In 2014, King’s AgriSeeds had a customer in Belleville, Pennsylvania report rootworm problems with a Bt corn hybrid, possibly suggesting rootworm resistance to the trait. This began an investigation into a trend toward resistance that more customers are struggling with.
We notified Dr. John Tooker, Associate Professor of Entomology at Penn State University. Dr. Tooker looked into this matter, collected other information and reported back to us about this situation.
While the situation is complicated, the population of western corn rootworms from Belleville appears to have developed partial resistance to the Bt traits that were present in those fields. In lab feeding tests on Bt corn, rootworms from the Belleville population survived about three times better than rootworms known to be completely susceptible to the Bt toxin. However the rootworms from Belleville also only survived about half as well as a population of rootworms known to be resistant to the Bt trait. This means that the Belleville population was somewhat, but not fully, resistant to the Bt traits.
To avoid populations of rootworm that may develop resistance to Bt traits, Dr. Tooker encourages rotating corn with soybeans or alfalfa, and not growing more than two years of continuous corn. The problematic fields in Belleville were in corn for just three years. It seems that nearly any population of rootworms exposed to continuous corn could develop resistance; it is unlikely that the Belleville incident was an isolated event.
“If growers do not use proper crop rotation, they should use either a corn hybrid with a two mode of action trait brand such as EZ Refuge (MCT5375) or Duracade (MCT4058), or use a soil insecticide,” according to Tim Fritz, owner of King’s AgriSeeds.
Rootworm resistance to the transgenic Cry Protein (Cry3Bb1) is the main component of Bt resistance and has been observed in several Midwestern states, including Illinois, Iowa, Minnesota, Nebraska, and South Dakota.
If we grow corn after corn with the same trait, resistance to the Bt trait builds up over time.
Farms that grow continuous corn year after year, or have short rotations with corn, (without changing-up modes of control) are providing the right conditions for rootworm to develop resistance. This issue provides a good reminder that relying on agronomic fundamentals (such as crop rotation and rotating insecticide chemistry by using a diversity of rootworm control insecticides) may be just as effective, or more so, than traits.
Long term (greater than 3 years) continuous corn can greatly increase rootworm and other pest problems. Corn rootworm damage is seldom a problem in field corn when the corn is rotated to a different crop within three years after the field is first planted to corn.
The two main types of rootworm to be aware of are Northern and Western Rootworm. Western rootworm usually hatches earlier, and has shown resistance to the Bt trait in the Midwest.
Seed treatments do not appear that effective against rootworms, and can be influenced by basic agronomic practices. Most insecticide seed treatments are effective for only 2-3 weeks, so early-planted corn whose emergence and early growth may be slowed by cool weather may be susceptible to rootworms.
In Southeastern Pennsylvania rootworms hatch in late May to early June, at which point the seed treatment is unlikely to provide protection for early-planted corn.
In Central Pennsylvania eggs typically begin hatching between June 1 and June 15. Hatching may occur 7 to 10 days later in northern cooler parts of the Pennsylvania or at higher elevations. The total hatching period is typically three to four weeks long.
To complicate things further, hatching is related to soil temperature. A cooler, wetter year like 2014 probably delayed hatching, further reducing effectiveness of seed insecticide treatments because the effectiveness of the seed treatment dwindles with time. And cool soil temperatures will slow seed germination, meaning the seed is in the soil longer before emerging. This makes the seed and its early root development more vulnerable to rootworms once they hatch and begin feeding. Also, excessive amounts of rain in spring can flush seed treatments from the seed zone.
Be aware that seed treatments containing Poncho or Cruiser 250 are not labeled to control rootworm; only 1250 doses of the two insecticides claim to have some effectiveness against them. Now I have seen that even the C-1250 is also not working as effectively at some locations, and it seems the rootworms are building up a tolerance for it.
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