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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 leaf damage.

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.

Source: 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. doi:10.1016/j.foreco.2011.10.006

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