"So far we have found no resistant organisms."
Bt maize produces a substance which is highly effective in controlling its main pest, the European corn borer . Plant pests, however, are adaptable. There are concerns that resistant European corn borers could rapidly increase in areas where Bt maize is extensively grown, because they have a survival advantage. A research project at the Federal Biological Research Centre (BBA) in Darmstadt is working on methods to monitor the development of resistance in the European corn borer. The aim is to detect resistant corn borers in good time, so that countermeasures can be taken. GMO Safety spoke to Gustav Adolf Langenbruch from the Institute for Biological Control at the BBA.
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GMO Safety: Dr. Langenbruch, according to a study which has just been published in the USA, not a single resistant pest has been found there to date, despite the fact that Bt plants have been grown in the USA for several years. Many experts have found this rather surprising. Like your American colleagues, here in Germany you are looking for European corn borers which have developed resistance to Bt maize. Can you outline your findings so far?
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Langenbruch: So far we too have found no resistant organisms. The descendants of larvae found on Bt maize plants last year are currently being tested in our laboratory. Some of the F2 generation have indeed survived the biotest with the Bt toxin , but in the subsequent generation most of the insects then died. We have observed that the surviving insects scarcely produce any eggs. This is a direct side effect of absorbing the Bt toxin.
However, we are having unexpected difficulties finding resistant organisms, because in the Bt maize fields there are more plants than originally anticipated which produce no or only very small quantities of Bt toxin in their cells. This means that we are finding a number of live larvae in the Bt maize field, but they are not necessarily resistant. We test all insects where possible, which is of course very labour-intensive.
GMO Safety: Are there any other strategies for finding resistant organisms and studying their resistance mechanisms?
Langenbruch: We have also attempted to provoke a resistance to this toxin in the laboratory by continually feeding Bt maize or the Bt toxin it contains. But it is not that simple. Dosing the insects with the Bt toxin puts them under enormous stress. Under such conditions pathogens can then strike, such as microsporidia. These are micro-organisms which attack many of the caterpillar’s organs. They multiply rapidly and weaken the caterpillars, eventually killing them. In some cases microsporidia destroy entire laboratory cultures in this way.
Even if it were possible to obtain resistant organisms in laboratory cultures, this method would be of only limited use, because the resistances created in the laboratory may not be the same as those which are relevant in the field.
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GMO Safety: What is the significance of the findings to date for future work in this area? Isn’t the fact that no resistant organisms have been found something of an all-clear signal? Langenbruch: I was also surprised by the American findings, actually, but resistant organisms can still surface at any time. We must carry on looking. There is one thing that we should not lose sight of here: The European corn borer originated in Central Europe and was introduced to the USA before 1920. The population there can be traced back to relatively few individuals. So it is possible that resistance genes could be present in the European populations, which never in fact made it to North America. This is why many scientists in the USA stress the importance of looking to Europe to observe the true potential for the development of resistance. GMO Safety: How common could resistance genes be in the European corn borer? Langenbruch: So far we have not found a single insect with resistance genes. However, using statistical methods we can calculate the maximum rate of occurrence of such resistance genes. On the basis of our data, the probability of occurrence of a single resistance allele is less than 6.9 x 10-2. However, the prerequisite for preventing resistance using the high dose/refuges strategy is a probability of less than 1 x 10–3. This means that we have not yet been able to demonstrate that this precondition exist in Germany. GMO Safety: What would happen if you were to find the first resistant insect in the field? Langenbruch: Well, that would be a warning shot, but first we would have to find out what type of resistance we had found. If it were a dominant resistance, then all insects with just a single resistance allele would be able to colonize the Bt maize fields. But it’s much more likely that it would be a recessive resistance gene. In that case only insects carrying both resistance alleles would survive. If the resistance alleles were quite scarce in the population, the refuges/high-dose strategy would definitely provide an opportunity to considerably inhibit the spread of resistance, at least. But one resistance is not necessarily the same as another. From experiments in the laboratory we know that there can be huge differences. There are cases of “low” resistance, where insects can tolerate about 10 times the amount of Bt toxin compared with other members of the same species, whereas others can survive 10,000 times the amount. And there are also cases where populations with high resistance have developed within a short space of time from populations with low resistance. Another factor can also play a key role in predicting the spread of the resistance in question: the general fitness of these insects. The insect often pays a price for resistance, because it is associated with negative characteristics. For example the rate of reproduction could be reduced. So each case has to be examined individually to work out the probability of a specific resistance spreading. |
GMO Safety: Where in general do you see weaknesses in current procedures for slowing down or completely preventing the spread of resistances to Bt plants?
