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Angie Peltier

Angie Peltier
Former Extension Educator, Commercial Agriculture

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Hill and Furrow

Current topics about crop production in Western Illinois, including field crops research at the NWIARDC in Monmouth.
Figure. Counties shaded red have confirmed populations of Western corn rootworm that have evolved resistance to the Cry3Bb1 Bt trait. Counties highlighted in red had severe Western corn rootworm damaged first year corn in 2013 (Drs. Mike Gray and Joe Spencer).
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Western corn rootworm populations resistant to Bt (Cry3Bb1) close to home

Posted by Angie Peltier - Insects

Bacillus thuringiensis (Bt) is a bacterium. It naturally produces toxin proteins that create fatal holes in the digestive system of susceptible insects. In 2003, transgenic corn hybrids expressing a bacterial Bt toxin (Cry3Bb1) in root tissue with activity against the Western corn rootworm (WCR) were released for sale to corn producers. Since this time, seed corn companies have created corn hybrids that express one or more Bt traits with activity against Western corn rootworms (mCry3A, Cry34/35Ab1, eCry3.1Ab).

Transgenic Bt: "low-dose events". From a resistance management standpoint, whether it is a fungicide, a herbicide or an insecticide, it is important to hit pest populations with a full rate of a pesticide. Unfortunately, this does not always occur with transgenic Bt proteins in corn roots. Even before WCR populations evolved resistance to the Cry3Bb1 Bt trait, there were larvae that would survive and feed to complete their life cycle. This is because the concentration of Bt proteins that are expressed in root tissue may be lower than the concentrations expressed above ground. The concentration of Bt in root tissue tends to decline with time as corn plants develop. Consequently, the portion of the WCR population that hatches later is exposed to lower concentrations of the Bt toxin.

Populations can shift over time. The weeds, pathogens and pest that we battle in our farm fields are diverse populations. WCR populations have shifted to overcome many of the chemicals and tactics that producers have employed to manage them: broadcasted insecticides (chlorinated hydrocarbon, carbaryl and methyl parathion, carbofuran) and crop rotation. Due to random genetic mutations, individual WCR larvae may hatch that are able to better survive exposure to the Bt toxin, feeding happily on corn roots carrying the Bt trait regardless of when they were exposed (early-season under higher Bt concentrations or later on). As adults these individuals will mate, passing on their Bt-tolerance genes to their offspring. Rootworm populations shift over time as those that are still sensitive to the Bt toxin produce fewer offspring than those that are tolerant. It may not be until several years later, that the results of these population shifts may be readily observed through severe feeding damage.

In 2011, Iowa State University Entomologist Dr. Aaron Gassmann published an article detailing a study in which he and his colleagues confirmed WCR populations resistant to the Cry3Bb1 Bt trait (ex. YieldGard VT Triple) originally collected in 2009 from several counties in Eastern Iowa.

Resistant populations had developed in fields that had been planted to corn carrying the same below-ground Bt-trait (Cry3Bb1) for 3 or more consecutive years. Deploying the same trait in the same field for multiple years in a row may have increased the speed at which the population shifted to contain more Bt-insensitive individuals.

Since this original article was published, Dr. Gassmann's team found that certain field-collected WCR populations were resistant to two Bt toxins: Cry3Bb1 and mCry3A.

Bt-resistant Western corn rootworm populations confirmed in 5 Illinois counties. Since this time, University of Illinois Entomologist Dr. Mike Gray and Illinois Natural History Survey Entomologist Dr. Joe Spencer have received reports of significant feeding damage on corn carrying the Cry3Bb1 trait throughout Illinois.

If Dr. Gray is called to investigate a suspected Bt-failure, the first thing that he does (in a county in which Bt-resistant WCR populations have not yet been identified) is make arrangements to go and visit the field. At the field Dr. Gray uses test strips to determine that the damaged corn hybrid is indeed expressing the WCR Bt trait and also carefully collects adult beetles. These beetles are transported back to the lab and are fed until they mate. Eggs are collected for further study.

After a cool, dormant period in soil, eggs hatch and larvae from populations both never exposed to Bt and those suspected to be Bt-resistant are placed onto Bt and non-Bt corn seedlings. Larval survival is measured after 17 days of incubation on live plants.

Using this methodology, scientists in Illinois have confirmed WCR populations resistant to the Cry3Bb1 protein in five Illinois counties: Henry, Whiteside, Mercer, McDonough, and Sangamon Counties (Figure).

What can I do if I notice significant feeding in-season? If a farmer notices significant feeding damage on roots, there are no treatments that can be applied in-season to rescue the current year's corn roots. Insecticides can be applied if corn rootworm adult populations are significant and they are clipping silks before pollination in complete.

It may be little consolation in the current growing year, but observing significant feeding damage on Bt hybrids is essential information for prudent WCR management in future growing years.

What can we do to slow the development of resistance? Producers are encouraged to develop a long-term integrated pest management strategy to manage WCR and reduce the speed at which populations may shift.

  • Crop rotation: As initial development of Bt-resistant WCR populations occurred in continuous corn fields, rotating crops in recommended. (Note: Drs. Gray and Spencer visited corn fields in Kankakee and Livingston Counties that had sustained significant feeding damage (Figure). These fields had been planted to soybean the year before. It is not yet known whether rotation-resistant Bt-resistant WCR populations have been confirmed.)
  • Combining an in-furrow soil insecticide with a non-Bt corn hybrid. As in-furrow insecticides are applied in bands, the untreated soil between rows acts as a natural refuge, in which a significant fraction of the WCR larvae remain unexposed to insecticides. This may be one reason why WCRs, which have evolved over time to combat many management tactics, remain susceptible to in-furrow insecticides.
  • Rotating pyramided Bt hybrids. Planting corn hybrids that express more than one Bt toxin (pyramided) is similar to tank mixing fungicides or herbicides with different modes of action and will likely slow the speed at which resistance develops in the rootworm population. Similar to tank mixing glyphosate with another herbicide and spraying it to control a glyphosate-resistant marestail population (control effective relies on the non-glyphosate herbicide), control is relegated to the non-Cry3Bb1 trait when a pyramided hybrid containing Cry3Bb1 planted in a field in which the WCR population is Cry3Bb1-resistant.
  • Consider an adult suppression program in fields that are heavily infested with WCR beetles. Additional information about scouting and thresholds can be found here and here.

For more information on this literally evolving story, follow Dr. Mike Gray's articles in the University of Illinois Bulletin. Previous Bulletin articles that may be of interest include:

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