London (4.1.10)
– Just as with the development of resistance to weed-killers (http://www.cropgen.org/article_285.html),
so insects can become resistant to insecticides.
This possibility was well recognised when-GM insect-resistant cotton and maize
were first developed by inserting one (or more) of a group of genes from the
soil bacterium Bacillus thuringensis coding for certain proteins
which are very toxic to the caterpillars of lepidopteron insects. The offending
insects included the European corn-borer, sometimes causing extensive damage
to maize, and the boll-weevil which has ravaged cotton plantation. The origin
of the genes in B. thuringensis has given rise to the term “Bt-“
to describe plants made resistant to insects in this way.
Sooner or later, it was reasoned, Bt-resistant insects might arise by chance
and proceed to multiply, encouraged the absence of their sensitive relatives
which had been killed off by the toxin; a totally resistant population would
ultimately have arisen. A management procedure was therefore devised based
on “refugia”, relatively small areas of the original non-Bt-variety
of the crop sown near to the Bt-plants. The refugia would serve as sources
of significant susceptible insect populations; these, it was reasoned, would
mate with any resistant mutants which might arise among the Bt-plants not
far away. So long as resistance was a recessive genetic characteristic, mating
of a resistant mutant with an original sensitive individual would ensure that
their progeny remained sensitive. In this way, the likelihood of a resistant
population developing was much diminished though not totally abolished: essentially
nothing in biology is 100% certain. Readers interested in learning more will
find a very readable discussion in Federoff and Brown (1).
The standard procedure, obligatory for farmers using Bt-crops, has been to
devote 20-50% of the sown area for the particular crop to the non-GM and hence
insect-susceptible form; the percentage depends on expected the severity of
infection. The refugia can take one of several formats (whole fields, blocks
within fields, or several rows within the field); if whole fields, they must
be within half a mile (0.8 km) of a Bt-field
This management process, in train since the mid-1990s, has indeed been extremely
successful. Reports of any resistant insects are sparse and, so far at any
rate, have not become more than minor local problems. The farmers do, of course,
suffer damage among the non-Bt crops but the overall benefit for the benefit
provided by Bt is large enough for them to be willing to accept that. Farmers
are allowed to control the insects in the refugia, for example, by spraying
but not by in-plant techniques (like Bt) which might generated resistant forms;
Bt-based sprays are also to be avoided. Most farmers growing Bt-crops, at
least in well-policed countries, have adhered to the management procedures,
with the numbers failing to do so declining year on year.
Indeed, so successful has been the refugia regime that there is now a move
to reduce the required area from 20% to 5%, at least for Monsanto’s
new SmartStax maize (2), expected to be ready for planting in 2010. The company
is estimating a yield benefit of 4-10% on the farm and is planning to have
ready enough SmartStax seeds for 4 million acres, so farmers should be able
to run their own trials on this new seed during the 2010 season. SmartStax
contains several traits to protect against above-ground insects as well as
below-ground pests and also provide broad herbicide tolerance for weed control.
Now available for maize, cotton, soybean and other crops are likely to follow.
Papers from the Entomological Society of America (3, 4) look at the issue
in more detail. Bruce Tabashnik, the senior author, is quoted as saying: “Resistance
is not something to be afraid of, but something that we expect and can manage
if we understand it. Dozens of studies monitoring how pests have responded
to Bt crops have created a treasure trove of data showing that resistance
has emerged in a few pest populations, but not in most others. By systematically
analyzing the extensive data, we can learn what accelerates resistance and
what delays it. With this knowledge, we can more effectively predict and thwart
pest resistance.”
The authors conclude that the refugia strategy can slow the onset of resistance
by, as expected, reducing the chance of insect mating allowing spontaneous
resistant forms to pass on that ability to the next generation. Crops containing
two or more Bt-toxins, as in the case of SmartStax, as more effective at controlling
insect resistance. They go on to note that, nearly 15 years after Bt-crops
were first introduced on a commercial scale, most insect pest populations
remain susceptible although a few cases of resistance have indeed shown up
in the field.
Sources:
1. Nina Federoff and Nancy Marie Brown (2004). Mendel in the kitchen.
Joseph Henry Press, Washington DC (see pages 209-221)
2. Corn refuge acres decrease from 20 to 5 percent. Seeking Alpha
(9.12.09) (http://seekingalpha.com/instablog/362794-t-marc-schober/39158-monsanto-corn-refuge-acres-decrease-from-20-to-5-percent)
3. Insect resistance to Bt crops can be predicted, monitored, and managed.
Entomological Society of America (2009) (http://www.entsoc.org/resources/press_releases/2009_btcrops.htm)
4. Bruce E. Tabashnik, J.B.J. Van Rensburg and Yves Carrière (2009).
Field-evolved insect resistance to Bt crops: definition, theory, and data.
Journal of Economic Entomology, 102(6): 2011-2025 (http://www.entsoc.org/btcrops.pdf)
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