Corn rootworms have been around as long as corn has been grown in our area. They can cause severe damage to corn fields by destroying roots and thereby inhibit nutrient uptake and standibility of the corn plant. We tend to see significant numbers of corn rootworms show up when we plant corn-on-corn for a number of years. Planting corn year after year has become very popular in recent years because of high prices and excellent yields. Corn rootworms have developed resistance to many control tactics over the last 50 years including crop rotation and soil and foliar applied insecticides.
Bt corn came on the market back in the late ninety's as a possible solution to many insect problems. In its simplest form, Bt corn is genetically modified to, in effect, kill specific insects that consume it. Bt was first introduced as an answer to European Corn Rootworm in the mid-1990's and proved to be a workable solution that has virtually eliminated the corn borer problem as we know it. A few years ago, corn hybrids were introduced that had the Bt-RW (Rootworm) trait that proved to be effective against the western corn rootworm (yellow colored beetle with black stripes). That is until last year and now in 2012; we have seen significant resistance in the form of Corn Rootworm infestations that threaten the whole field.
So, how did this happen? Part of the problem lies with corn hybrid selection. Because of the potential for corn rootworms to develop resistance to BT-RW traits, the EPA (Environmental Protection Agency which tests and governs all releases of new pesticides) registration for each trait outlines an insect resistance management (IRM) program that growers agree to implement when they sign the technology agreement. The heart of resistance management involves planting a refuge (non Bt-RW hybrid) that will produce susceptible beetles to mate with beetles emerging from the Bt-RW hybrids and dilute the accumulating resistance genes. Capability of western corn rootworm to develop resistance to Bt-RW has been demonstrated under laboratory conditions. Since the sale of these traits began, it has been a question of when and not if resistance will show up. The key factors influencing speed of resistance include: repetitive use of the same traits in a field and lack of a refuge.
Tina LeBrun and Wayne Schoper
Performance Problems what to do
If you note unexpected levels of rootworm injury or lodging, or suspect performance problems in any Bt-RW please follow these steps:
1. Consult your planting notes to confirm hybrids and trait placement.
2. Scout where problems are occurring in the field. Look at lodging. Dig a few roots to verify corn rootworm damage. Note the corn rootworm species and relative abundance in the field.
3. Contact your seed dealer to report unexpected corn rootworm injury. Arrange to have an independent ag professional familiar with corn rootworm present for company field visits.
4. Each company has its own internal reporting process but expect these elements:
* Ground truth of planting information of tissue testing with gene check kits.
* Digging roots both Bt-RW and refuge hybrids to verify excessive root feeding and explore other potential causes of lodging.
* Field history information including cropping history and management practices.
* If verified, the company may collect beetles, if present, to verify resistance and may request management steps for the following corn crop.
5. If you experience Bt-RW performance issues, check out the University of Minnesota's website: www.extension.umn.edu/cornrootworm/
There you can get the latest updates on what is happening. If you wish, you can also report your location and extent of the problem so that this information can be used to put together a database of the extent of the problem in Minnesota.