of hatching the following spring, they lay dormant that year.  They will then hatch the second spring when the field has been rotated back to corn.  In both cases of the WCR variant and the NCR extended diapause, these life cycles prevent crop rotation from being an effective management tool.  

Management

If you are experiencing rootworm pressure on first-year corn, you are either dealing with a WCR variant or the NCR extended diapause.  This is one reason why it is important to control volunteer corn in soybeans.  The more volunteer corn there is, the more it will attract the WCR beetles to feed on the corn and then lay eggs in the soybeans.  You will need to either begin using an insecticide, or the Genuityâ SmartStaxâ trait – which is a dual mode of action below ground, or perhaps begin using both.  If using an insecticide, read and follow the label.  Choose an insecticide that has great efficacy on rootworms and use the appropriate rate.  While neither option is a silver bullet, the traited corn will provide the most consistency.  I have seen insecticides fail because of dilution in excessively wet years and because of lack of activation in excessively dry years.

If you are experiencing rootworm pressure on traited corn, you will need to up your game, so to speak.  Since rootworms are incredibly adaptable, you will need to be even more adaptable to stay ahead of them.  Here are some things to consider.

  1. Consider using an insecticide application to control adult rootworm beetles so the females do not lay eggs.This option is a possibility, but the efficacy can be very low because adult emergence from the soil continues for up to six weeks and because beetles will fly from field to field.The more important issue is protecting this year’s crop.Corn needs at least ½ inch of silks in order to pollinate and fertilize the kernels properly.If adult beetles are clipping silks shorter than that during pollination, the adult beetles will need to be controlled.
  2. Weed control – start clean, stay clean.Roots of grass weeds act as an alternative host for rootworm larvae.Rootworms can feed and grow on these alternative hosts without digesting the Bt proteins from Genuityâ SmartStaxâ hybrids in the corn roots.Once the grasses are killed, the developmentally advanced larvae will move to the corn roots and it will require more root feeding to control them.
  3. Proper hybrid selection and placement is very important.Make sure you are planting hybrids that have strong root strength and make sure the maturities of your hybrids are properly adapted to your area.Also take steps to ensure you are avoiding soil compaction and be diligent to match your hybrid to your soil type and management practices.
  4. Monitor the rootworm beetles.This is best done by trapping them.Taking a quick walk through a field in the middle of August looking for rootworm beetles is not effective.Set beetle traps in continuous corn fields as well as bean fields.If you are trapping several beetles in these fields, chances are high of having heavy rootworm pressure next year.Corn fields with heavy pressure would be a good option to rotate to beans.Soybean fields will need to be planted to corn with the Genuityâ SmartStaxâ trait, or perhaps even kept in soybeans for another year.I know beans-on-beans is not a popular option, but it might be the best option in light of heavy rootworm pressure.

Corn rootworms are an important economical pest to control.  We have seen fewer effects from rootworms the past couple of years.  This is due to the combination of being diligent in how we attack this pest as well as the weather helping us out.  In some areas the rootworm population has been less.  In other areas timely rains have compensated for the rootworm feeding.  In dry years where plant available moisture is low, rootworm feeding will exacerbate the problem and will impact yields significantly.  We need to continue to be vigilant against rootworms.  If you are seeing any issues in your fields, please do not hesitate to contact your LG Seeds District Sales Manager or Agronomist.  We would love to help!

Sources and Additional Information:
  1. http://crops.extension.iastate.edu/cropnews/2016/06/guidelines-using-sticky-traps-assess-corn-rootworm-activity
  2. https://extension.entm.purdue.edu/fieldcropsipm/insects/corn-rootworms.php

 Note: The information in this issue is based upon field observations and third party information. Since variations in local conditions may affect the information and suggestions contained in this issue, LG Seeds disclaims legal responsibility therefore. Always read and follow label instructions.
Genuityâ and SmartStaxâ are trademarks of Monsanto Technologies LLC.
LG Seeds and design are trademarks of SCA Limagrain.

Download a copy of this Technical Bulletin: Tech_342 - Corn Rootworm - Life Cycle and Management