GMO Myths and Truths

5.4 Myth: GM Bt crops reduce insecticide use
Truth: GM Bt crops merely change the way in which
insecticides are used
GM proponents claim that GM Bt crops reduce
insecticide use, as farmers do not have to spray
chemical insecticides. But this claim does not
stand up to analysis, since the Bt gene turns
the plant itself into an insecticide and because
pest adaptation makes the GM pesticide
less effective over time, making it necessary
for farmers to revert to the use of chemical
pesticides after just a few years. The genetically
modified insecticide is present in active form in
every part of the crop, including the parts that
people and animals eat.
So Bt crops do not reduce or eliminate
insecticides. They temporarily change the
type of insecticide and the way in which it is
used – from sprayed on, to built in. But in the
long term, use of chemical pesticides must be
resumed, as long as the industrial agricultural
model is followed.
Even if we choose to ignore this factor and only
consider the temporary reduction in chemical
insecticide sprays due to Bt crops, the figure is
unspectacular (see 5.2, above) – a reduction of
64.2 million pounds (29.2 million kg) over the
first thirteen years of GM crop cultivation in the
United States. This reduction is swamped by the
massive increase in pesticide use resulting from
the adoption of GM herbicide-tolerant crops,
which has caused farmers to spray 383 million
more pounds (174 million kg) of herbicides than
they would have done in the absence of GM
herbicide-tolerant seeds (herbicides are technically
pesticides). 9
Even the modest reduction in chemical
insecticides attributed to GM Bt crops is proving
unsustainable, due to the emergence of pests
resistant to Bt toxin and secondary pests, as
explained below. Moreover, there is a question
mark over whether Bt crops can truly be said to
have reduced chemical insecticide use in view of
changes in the types of insecticides used and in
the methods of application.
5.4.1. Resistant pests are making Bt
technology redundant
GM Bt insecticidal crops express the Bt toxin
in every cell for their entire lifetime, constantly
exposing pests to the toxin. This is different from
the traditional use of natural Bt as a spray, where
the targeted pests are only exposed for a brief
period before the Bt breaks down in daylight.
Exposing pests to a pesticide for long periods
of time inevitably speeds up the emergence of
resistant pests, since selective pressure eliminates
all but the most resistant pests, which then
reproduce and pass on their genes.
For this reason, Bt crop technology sometimes
enjoys short-term success in controlling pests
but is soon undermined by the emergence of
pests resistant to the toxin. 43,44,45 By 2009, the
western corn rootworm had evolved resistance
to a Bt maize specifically engineered to target the
pest that was first commercialised only six years
previously. 46 Bt-resistant rootworm populations
have been reported in Iowa 46,47 and Illinois. 48
5.4.2. The “refuge” concept breaks
down
Farmers are encouraged to plant “refuges” of non-
Bt crops as a resistance management strategy to
delay the emergence of Bt-resistant pests. The
idea is that the non-Bt crop acts as a refuge where
Bt-susceptible pests can survive, ensuring the
existence of a population of Bt-sensitive pests
to mate with any Bt-resistant pests that survive
in the adjacent field where the Bt crop is under
cultivation. The theory is that the Bt-susceptible
pest population will dilute out the Bt-resistant
population that survives in the Bt crop, assuring
that the predominant population is Bt-susceptible.
But a study on rootworm resistance in Iowa
found that refuges were redundant in the case of
substantial Bt-resistant rootworm populations, as
the pests were able to live and reproduce in Bt maize
fields. The study concluded, “Even with resistance
GMO Myths and Truths 77