Feb 12, 2008

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Research on the development of resistance in six pests

Cotton pest develops resistance to Bt plants: “evolution in action”

For more than ten years, millions of hectares of genetically modified maize and cotton plants have been planted worldwide. Apparently for the first time, pest insects in nature now have developed resistance to the Bt toxin formed in such plants: cotton bollworms found between 2003 and 2006 in the USA display significantly lower levels of sensitivity to the toxin as in previous years. With regard to all other cotton and maize pests under investigation, the Bt toxin proved to be unchanged in its effectiveness.

Bruce E. Tabashnik, a renowned insect researcher at the University of Arizona in Tucson, has addressed the question of whether the Bt toxin applied in maize and cotton plants may lead to the evolution of resistance by the relevant “target organisms” - in this case, a variety of chewing insects. It is no surprise that, after a while, pests can develop biological strategies against insecticidal agents and become thereby insensitive: as a rule, even advantages that have been established in a plant by conventional breeding methods only have a limited time span of effectiveness.

Resistance in bollworms, sensitivity in other pests remains unchanged

Since 1996, Bt maize and Bt cotton have been grown worldwide on a large scale. In some regions, they enjoy virtual exclusivity. Experts such as Tabashnik had expressed surprise at the lack of evolution of resistance in pest populations since the genetically modified cotton and corn were grown commercially starting in 1996. However, pests now have been found in nature that indeed have envolved resistance to the Bt toxin present in genetically modified cotton plants.

A group of scientists associated with Tabashnik evaluated studies from Australia, China, Spain and the USA that were concerned with the evolution of resistance to Bt toxins by six important insect pests. The results have been published in the current issue of Nature Biotechnology (February 2008).

• In the case of one insect, the cotton bollworm (Helicoverpa zea), a resistance could be proven to have arisen in nature according to the research article. Such insects were found in cotton fields in the southeast of the USA.
• With regard to other pests, sensitivity to the Bt toxin was proven to be almost fully unchanged. Insects under investigation included other cotton pests such as the pink bollworm (Pectinophora gossypiella), which is the most significant pest in the southwest of the USA, as well as the corn borer (Ostrinia nubialis), which is widespread in Europe and threatens maize growers.

Tabashnik and his colleagues cited extensive information from two studies that were published in 1999 and 2006. Insect samples were collected in fields. Subsequently in the laboratory, the concentrations of Bt toxins necessary to kill one half of the pests were investigated. Analysis of the resulting data indicated that, in order to be killed, bollworms collected from some sites in the US states of Arkansas and Mississippi in 2003 and 2004 needed to consume significantly more Bt toxin than did insects that were collected between 1992 and 1993, i.e., before the market entry of Bt cotton.

“The laboratory research substantiates the first case of resistance to the Bt toxin of a transgenic plant having arisen in nature. What we are able here to observe is evolution in action,” declared Tabashnik. However, such resistance has been noted in only one pest - and also only in one geographical region of the USA. According to Tabashnik, “Our results contradict the worst-case scenarios of some experts under which resistance to Bt plants was expected in three years.”

No resistance yet in the European corn borer

The single genetically modified plant that commercially is cultivated in Germany is Bt maize. The results observed in research on the cotton bollworm cannot simply be extrapolated to the European corn borer and this pest has been monitored for years with regard to the possible evolution of resistance. Between 2001 and 2003, a working group of the Institute for Biological Plant Protection of the BBA in Darmstadt (the Federal Biological Institute for Agriculture and Forestry, since 2008 known as the Julius Kühn Institure, JKI) investigated corn borer populations in fields of Bt maize. This research was executed within the framework of safety research sponsored by the Federal Ministry of Education and Research (BMBF) and found no resistant insects in the cultivation area of Oderbruch. As already was the case in a similar study in 2003, Tabashnik also was unable to discover a single resistant European corn borer.

Effective resistance management

For the cultivation of Bt plants in the USA and Canada, measures are prescribed that are aimed at preventing the evolution of resistant pest populations. For example, “refuge areas” should be established that are planted with conventional varieties. In these areas, pest insects with unchanged sensitivity to Bt toxins should be allowed to multiply and thereby possibly to mate with individuals that have developed resistance. The resulting generation of insects would again be sensitive against the Bt toxin. Tabashnik’s results have shown that bollworm resistance to Bt cotton evolved most quickly in areas where the proportion of refuges was lowest. Furthermore, the results suggest Bt plants should contain the Bt toxin in quantities which exceed by far the quantities necessary for killing of non-resistant insects. The eradication of even moderately resistant pests is ensured thereby (this combination is known as the high dose/refuge strategy).

Another strategy towards the prevention of resistance is the cultivation of plants that contain several different Bt agents within each plant. Even when pests evolve resistance towards one agent, another agent will remain effective nonetheless. The evolution of resistance towards two agents simultaneously is extremely unlikely.