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Insect Pressure on Corn Acres

Jul 22, 2021

Corn rootworm pressure is showing up in our geography, and depending on differing management styles, some areas are seeing heavier pressure than others. So why is pressure so strong this year? We did not see a harsh winter to freeze out any eggs deposited in the soil. Additionally, this spring, we didn't get enough moisture to drown a good portion of the larvae out. We were pretty dry going into planting and stayed dry for a while after that. The heat and lack of moisture added up to a very conducive environment for corn rootworm, and today, at tassel and pollination time, we are seeing activity that could end up being devastating for the crop.
 
Here in southern Minnesota we see two different species of corn rootworm - the northern corn rootworm (Diabrotica barberi) and the western corn rootworm (Diabrotica virgifera virgifera).

CRW.png

Northern corn rootworm can vary in color from yellowish/light green when it is newly emerged to a darker green as it ages and feeds, as seen above.  It has no markings on it, other than being green. An easy way to remember the “NO”rthern corn rootworm is green, is that it has “NO” distinguishable markings. Western corn rootworm appears to be tallow to green in color with 3 black stripes on their forewings. The male western corn rootworm beetles’ wings often have less distinguished striping, and more black pigmentation, as pictured on the right side.

CRW-(4).png
Both northern and western corn rootworm larvae are very similar in appearance, being about a ½ inch long when fully grown. Larvae go through three instars that each last seven to ten days. During these three instars, they feed on corn roots. After going through three instars, the larvae pupate in the soil, where they are changing into an adult beetle. During this stage they do not feed on roots.  
 
Northern and western corn rootworm only have one generation per year, with eggs being deposited in the soil from mid-summer through fall. The eggs then overwinter, and then start hatching around late May to early June depending on environmental conditions. The larvae stages described above, will then begin to emerge as adult beetles by late June to early July. About two weeks after emerging, females will lay eggs.
 
During this time, the beetles will feed on leaves, pollen, and green silks of the corn plant. Due to an earlier emergence this year, we may see pollination issues in some areas due to the beetles feeding on silks, also known as silk clipping, and disrupting the pollination process. A result of this may be “zipper ear”
CRW-(2).png
What are areas to watch out for?
  • Corn on corn acres
  • Hybrids that do not have a below ground protection trait
  • Acres where an insecticide was not applied
 
These are only obvious red flags, but all acres should be scouted to ensure no pressure is found. The northern corn rootworm has a trait called “extended diapause”, where eggs with that trait can remain dormant in the soil for two winters and hatch the second season if on rotated ground. Therefore, if you have had pressure and switch to a soybean rotation, that does not necessarily equate to being rid of the corn rootworm issue the following year. Doing corn rootworm digs and floats, as well as putting out sticky traps will help indicate what kind of pressure your field has and what management decisions to make.
 
Using a hybrid that has underground trait protection is a key management decision to protecting your crop. SmartStax (SS) or Qrome (Q) hybrids have underground protection in addition to above ground protection to defend against corn rootworm. Products like Dekalb’s 56-65SS and Brevant’s B98R95Q are both hybrids that have defense mechanisms against corn rootworm feeding. The 106-day hybrid and the 98-day hybrid, respectively, are both in our NuWay Agronomy replicated test plots this year and we are excited to bring them to harvest. If using a DoublePRO (VT2P) or AM product, it is always recommended to be used on corn-soybean rotated ground. DoublePRO and AM products provide only above ground insect protection. An insecticide can be added to provide additional protection for those hybrids.
 


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