Langenbruch: Farmers in the USA who grow Bt crops must plant at least 20 percent of their acreage with conventional crops, which they are not allowed to treat with insecticides. These areas represent the refuges stipulated in statutory resistance management. But who ensures that these regulations are observed? According to what one hears from the USA, many farmers are in practice very slack. The task of monitoring compliance with these regulations should not lie solely with the plant manufacturers. Bacillus thuringiensis (Bt) is a highly significant and extremely valuable natural resource for pest control, which should under no circumstances be jeopardised.
GMO Safety: Could a resistance to a specific Bt plant affect plant protection measures in other plants?
Langenbruch: The answer is yes, especially with organic farming. For example, in the USA the European corn borer is also to some extent a potato pest. If the European corn borer became resistant to Bt maize, then in certain arable areas this resistance would also affect organic potato crops.
Cabbage pests are another example. Different species of cabbage have the same pests, including cabbage white butterflies, diamond-black moths and cabbage moths. If these pests developed a resistance to a Bt cabbage variety, there would be serious implications for pest control in all varieties of organic cabbage. It’s important to realise that organic farming is heavily dependant on Bt pesticides. There are hardly any good alternative methods of control for organic growers. Therefore when using Bt technology it is important not to lose sight of the cross combination of pests and crop species.
GMO Safety: How do you see the future for Bt technology?
Langenbruch: Based on what has been said so far, I advocate a cautious approach to Bt technology. One should not indiscriminately transfer the results of research into one transgenic strain of a plant species to other transgenic strains of the same plant species or even to transgenic strains of other plant species. With the Bt maize grown in Germany today, which produces the Cry 1Ab toxin, the potential problems are still manageable. Of the two biological methods of control available for the European corn borer, the Bt pesticide is the one that would be affected by any such resistance in the corn borer. It would probably become less effective in controlling these European corn borers. The use of the parasitic wasp, trichogramma, which is currently employed on 10,000 hectares in Germany, would not be at risk. Even if this method continues to improve, it already achieves an efficiency factor of 70-80 percent, especially in weaker populations. In this respect, in some cases it only marginally inferior to chemical pesticides. It is certainly more expensive, but the state is so serious about environmental protection that it will make the necessary financial adjustments.
But allow me to pursue another line of reasoning for a moment: When controlling the European corn borer, we should not focus solely on Bt maize. There are a range of methods, including soil conditioning, harvesting methods and crop rotation, which can keep this pest under control to a certain extent.
There is one option for the use of Bt maize that I think is worth considering. I personally believe that Bt maize is the best weapon we have against the European corn borer. This transgenic plant is 99.9 percent efficient in controlling the pest. We know that the pest is spreading further and further north; it has already reached North Rhine-Westphalia and Saxony-Anhalt. So I imagine that growing a continuous belt of Bt maize at the limit of migration of this pest would at least considerably slow down its advance. This could provide long-term protection for large areas of maize cultivation like Münsterland and parts of Lower Saxony. It could work.
GMO Safety: Thank you for talking to us, Dr. Langenbruch.
GMO Safety: More on this topic
- SiFo Project: Investigations into the early detection of the development of resistance in the European corn borer and understanding the resistance mechanisms
- Science live: Wanted: Resistant European corn borers!
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