Genetically modified crops that produce insect-killing proteinsfrom the bacterium Bacillus thuringiensis (Bt) have reduced reliance on insecticide sprays since 1996. Theseproteins are lethal to some devastating crop pests, but do not harmmost other creatures including humans. Yet, just as insects become resistant to conventional insecticides,they also can evolve resistance to the Bt proteins in transgeniccrops. To delay pest resistance to Bt proteins, the U.S.
EnvironmentalProtection Agency, or EPA, has required farmers to plant”refuges” of crops that do not produce Bt proteins nearBt crops. Refuges are planted with standard, non-Bt crops thatpests can eat without ingesting Bt toxins. Planting refuges promotes survival of susceptible pests. Ifsusceptible pests greatly outnumber resistant pests, resistantindividuals are unlikely to mate with each other and produceresistant offspring. But how much refuge acreage is enough? In an article appearing in the June 2012 issue of the Journal of Economic Entomology , authors Bruce Tabashnik from the University of Arizona and FredGould from North Carolina State University conclude the EPA shouldmore than double the percentage of corn acres planted to mandatedrefuges to delay insect resistance, encourage integrated pestmanagement, or IPM, and promote more sustainable crop protection.
To slow resistance in the western corn rootworm ( Diabrotica virgifera virgifera ), a beetle that is one of the most economically important croppests in the U.S., the EPA currently requires 20 percent of thetotal acreage being set aside as refuges for corn producing one Btprotein (Cry3Bb1), and a 5 percent refuge portion for corn thatsimultaneously produces two different Bt proteins. However, the authors note that this adaptable pest has rapidlyevolved resistance to Cry3Bb1 in some areas of the U.S. Corn Belt.For Bt corn to remain effective against rootworms, they recommendincreasing refuge requirements to 50 percent for corn producing oneBt protein and 20 percent for corn producing two Bt proteins. “Corn rootworms can cost U.S. farmers close to $1 billion eachyear. ABL Laminated Tube
Bt corn has helped to reduce these costs and to decreaseinsecticide sprays, but evolution of resistance by the pests candiminish or even eliminate these benefits,” said Tabashnik,who heads the department of entomology in the UA College ofAgriculture and Life Sciences. “To delay pest resistance and sustain the benefits of Bt corn,we recommend planting more corn that does not produce Bt toxinsactive against rootworms. This refuge strategy allows thesusceptible pests to survive and has worked to slow resistance ofother pests to Bt crops.” “Most of the corn seed currently produced in the U.S. istransgenic and includes genes for insect control,” said Gould.”Enlarging refuges will require more seed without cornrootworm control genes. Cosmetic Plastic Packaging
This shift in production will take time, sothis process should begin immediately.” In addition to increased refuge sizes, the authors write that thebest way to postpone resistance is to use IPM, in which Bt corn iscombined with other control tactics such as crop rotation andjudicious use of insecticide sprays. “We advocate greater use of integrated pest management, whichis a common sense approach based on the best available combinationof tactics,” Tabashnik said. “The goals are to limit pestdamage, maximize farmer profits and preserve environmental quality.Maintaining the effectiveness of Bt toxins can help us achievethese goals.” “We’re seeing the early signs of rootworm resistance to Btcorn, which fit predictions from evolutionary theory andexperiments in the lab and greenhouse,” he added. The paper indicates rootworm resistance to Bt corn was firstdetected in 2009 in Iowa; six years after sales of rootworm-killingBt corn began in the U.S. Pharmaceutical Flexible Packaging
and only one year after this type of Btcorn was first planted on more than 25 million acres. According to Tabashnik, Cry3Bb1 is effective enough to beeconomically useful, but not effective enough to meet the so-calledhigh dose standard, the ability to kill at least 99.99 percent ofsusceptible pests and also nearly all of the hybrid pests that areproduced when resistant pests mate with susceptible pests. “When Bt crops meet the high dose standard, resistantindividuals are extremely rare, and smaller refuges work fine,because you might have one resistant insect in a million. In thiscase, a 20 percent refuge provides enough susceptible individualsto dilute that rare resistance.” But plants with Cry3Bb1 allow survival of 1 to 6 percent of pests,which is expected to quickly select for resistance. “A single farm can have millions of these beetles,”Tabashnik explained.
“If 1 to 6 percent survive on Bt corn,you have tens of thousands of potentially resistant insects and therefuge needs to be much bigger.” Tabashnik’s research has shown that in Arizona, Bt cotton meets thehigh dose standard against pink bollworm and the small refugestrategy has prevented resistance for more than a decade. On the other hand, Tabashnik pointed to a case in Puerto Rico,where adequate refuges were not planted. Within a few years, thepests evolved resistance and devoured the Bt corn plants. Thebiotechnology companies voluntary stopped selling Bt corn seedthere, but five years later, the insects remain resistant to thetoxin. Although biotech companies recently starting selling some varietiesof Bt corn that produce combinations of Bt toxins, Tabashnik said,the resistance to one toxin still raises concerns.
“You can think of the multi-Bt toxin approach as a pyramid:The base has to be stable. If one of your building blocks, which issusceptibility to Cry3Bb1, is crumbling, you have a problem.Resistance to any one toxin jeopardizes the effectiveness of thewhole system.” “We’re at a tipping point where decisive action can providelong-term benefits and avoid loss of an environmentally friendlytool for pest control.” Et Cetera Extra Info Reference: “Delaying Corn Rootworm Resistance to Bt Corn,” Journalof Economic Entomology, published by the Entomological Society ofAmerica.