Managing insect resistance
Researchers from CIRAD (French Agricultural Research Centre for International Development) and the University of Arizona have hinted that due to the increase of areas planted with transgenic crops that produce insecticide proteins, insects are becoming increasingly resistant.
The major question which now remains is why in some cases, do insects adapt in less than two years, while in others they still have not managed to adapt after fifteen years?
According to CIRAD, it is all a question of managing resistance, where farmers have the means to slow the development of resistance and all that remains is to implement them.
CIRAD further says that since 1996, farmers worldwide have planted more than 400 million hectares of genetically modified maize and cotton to produce the insecticidal proteins from the bacterium Bacillus thuringiensis (Bt). These plants have helped reduce pesticide use, although as the theory of evolution would have it, leading to the appearance of resistant insects. With the benefit of hindsight, it is now possible to assess the speed at which insects become resistant and, above all, to understand why.
A CIRAD researcher and his peers from the University of Arizona recently analysed 77 studies involving field monitoring of resistance to Bt crops. They found 24 cases in eight countries, concerning 13 insect pest species (lepidopterans and coleopterans) targeted by six Bt toxins. They based themselves on the principle that a population was considered resistant if 50percent of the individuals in it had become resistant, i.e capable of surviving the ingestion of such toxins.
The researchers observed five cases of resistance to Bt crops in 2010, against just one in 2005. Three of the five cases (Diabrotica virgifera virgifera and Spodoptera frugiperda on maize and Helicoverpa zea on cotton) were in the USA, which accounts for almost half the area planted with Bt crops worldwide, and the others (Busseola fusca on maize and Pectinophora gossypiella on cotton) were in South Africa and India.
While the increase in the number of cases of resistance can be put down to the increase in the area planted with Bt crops – from 1.1 to 66 million hectares between 1996 and 2011 – hence in cumulated pest exposure to toxins, the researchers also observed variations in the speed at which resistance appeared.
In some cases, resistance developed within two years, while in others, it still had not been detected after 15, which led the authors to look at the conditions in which resistance appears, and above all at the factors that slow its appearance.
The researchers confirmed that the efficacy of Bt crops has a better chance of lasting if resistance genes are initially scarce within the insect population and if the inheritance of that resistance is recessive, in other words if only insects with two copies of the resistance gene survive on Bt plants.
However, the authors showed that the more efficacy of Bt crops is sustained the more steps are taken to manage the development of resistance. For instance, they demonstrated the merits of “refuge zones”. These zones, planted near Bt crops, contain non-modified plants, which thus do not have the Bt genes and do not produce toxins. They enable toxin-sensitive insects to survive and mate with resistant insects to produce sensitive progeny.
According to CIRAD, setting up such refuge zones is a key part of insect pest control. Numbers must be precisely calculated in line with the toxin dose produced by the Bt crop and the initial frequency of resistance genes in the insect population.
The research also shows that this way, it is possible to slow the appearance of resistance with just a small number of refuges provided toxin doses are high and resistance gene frequency low. If not, it is necessary to install large numbers of refuges, otherwise resistance develops fast.
However, the researchers observed that in many cases, despite strict regulations, farmers do not set up sufficient numbers of refuges to control the development of resistance.
In Australia, where the regulations are strictly applied, fewer than 1percent resistant individuals have been found in H. armigera and H. punctigera populations on Bt cotton, while in the southern USA, where the regulations are much less strict, more than 50 percent resistant individuals have been detected for certain H. zea populations under the same conditions.
The researchers also found that in the past ten years, farmers have switched from growing first-generation transgenic crops, which produce just one Bt toxin, to plants that produce two. However, again, precautions have to be taken to ensure the efficacy of these so-called “pyramid” plants.
CIRAD says that these plants in fact work best if they are not grown at the same time as plants that produce just one toxin, as is the case in Australia, and provided resistance to one of the toxins is not already established, otherwise the benefits are considerably reduced, as shown by the resistance of H. zea to one of the Bt cotton toxins in the USA.
The research shows that even with this arsenal of control methods, it is vital to manage resistance. It is inevitable that insects will adapt to Bt crops, and those crops will see their efficacy wiped out sooner or later by the resistance developed by those insects. The aim is therefore to slow the development of resistance through integrated pest management, which combines transgenic plants and control of resistance within insect populations.
(Source – http://www.agriculturalreviewonline.com/index.php/farm-management/385-managing-insect-resistance)