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New Scientist March 8 1997

 

RESISTANCE to the most important biopesticide now in use could evolve far faster than farmers had hoped. A new study of diamondback moths exposed to toxins made by the bacterium Bacillus thuringiensis suggests that widespread resistance could emerge in just a few years. The various strains of B. thuringiensis produce about a hundred Bt toxins. "But only a fraction of these wig kill any one pest, so pesticide-makers have a limited arsenal," says Bruce Tabashnik of the University of Arizona at Tucson, who led the research. "They know that resistant strains of insects will evolve, but they have been hoping that a separate genetic change would be needed to confer resistance to each toxin." In the diamondback moth, Plutella xylostella, a major pest of cabbages and other leafy crops, this does not seem to be the case. "Our work shows a single gene in resistant moths confers resistance to four different Bt toxins, and this gene appears to be far more common than anyone believed," says Tabashnik. While at the University of Hawaii, working with colleagues elsewhere in the US and in Canada, Tabashnik took moths from two groups-a field population that had been heavily sprayed with Bt toxins, and a lab population that had not been exposed to the toxins for at least 100 generations. In a series of experiments in cross-breeding, they found that the offspring of survivors of exposure to a single toxin were resistant to all four toxins. The results are published in the current issue of Proceedings of the National Academy of Sciences. Most alarmingly, the researchers found that 21 per cent of moths in the isolated lab population had the resistance gene. Geneticists had previously estimated that Bt resistance genes would be carried by as few as 1 in 10 000 insects. Jonathan Beard

Editorial New Scientist March 8 1997

IT IS always a tragedy when a gift from nature is squandered, whether it is a river dead from pollution or a forest laid to waste for timber. Let's hope that the biotechnology industry is not about to throw away the enormous potential of a simple bacterium, Bacillus thuringiensis, in its haste to get to market. Properly used, the bacterium could provide a wonderful way to control insect pests and help farmers to gain high yields without synthetic pesti- cides. Misused, however, and pests will grow resistant to it and its power will be gone for good. Overeager gene [email protected] are already rushing in where cautious ecologists fear to tread. The latest research suggests that it's time to slow them down (see p 5). B. thuringiensis is a soil bactenum with a remarkable ability to fight insects. If it gets inside the gut of an insect larva, it grows and multiplies, producing powerful crystalline toxins which eventually paralyse the gut and kill the insect. No one knows why the bac- terium does this, but it's proving very useful because different strains of the bacterium kill different insects while none of them harm other creatures. In fact, sprays containing the bacterium make excellent biocontrol agents and have become popular for their natural, environmentally friendly ability to control pests. Many organic farmers have come to rely on them. Not surprisingly, biotechnology companies have also long been excited about the potential of the bacterium. There has been a rush to iso- late the genes that produce the various bacterial toxins and to patent them as fast as possible. Arguments over who really owns what are already making the lawyers happy: in the US, at least six companies are involved in legal dis- putes over Bt technology. If they are placed into the genome of crop plants, the Bt gene should give them a natural immunity to insect pests. The toxin genes are currently under test in crops such as cotton, maize, tobacco, soya bean, potato and tomato and many appear promising. If all goes well, Bt gene crops should cut the need for chemical pesticides, protect the environment and increase crop yields. Unfortunately, all may not go well. In the rush to get products to market, the Bt gene may start popping up in crops everywhere. Insect pests will then be under enormous pressure to develop resistance to the toxins. So after a brief boom, everyone would have lost.

Biotechnologists have sought security in the numbers of different Bt genes. There are many varieties of the bacterium and the toxin it pro- duces. So they argue that even if a pest becomes resistant to one strain, it should be possible to control it with another. Unfortunately, that cosy thought has been undermined by researchers in Tucson who have found that in one pest-the dia- mondback moth-a single gene change can confer resistance to four different Bt toxins. That means the scope for rapid spread of resistance might be much higher than thought. If indiscriminate use of Bt crops rushes ahead, resistance may spread just as rapidly. Biotechnology companies do have ideas about how the spread of resistance might be slowed. Areas where Bt crops are grown could altemate with areas containing ordinary crops. if a Bt-resistant strain appears, it should breed with strains which are not resistant from the surrounding areas. The Bt-resistant strain should therefore not spread. All well and good. The trouble is we don't know nearly as much about managing resis- tant pests as we do about cloning genes. And we don't know how farmers will use the new varieties. First experiences with Bt crops aren't reas- suring. The agrichemicals giant Monsanto tried its first commercial planting of Bt cotton last year and many pests survived. If these tum out to be the more resistant insects and they go on to breed, then it is easy to see that farmers may have accidentally begun selecting for super- resistant strains. Several things must be done. We need more research on resistance and pest behaviour- and quickly. Without it, a few years from now and the potential benefits of Bt could be gone for ever. And also there must be tighter regulation. Currently, new biocontrol agents are examined much less closely than new pesticides. That's because we already know the dangers of pesti- cides. The impact of biocontrol agents needs to be thought through too. Numerous other biocontrol agents are wait- ing in the wings. Viruses, nematode worms and fungi are all being studied, as well as numerous kinds of toxins from plants. They have real potential to provide bigger crop yields without damaging the environment. Let's not waste what nature offers by giving the pests a head start.


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