Posted: June 2014
Insects feed less on insect-resistant aspen leaves, but the trees don’t
grow larger than unmodified aspens
Biotechnologists have genetically modified aspen trees to produce a toxin
(Bt toxin, originally isolated from the bacterium Bacillus
thuringiensis) that should kill certain insects when they feed on any
part of the tree. Axelsson et al. did an experiment under “semi-natural
conditions” to see whether Bt aspen trees do in fact fare better
than unmodified aspens. They worked with two different lines of Bt
aspens — one of which (Bt17) expressed more Bt toxin than the other
(Bt27) — as well as wildtype aspens. Both the wildtype and Bt
aspens were originally cloned from the same parent plant. After initial
propagation in a greenhouse, the trees were transferred into pots that were
placed outdoors (in Sweden). The researchers observed and analyzed insect
predation on the leaves of the trees. In addition, half the trees of all
types were given a high level of fertilizer while the other half received
a low level of fertilizer.
As expected, the Bt aspens that produced higher levels of the toxin
(Bt17) suffered significantly less leaf damage than did the unmodified
aspens. There was little difference between leaf damage on the unmodified
aspens and the Bt aspens expressing low amounts of the toxin
(Bt27). Overall leaf damage was not extensive. Whether the trees had been
given high or low amounts of fertilizer had no affect on the degree of
Although one might assume that with less leaf damage the Bt aspens
would grow faster and/or larger (greater productivity), this was not the
case: “Although the Bt-modification did decrease leaf damage ...
biomass production was not enhanced in either of the two insect-resistant
lines.” The unmodified aspens grew just as well even though they
experienced more leaf damage.
Surprisingly, one insect, the leaf rolling weevil (Byctiscus
populi), which belongs to an insect order that is “presumably targeted
by these insect-resistant lines,” used in a similar way both the
insect-resistant leaves and the unmodified leaves to deposit eggs and for
feeding. This insect can be a serious pest in commercial aspen
plantations. Evidently, it is not a given that the insect-resistant GM
trees will protect the trees from all the targeted pests.
Axelsson, E. P., J. Hjältén, C. J. Leroy et al. (2011). “Performance of
Insect-resistant Bacillus thuringiensis (Bt)-expressing
Aspens under Semi-natural Field Conditions Including Herbivory in Sweden,”
Forest Ecology and Management vol. 264, pp. 161-71.
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