[PHNUTR-L] Insects develop resistance to engineered crops

[Kathrynne Holden, MS, RD]

Colleagues, the following is FYI and does not necessarily reflect my own
opinion. I have no further knowledge of the topic.
------------------------
Public release date: 17-Jun-2005
http://www.eurekalert.org/pub_releases/2005-06/cuns-idr061705.php

Contact: Blaine Friedlander
bpf2 at cornell.edu
607-254-8093
Cornell University News Service

Insects develop resistance to engineered crops

ITHACA, N.Y. -- Genetically modified crops containing two insecticidal 
proteins in a single plant efficiently kill insects. But when crops 
engineered with just one of those toxins grow nearby, insects may more 
rapidly develop resistance to all the insect-killing plants, report 
Cornell University researchers.

A soil bacterium called Bacillus thuringiensis (Bt), whose genes are 
inserted into crop plants, such as maize and cotton, creates these 
toxins that are deadly to insects but harmless to humans.

Bt crops were first commercialized in 1996, and scientists, critics and 
others have been concerned that widespread use of Bt crops would create 
conditions for insects to evolve and develop resistance to the toxins.

Until now, it has not been shown if neighboring plants producing a 
single Bt toxic protein might play a role in insect resistance to 
transgenic crops expressing two insecticidal proteins.

"Our findings suggest that concurrent use of single- and dual-gene Bt 
plants can put the dual-gene plants at risk if single-gene plants are 
deployed in the same area simultaneously," said Anthony Shelton, 
professor of entomology at Cornell's College of Agriculture and Life 
Sciences and an author of the study, which was posted online June 6 in 
the Proceedings of the National Academy of Sciences (PNAS) and is in the 
June 14 print edition of the journal. "Single-gene plants really 
function as a steppingstone in resistance of two-gene plants if the 
single gene plants contain one of the same Bt proteins as in the 
two-gene plant."

Cotton and maize are the only commercial crops engineered with Bt genes. 
In 2004 these crops were grown on more than 13 million hectares (about 
32 million acres) domestically and 22.4 million hectares (more than 55 
million acres) worldwide. After eight years of extensive use, there have 
been no reports of crop failure or insect resistance in the field to 
genetically modified Bt crops, Shelton said. Still, several insects have 
developed resistance to Bt toxins in the lab, and recently, cabbage 
loopers (a moth whose larvae feed on plants in the cabbage family) have 
shown resistance to Bt sprays in commercial greenhouses.

So far, only diamondback moths, which were used in this study, have 
developed resistance to Bt toxins in the field. The resistance resulted 
from farmers and gardeners spraying Bt toxin on plants for insect 
control, a long-standing practice. While Bt toxin sprayed on leaves 
quickly degrades in sunlight and often does not reach the insect, 
genetically modified (GM) Bt plants express the bacterium in the plant 
tissue, which makes Bt plants especially effective against insects that 
bore into stems, such as the European corn borer, which causes more than 
$1 billion in damage annually in the United States.

In greenhouses at the New York State Agriculture Experiment Station in 
Geneva, N.Y., the researchers used three types of GM broccoli plants: 
two types of plants each expressed a different Bt toxin, and a third -- 
known as a pyramided plant -- expressed both toxins. Elizabeth Earle and 
Jun Cao, co-authors of the PNAS paper and members of the Department of 
Plant Breeding and Genetics at Cornell created the plants.

For their studies, the researchers employed strains of diamondback moth 
that were resistant to each of the Bt proteins. The combination of Bt 
plants and Bt-resistant insects allowed them to explore the concurrent 
use of single- and dual-gene Bt plants in a way that could not be done 
with cotton or maize, although their results are relevant to these 
widely grown plants.

First, the researchers bred moth populations in which a low percent of 
the moths were resistant to a single Bt toxin. The insects were then 
released into caged growing areas with either single-gene plants, 
dual-gene plants or mixed populations and allowed to reproduce for two 
years.

The researchers found that after 26 generations of the insect living in 
the greenhouse with single-gene and dual-gene plants housed together, 
all the plants were eventually damaged by the insects, because over 
time, greater numbers of insects developed resistance to the plants' 
toxins. However, in the same two-year time frame, all or almost all of 
the insects died when exposed to pyramided plants alone.

"It's easier for an insect to develop resistance to a single toxin," 
said Shelton. "If an insect gets a jump on one toxin, then it becomes 
more rapidly resistant to that same toxin in a dual-gene plant. And when 
one line of defense starts to fail, it puts more pressure on the second 
toxin in a pyramided plant to control the insect," Shelton added.

While single-gene Bt plants are most prevalent, industry trends suggest 
that pyramided plants may be favored in the future. In Australia, the 
use of single-gene Bt cotton was discontinued two years after farmers 
began planting dual-gene cotton in 2002. In the United States, companies 
introduced dual-gene cotton in 2003, but single-gene varieties remain on 
the market.

"Single-gene Bt plants have provided good economic and environmental 
benefits, but from a resistance management standpoint they are inferior 
to dual-gene plants. U.S. regulatory agencies should consider 
discontinuing the use of those single-gene plants as soon as dual-gene 
plants become available," Shelton said. "And industries should be 
encouraged to create more dual-gene plants."

Along with effective insect control, pyramided plants have an added 
advantage of requiring a smaller refuge -- a part of the field where 
non-Bt plants are grown. Refuges create opportunities for Bt-resistant 
insects to mate with other insects that do not have resistance. The 
offspring of such a mating will be susceptible to the toxins.

"Having a refuge is a good management strategy, but it is not suitable 
for small farmers in China and India," said lead author Jian-Zhou Zhao, 
a senior research associate in entomology at Cornell. "The two-gene 
strategy is more suitable in developing countries like China where 
farmers have an average of half a hectare (1.2 acres) of land, much less 
land than American farmers, and not enough to spare for refuges."
###
A U.S. Department of Agriculture Biotechnology Risk Assessment Program 
grant supported the study.
-- 
Kathrynne Holden, MS, RD < fivestar at nutritionucanlivewith.com >
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