Corn rootworms inflict more than $1 billion annually in lost revenue and control costs. PRI insect behaviorist Joseph L. Spencer regularly travels across Illinois collecting corn rootworms, studying their behavior, ecology, and their growing resistance to pest management, particularly resistance to Bt corn hybrids, and crop rotation.
This year, Spencer has collected corn rootworms from northern Illinois and around Champaign County.
“We were hoping to get a few more populations of western and northern corn rootworm (i.e. WCR & NCR) beetles from western and southern Illinois, but the abundance in the collection areas wasn’t great enough. We’re bringing these populations into the laboratory so that I can collect their eggs,” said Spencer.
After storing the eggs for a simulated winter, he warms batches of eggs and uses the larvae that emerge in on-plant bioassays to evaluate a population’s susceptibility to the various Bt toxins that are expressed in Bt corn hybrids currently available to Illinois farmers.
Bt toxins are toxic crystal proteins originally isolated from various strains of the soil microbe Bacillus thuringiensis. When eaten by susceptible pests, they bind and damage the lining of their digestive system causing death. These crystal proteins, or cry proteins, have very specific activity against particular pests. When the genes that encode these proteins are engineered into corn hybrids, the plants express the Bt toxins and kill just the target pest.
New types of rootworm-resistant corn hybrids will be commercialized in a couple of years. They will incorporate a novel mechanism to disrupt critical rootworm cell functions and still express some Bt toxins. Spencer doesn’t wait to test how susceptible the WCR & NCR populations are to some of those new products (and current Bt toxins for which resistance is growing).
“I’m also interested to learn how corn rootworm population susceptibility is changing over time,” said Spencer. “I’ve been bioassaying Champaign County populations for six or seven years and seen dramatic changes over that period.”
According to Spencer, growing resistance to most Bt toxins hasn’t recently been a problem for local growers because rootworm populations have been low thanks to wet springs, late planting, and other problems that have kept WCR larvae well below abundance thresholds where their feeding can have a significant economic impact on corn yield.
“When there aren’t enough hungry larvae to damage corn roots, it doesn’t matter whether or not they are resistant to Bt toxins,” said Spencer.
According to Spencer, evidence of growing resistance to the last generally effective Bt toxin and an increasing WCR abundance suggests that Illinois’ period of good fortune may be coming to an end.
“The situation is troubling for a couple of reasons,” said Spencer. “It will be a few years before any new product (that corn rootworms have not been exposed to will be available). Farmers need to be careful about protecting the effectiveness of the currently available Bt corn hybrids by only using Bt when it is necessary.”
According to Spencer, the plan for long-term sustainability of the new products still includes some reliance on existing Bt toxins to kill pests. However, If none of those toxins have appreciable remaining efficacy in a couple of years, the new product will be under very heavy selection pressure which will speed the development of resistance to it. While various companies have new crop protectant traits in development, the timeframe for discovery, testing, approval and commercialization is long and very costly.
“There aren’t a lot of new traits or products waiting in the wings beyond what is coming soon, and these circumstances make it all the more important that we treat new technologies carefully and use them as one of several options in a long-term, integrated approach to pest management,” said Spencer.
The Illinois Natural History Survey (INHS) is a division of the Prairie Research Institute at the University of Illinois Urbana-Champaign.