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Posted: June 2014

“Leaf Litter from Insect-Resistant Transgenic Trees Causes Changes in Aquatic Insect Community Composition”

There are concerns that genetically modified trees could have long-term unexpected effects on the environment. This is because trees have such a long life span and influence the long-term structure of the forest ecosystem and strongly influence surrounding ecosystems.

In Bt trees that have been modified to produce toxins that kill certain insect larva, every part of the tree contains the Bt toxin. One concern is that the leaves that fall from the trees each autumn and find their way into streams, rivers, and lakes, could affect the community of insects that feed on dead leaves, an effect that in turn could have ripple effects throughout the aquatic ecosystem.

Axelsson et al. (2011) carried out a field experiment in Sweden to see whether leaf litter from Bt aspen trees differentially affected the aquatic insect communities that colonize leaf litter from genetically modified trees on the one hand, and unmodified trees on the other. They worked with two different lines of Bt aspens, one of which (Bt17) expressed more Bt toxin than the other (Bt27), and wildtype aspens. All (both the wildtype and Bt aspens) were originally cloned from the same parent plant and were propagated and grown in a greenhouse.

Leaf litter from the trees was collected, air dried, placed in small mesh bags, and submersed in a stream. The bags (a total of 99) were harvested after one, three, six and twelve weeks. After harvest, the chemistry of the leaves was analyzed, and insects from three different orders that are known to be important in leaf litter decomposition were collected and identified.

They found a total of 27 species of insects from these three orders in the leaf litter. “The insect communities on Wt [wild, unmodified aspens] were significantly different from both the Bt17 and Bt27 litter, but we did not find significant differences between the two Bt groups. The observed community differences are manifested in a 33% and 25% increase in average abundance on Bt17 and Bt27 litter packs, respectively.” Five of the species were found only on the unmodified leaves, while two of the species were found only in the Bt leaf litter. But since these species were only present in small numbers, the main difference between the GM and unmodified leaf litter was a shift in the respective abundance of individuals belonging to the more dominant species.

It was not at all clear to the researchers why this difference in insect communities (a difference they call “robust”) occurred between the genetically modified and unmodified leaf litter. The leaf chemistry analysis of the leaf litter revealed no significant differences nor were there other indicators that the decomposition of the leaves occurred differently. They could not account for the “large unexplained variation in the insect community data set.”

As the author’s remark, such a shift in the aquatic insect community due to leaf litter could have “ecological consequences that [are] not readily predictable.” The authors conclude,
Our findings highlight the importance of further study on the causes and effects of Bt-expressing plants in a variety of environments and suggest that forest management decisions that include insect-resistant trees may have unforeseen effect on nontarget ecosystems such as forest streams. This should be taken under consideration in future commercial applications of GM trees.

Source: Axelsson, E. P., J. Hjältén, C. J. Leroy et al. (2011). “Leaf Litter from Insect-Resistant Transgenic Trees Causes Changes in Aquatic Insect Community Composition,” Journal of Applied Ecology vol. 48, pp. 1472-9. doi:10.1111/j.1365-2664.2011.02046.x

Copyright 2014 The Nature Institute.

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