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Development of resistance in the European corn borer and Western corn rootworm

Bt plants: Good resistance management means no resistant pests

To the astonishment of many practitioners, the Bt concept is still effective 15 years after it was first commercialised. So far, hardly any resistant pests have been found. The resistance management strategy practised until now has proved successful and resistant pests are found only where it is not practised systematically. This is the finding of a recent study by American ecologists. However, unlike with caterpillars, such as those of the European corn borer, there are some indications that the Western corn rootworm, a beetle, may have adapted much faster than expected to the Bt toxin that targets it.

European corn borer larva in a maize cob.  Sometimes the corn borer larvae even eat their way into the cobs.

European corn borer (Ostrinia nubilasis): No resistant pests found in the field so far in places where resistance management is practised.

Western corn rootworm (Diabrotica vigirfera): First resistant pests found?

Andow, David

Dr David Andow (University of Minnesota, St. Paul) has looked at the ecological impacts of GM crops in a number of studies.

Are Bt pyramids the answer? In the USA and other countries there are now Bt plants on the market that produce several different variants of the Bt protein. Since the pests now have to overcome each of them, the idea is that this will again significantly delay the emergence of resistant pest populations. For Huang, Andow and Buschman, these ‘pyramids’ of Bt proteins are an additional measure, but they are no substitute for the high-dose/refuge resistance management strategy. This is a view shared by the insect researchers at Iowa University: although their research shows that it will take longer for the Western corn rootworm to crack several Bt protein variants, they believe it is still necessary to improve the current resistance management strategy.

In fact, it is only a matter of time before pests develop resistance to an active ingredient used to control them. This applies to chemical pesticides as well as to the Bt concept, which is used primarily in maize and cotton. The genetically modified plants produce an active bacterial agent (Bt protein) that, depending on the variant, targets specific insect pests – moths or beetles. Each year, genetically modified Bt crops are planted on around 60 million hectares worldwide.

Experienced plant breeders and agronomists have never doubted that within the range of genetic variation there would be some target individuals that would prove more or less resistant to ‘their’ Bt protein. While for genetic engineering opponents this provides one more reason to reject genetically modified Bt plants, practitioners are looking for ways of delaying the resistance breakthrough for as long as possible. They have developed a resistance management strategy based on two key elements: firstly, the plants need to produce enough Bt protein to kill pests with low levels of resistance (high dose). Secondly, refuge areas are provided near Bt crops. These are planted with conventional cultivars so that pests living there can mate with resistant insects from the Bt fields. Scientists assume that the resistance trait is recessive, which means that the progeny will only be slightly resistant, if at all.

This high-dose/refuge strategy has now been recognised all over the world. The American ecologists, Fangneng Huang, David Andow and Lawrent Buschman, conclude that if the major pests targeted by Bt maize and Bt cotton remain susceptible to Bt toxins after fifteen years of intensive use in the USA and Canada, it is thanks to the successful implementation of the high-dose/refuge strategy. They evaluated numerous international scientific studies on Bt resistance development in insect pests and published their findings in a scientific journal in spring 2011.

Until now, Bt-resistant pest populations have been clearly documented under field conditions in only three cases: a species of moth larva (fall armyworm, Spodoptera frugiperda) found to be resistant to Bt maize in Puerto Rico, the African stem borer (Busseola fusca) found to be resistant to Bt maize in South Africa, and the pink bollworm (P. gossypiella) found to be resistant to Bt cotton in India. In all three cases, the researchers found there had been failures in the resistance management: a lack of sufficient refuge, and failure to use high-dose Bt cultivars. In India, for instance, illegally propagated Bt cotton seed had been used, in which the levels of Bt protein were too low to kill pests with low levels of resistance.

Systematic implementation of the high-dose/refuge strategy everywhere is, in the view of Huang, Andow and Buschman, a proven means of significantly delaying field resistance to Bt crops. However, the authors concede that it is much harder to establish effective resistance management in developing countries, where farmers are less well educated and monitoring is patchy.

Another recent study raises doubts about whether resistance management has in fact been as successful as Huang, Andow and Buschman claim in all cases. Insect researchers at Iowa State University in Ames (USA) have for the first time found Corn root worm that have developed field resistance to Bt protein. Whereas almost all Bt crops target moth or butterfly caterpillars, the Western corn rootworm is a beetle whose larvae eat the roots of maize plants. In the USA, lots of maize varieties produce a special variant of Bt protein in their roots that targets this pest.

The scientists investigated Bt maize fields in Iowa that had visible rootworm feeding injuries and compared them with fields of healthy maize plants. The Western corn rootworms collected from the soil of the fields in question were examined more closely in the laboratory. Beetles from the damaged fields were significantly more resistant to the Bt protein produced in the maize plants (Cry3Bb1). The pests were so resistant that they caused significant damage on the fields despite the Bt maize.

The explanations put forward so far are hypotheses rather than undisputed causes. There are some indications that resistance to the Bt protein in the Western corn rootworm is a dominant rather than a recessive trait. This would make refuges ineffective – unlike those used with moths, including the European corn borer. It is also possible that not enough Bt protein is produced in the root area of Bt maize plants to kill off the beetle larvae that possess low levels of resistance. A lack of crop rotation could also encourage the development of resistance.

To prevent the Bt concept losing its effectiveness as a means of controlling the Western corn rootworm, the scientists from Iowa University recommend developing better-adapted resistance management strategy and developing more integrated management solutions for pests targeted by Bt maize.

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