London (20.5.10) –
Every technology needs proper and effective management. A car is a fine invention,
a joy and a pleasure to its owner. But unless that owner knows how to drive
it and to do so safely, he will most probably wreck the car and quite possibly
kill someone else in the process. With aeroplanes it can be even worse. Moreover,
while it is often possible to work out before use with a new technology many
important aspects of management it is inevitably the case that others become
clear with use, particularly if the circumstances of use themselves change with
time: once there was an occasional car travelling at maybe twenty miles an hour;
now there are millions, many doing over seventy.
So it is with GM crops. The earliest designs, insect resistance using Bt and
herbicide tolerance, were intended to deal specifically with two types of problem:
the ability of a plant to resist specific insect attack without the use of external
insecticides and a method for controlling weeds by making the crops themselves
resistant (tolerant) to a specific herbicide while the weeds remained susceptible.
There were recognised management (stewardship) procedures recommended or demanded
from the outset. For insect resistance it was recognised that the possibility,
perhaps even the certainty, existed that by chance variation and selection on
purely Darwinian lines resistant insects would arise sooner or later; in the
absence of further controls they would flourish in the ecological niche left
vacant by the disappearance of the susceptible pests the crop had been designed
to resist. It was accordingly recommended that cultivation procedures should
include refugia of non-insect resistant plants in which insects attacking the
plants would be subject to no evolutionary pressure to acquire resistance to
Bt; if the refugia were a sufficiently large proportion of the total plantings,
the chances were that any rare resistant insect arising among the Bt plants
would almost certainly mate with one of the many susceptibles from the refugia
rather than with a very rate resistant mate. Thus, the resistance property would
be lost in the next generation.
It worked. Last year, after some 15 years of Bt-maize and -cotton cultivation
on an ever increasing acreage worldwide, a paper by Bruce Tabashnik and others
(1) concluded that existing theories and strategies can be used to predict,
monitor, and manage insect resistance to Bt crops. Field outcomes are consistent
with predictions from theory, suggesting that factors delaying resistance include
recessive inheritance of resistance, abundant refuges of non-Bt host plants,
and two-toxin Bt crops deployed separately from one-toxin Bt crops.
Nevertheless, the unexpected can happen. A ten-year study in China, where 4
million hectares of Bt-cotton are now grown, has found that the population of
mirid bugs (insects of the Miridae family), previously only minor pests
in northern China, have increased 12-fold since 1997. The mirids are now a main
pest in the region; their rise in abundance being associated with the scale
of Bt cotton cultivation (2, 3, 4). Kongming Wu, an entomologist at the Chinese
Academy of Agricultural Sciences in Beijing, suspects that mirid populations
increased because less broad-spectrum pesticide was used following the introduction
of Bt cotton. "Mirids are not susceptible to the Bt toxin, so they started
to thrive when farmers used less pesticide," says Wu.
The mirid problem appears less severe in cotton than the depredations of the
lepidopteran pests which are controlled by Bt, needing perhaps five treatments
per season and often only one or two. Nor is the mirid problem limited to Bt
crops but further management procedures will need to be developed to deal with
a situation not originally foreseen. As elsewhere in human society, experience
is a major factor in refining management practice
A parallel situation arises with regard to herbicide resistant weeds which,
in some soya-growing areas, are becoming an embarrassment. Biological systems
are very opportunistic; if something can happen, sooner or later it probably
will. Should a weed by whatever mechanism (gene transfer or endogenous gene
mutation) acquire resistance to a herbicide such as glyphosate, the one most
used with GM herbicide-tolerant crops, it will be able to grow uncontrolled
in the vacant ground cleared of weeds by the action of that herbicide (5). The
reasons for these resistant weeds is fundamentally due to biological opportunism
but may also be assisted by inadequate rotations on the part of farmers. Planting
the same crop year after year (as has been the case of GM glyphosate-resistant
soya in Argentina because it has been economically so successful) means that
the same weeds are present each year with a greater risk of resistance arising
among them Another factor is a failure promptly to remove any resistant weeds
as soon as they show up but that does mean increased effort or modification
and improvement of agronomic practices (6).
The most effective way is likely to be the stacking with each plant of two or
even three genes, each specifying resistance to a different herbicide, coupled
with the use of those two or more herbicides to control the weeds. The likelihood
of a weed becoming resistant simultaneously to two or three herbicides is very,
very much less than the chance of acquiring resistance to a single chemical.
Such multiple stacked crops are now beginning to come onto the market although
they, in turn, give rise to other problems: the procrastination, particularly
in the European Union, associated with approving the stacked varieties and the
refusal to import a product which contains even the slightest measurable trace
of them until they have been formally approved.
That, of course, is another sort of management problem.
Sources:
1. 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.)
2. Yanhui Lu, Kongming Wu, Yuying Jiang, Bing Xia, Ping Li, Hongqiang Feng,
Kris A. G. Wyckhuys, Yuyuan Guo (13.5.10). Mirid bug outbreaks in multiple
crops correlated with wide-scale adoption of Bt cotton in China. Science
(http://www.sciencemag.org/cgi/content/abstract/science.1187881)
