Rootworms Resistant to Bt Gain Ground
In areas where resistance to both crop rotation and Cry3Bb1 is suspected, Gray advises using pyramid hybrids with their multiple traits or rotating traits. If using a suspect trait, in particular Cry3Bb1, he suggests Flageole’s practice of using an overlay of a soil insecticide for increased protection against lodging.
Bryan Johnston, Cabery Fertilizer, Cabery, Ill., was the one who asked Gray and Joe Spencer, an entomologist with the Illinois Natural History Survey, to look at problem fields.
Cabery has been running trials with granular insecticide on top of traited seed in anticipation of resistance. When rain and winds hit part of Cabery’s trade territory, lodging was severe.
Expanded scouting revealed extensive root feeding and lodging and also demonstrated the benefit of CRW insecticides.
“Fields can look normal when you drive by, but seen from the air or scouting into the fi eld reveals problems,” said Johnston. “It is important to dig the roots, as we had some lodging from hybrid failure due to poor root systems. We saw a major difference in lodging and feeding where a granular insecticide like Force was applied versus no granular. Others in the area reported the same.”
MULTIPLE MODES OF ACTION HELP
Although Johnston acknowledged the reluctance many growers have to reverting to CRW insecticides, he expects to be recommending a soil-applied insecticide next year.
“Growers have to realize the sure-deal of 100 percent protection by traits is not there anymore,” he said. “Traits can still bring value, but we definitely have to utilize multiple modes of action.”
Gray agreed that multiple modes of action are a good response. However, he advised being selective. “Our land grant community of entomologists is still pretty consistent on the question of insecticide and pyramid Bt hybrids,” said Gray. “It may be a negative when it comes to resistance management.
Where insecticides should be considered is where a trait has failed or been inconsistent and the grower still wants to plant a hybrid with that trait.”
Johnston recommended an insecticide with a pyramid hybrid may be necessary if resistance is suspected with one of the traits. Gray agreed that if a trait in a pyramid hybrid has been compromised, you are back to a single mode of action, little different from simply rotating to the other trait. He warned that even new traits, while welcomed, may not be the answer.
“I anticipate high levels of protection from the new Duracade hybrids that contain ECry3.1AB, a modified form of Cry3Bb1, being introduced by Syngenta in 2014,” said Gray.
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