RESISTANCE TO BT TOXIN SURPRISINGLY ABSENT FROM PESTS
September 2003
Nature Biotechnology, Vol. 21 No. 9 pp 958-959
Jeffrey L Fox
www.nature.com
Via AgBioView at www.agbioworld.org
Defying the expectations of scientists monitoring transgenic crops such as
corn and cotton that produce insecticidal proteins derived from
Bacillusthuringiensis (Bt), target insect pests have, according to US
Department of
Agriculture-funded scientists cited in this story, developed little or
no resistance to Bt crops thus far. The story says these findings suggest
that transgenic Bt crops could enjoy more extended, more profitable
commercial life cycles and that the measures established to mitigate
resistance before the crops
were introduced are paying off.
The diamondback moth is the only pest to have evolved resistance to Bt
sprays used by organic growers, but no pest has evolved resistance to
transgenic Bt crops in the field.
Entomologist Bruce Tabashnik of the University of Arizona (Tucson, AZ,
USA), whose research group recently completed a survey of this phenomenon
in collaboration with scientists from Cornell University (Geneva, NY, USA),
was quoted as saying, "If I'd gotten up seven years ago and said that there
would be no evidence of increased Bt resistance after Bt crops were planted
on 62 million
hectares [cumulative and worldwide], I would have been hooted off the
stage. No one predicted that there wouldn't even be a minor increase, which
is extraordinary."
Nor has Monsanto (St. Louis, MO, USA) seen any signs of resistance to
transgenic Bt crops, despite widespread use in a number of countries.
Graham Head, who is responsible for global coordination of insect
resistance management at Monsanto, agrees with Tabashnik's explanation of
these findings: "the use of refuges to manage resistance that tends to be
recessive and have fitness costs is a highly effective means of delaying
resistance," says Head
The primary resistance-preventive measure that farmers who plant transgenic
Bt crops are required to take is to set aside some acreage (see p. 1003) as
refuges on which they grow varieties of the same crop devoid of Bt. The
story says that carefully developed population genetics models indicated
that such Bt-free refuges would permit susceptible insects to survive and
swamp out resistant variants that might emerge from the pest population
feeding on Bt plants in nearby fields.