Resistance of Corn Rootworm to Bt Corn Discovered in Iowa

After receiving complaints from farmers, researchers at Iowa State University launched an investigation into several problem fields in Iowa. Farmers complained of heavy rootworm feeding in fields that were planted with a corn hybrid with Bt protection for rootworm. Corn with Bt protection has a gene from the bacteria Bacillus thuringiensis that confers resistance to insect feeding.

Insects were collected from problem fields across the state of Iowa along with samples from fields with no reported problems. The rootworm larva were grown in controlled conditions in the lab and were fed roots from plants expressing a single Bt trait (Cry3bb1, Cry34/35ab1) or no Bt trait (control).

The insect larva from the control fields were not impacted by the non-Bt trait corn hybrid and were controlled with all Bt traits, as one would expect. The rootworm larva from the problem fields were controlled with the Cry34/35ab1 trait but not the Cry3bb1 trait. This is the first reported case of coleopteran (beetle) resistance to Bt traits. The Cry3bb1 trait is found in YieldGard products.
An investigation into practices that led to resistance shows a similar story to other cases of pests developing resistance to a control measure. Problem fields were planted to continuous corn for at least three years with only the Cry3bb1 trait for rootworm control. The research paper that reported the insect resistance also cited that only 50% of the needed refuge was planted.

Thankfully for Darke County farmers this problem is currently only identified in Iowa. To avoid a similar problem, Darke County farmers need to heed several management recommendations. First, crops should be rotated to decrease the pressure of the Bt trait on the insect population. If corn will be planted for more than one season in a field, farmers should rotate Bt traits or plant a hybrid with multiple traits for rootworm control. Secondly, the needed refuge should be planted to help dilute any resistance that may be present in an insect population. Thirdly farmers should never rely on a single management practice for a pest problem. For more information on the resistant rootworm biotypes visit the CORN Newsletter at http://corn.osu.edu/ or the OSU Agricultural Entomology at http://entomology.osu.edu/ag/.

A similar problem with resistance to a management tactic is present in Darke County. Among other weeds like marestail and giant ragweed, the weed waterhemp is confirmed in Darke County to be resistant to both glyphosate (Roundup, Cornerstone, Buccaneer, etc…) and ALS inhibitor herbicides (FirstRate, Classic, Synchrony). In other states waterhemp is resistant to other herbicide families including the triazines (atrazine and simazine), PPO inhibitors (FlexStar, Cobra), and the HPPD and PD inhibitors (bleacher herbicides like Balance, Command, and Callisto). A single plant in Illinois was identified as resistant to glyphosate, triazines, ALS, and PPO inhibitors. This weed has the potential to increase a resistant portion of the population quickly if a single herbicide is used year after year.

To learn about waterhemp and to have a general discussion of weed management, visit the OSU Extension, Darke County, research site at the corner of Brock Cosmos and Arnett Roads near Rossburg on Tuesday, August 16, at 7pm. Signs will be posted to direct you to the site from the intersection. The site is about a mile east of Route 49 on Brock Cosmos or two miles west of Route 118. The site hosts 28 different herbicide programs in corn and soybean crops. There is no need to RSVP for the event but more information is available by calling the Extension Office at (937) 548-5215.