Bt Cotton Raises More Concerns About Genetic Engineering

Cotton farmers in Arizona have added a  potent  weapon  to  their
arsenal against an old enemy, the pink bollworm -­ a plant called
Bt  cotton  that  is  genetically  engineered  to produce its own
natural insecticide.  

But in the Aug.   5  Nature,  scientists  at  the  University  of
Arizona  in  Tucson raise concerns about the possible pitfalls of
using Bt cotton.  And those concerns, in turn, lead to some basic
worries about the safety of genetic engineering.  

Its "good news, bad news," according to Bruce Tabashnik,  head of
the UA entomology department and coauthor of the Nature paper.  

The article,  "Development time and resistance to Bt crops," with
UA assistant research scientist Yong-Biao  Liu  as  lead  author,
concerns  the  use  of  plant  "refuges"  to  help  delay  insect
resistance to the toxin in Bt cotton.  Scientists  worldwide  are
scrambling  to  learn  how  to  use this powerful technology most
effectively.  

To control pink bollworm  and  certain  other  cotton  pests,  Bt
cotton   contains  a  gene  transferred  from  the  bacterium  Bt
(Bacillus thuringiensis) that lets plants produce a safe, natural
insecticide.  When certain moth larvae feed on these plants, they
die.  

Because the toxin is lethal to caterpillars, but harmless to most
other organisms,  it is much safer for humans and the environment
than traditional insecticides.  

"Bt  cotton  is  working  phenomenally  well  in  Arizona,"  says
coauthor  Tim  Dennehy,   leader  of  the   Extension   Arthropod
Resistance Management Laboratory at the UA.  

The hitch is that caterpillars might quickly evolve resistance to
the  toxin  in  Bt  cotton,  thus negating its effectiveness.  To
delay pest adaptation to Bt cotton,  refuges of  ordinary  non-Bt
cotton  are  grown  to  yield  susceptible  moths  to  mate  with
resistant moths and dilute their resistance genes.  

"The bad news is that scientists  from  the  UA  and  the  United
States  Department  of  Agriculture  readily selected a strain of
pink bollworm in the laboratory  that  survives  on  Bt  cotton,"
Tabashnik said.  

"Further, resistant caterpillars develop more slowly on Bt cotton
than   susceptible   caterpillars  do  on  non-Bt  cotton.   This
developmental delay could quicken the evolution of resistance  to
Bt  by  increasing the chances of resistant moths mating with one
another,   rather  than  with  susceptible  moths  from  refuges.
However,  in  the field,  higher mortality associated with slower
growth of resistant caterpillars could  diminish  such  potential
negative effects." 

The  good news is that offspring produced by matings of resistant
and susceptible parents are killed by Bt cotton.   Also  positive
is  the  finding  that  even  the  resistant  strain suffered 63%
mortality on Bt cotton.  

"Refuges remain vital.  Our findings will help refine  strategies
for keeping this valuable technology effective," Tabashnik said.  

This  research was supported by the University of Arizona,  USDA,
Arizona Cotton  Growers  Association,  Cotton  Incorporated,  The
Cotton Foundation and Monsanto.  

[Contact: Bruce Tabashnik] 

06-Aug-1999 

http://unisci.com/stories/19993/0806994.htm