How to Manage Insect Resistance?

With the growth in the areas planted with transgenic crops that produce insecticide proteins, insects are becoming increasingly resistant in US. This was the conclusion drawn by a team of researchers from CIRAD and the University of Arizona in a review of the available scientific literature published recently.

But 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? It is all a question of managing resistance: farmers have the means to slow the development of resistance, all that remains is to implement them.

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 so-called Bt plants have helped reduce pesticide use, but have also, as the theory of evolution would have it, led 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.

In ten years, five times as many cases of resistant insects

A CIRAD researcher and researcher and his peers from the University of Arizona recently published a summary of the scientific literature available on this question, in which they 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 50% of the individuals in it had become resistant, ie capable of surviving the ingestion of such toxins.

The authors observed five cases of resistance to Bt crops in 2010, as 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 authors also observed variations in the speed at which resistance appeared.

Insects still sensitive after 15 years

In some cases, resistance developed within two years, while in others, it still had not been detected after 15. The authors therefore looked at the conditions in which resistance appears, and above all at the factors that slow its appearance.

First of all, as the theory of evolution would have it, they 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.

Refuge zones slow the development of resistance

But above all, they showed that the efficacy of Bt crops is more 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.

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.

In 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 authors 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 1% 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% resistant individuals have been detected for certain H. zea populations under the same conditions.

Transgenic plants with both toxins

Lastly, the authors observed 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.

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.

Managing resistance is still the cornerstone of pest control

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.