Pest resistance to Bt crops

posted in: Science | 13
Cotton Bollworm. Source: USDA

The issue of pest resistance to insect resistant Bt crops receives regular media attention, partly because anti-biotechnology lobbies use it as an argument to vilify GM crops. The German NGO testbiotech, in a recently published report commissioned by Member of the European Parliament Martin Häuslingof the Green Party, argues that because of potential resistance development, GM crops should not be allowed for cultivation in the EU:

There must be no large-scale, commercial cultivation of GE herbicide-tolerant or insecticide-producing crops. Such crop cultivation is unsustainable and will lead to a ‘race’ to step up their cultivation.

The idea that a particular technology should be banned if it cannot be used forever is dangerously misguided. The benefits obtained by millions of farmers thanks to Bt crops have been very real, perhaps most notably India but also in the European Union. Without GM, food prices would have been higher, environmental externalities more severe and health problems more common.

Pest resistance is a not a black and white concept where all pests are either completely resistant or not. What does it mean if some Bt-resistant insects are discovered in a field? As long as not all relevant populations of all relevant pest species in the field are completely resistant to the Bt toxins, Bt technology reduces insecticide use and enhances effective yield. Up until that point, Bt technology remains above the agronomic baseline of non-Bt seeds.

There is the theoretical possibility that emerging secondary pests might end up posing a greater problem than the primary pests did before the Bt technology was introduced. While there is some evidence that secondary pests such as aphids have increased in cotton fields, I’m not aware of any case where the pest pressure on the whole has increased as a result of Bt. Indeed, in absence of the insecticides that are reduced by using Bt, there is evidence that beneficial insects can return to farms.

Moreover, resistance to particular Bt toxins (or “Cry proteins”, so named because they form a crystal) is not the same as resistance to Bt technology in general. There are hundreds of naturally occurring Bt proteins as well as engineered chimeric Cry proteins with specific modes of action and varying effectiveness against different insect species. Different combinations of Bt proteins have already been approved for cultivation, and resistance to one toxin does not necessarily imply resistance to another. For example, transgenic broccoli engineered to produce Cry1C controls diamondback moth that is resistant to Cry1Ac. It is also not obvious that resistance development is as likely with some sets of Bt genes as with others. In addition, the potential to develop resistance may vary by insect species.

It is important to put the resistance issue in perspective. Resistance development in agricultural pests is not a specific problem of Bt or other GM crops. In the history of US agriculture, biological innovation has been key in the struggle against pests, if only to avoid regress (also known as the Red Queen effect where you have to run fast just to stay in one spot). The same applies to non-breeding strategies including chemical insecticides and biological control agents (sometimes called the “pesticide treadmill”). This is not to say that resistance development is desirable; rather, that its occurrence is not restricted to transgenic crops. Applying the logic of testbiotech, no conventionally bred insect-resistant plant variety and no chemical or biological insecticide should be allowed as they all carry proven risks of resistance development.

Strategies to delay resistance development

In contrast to anti-biotech groups, farmers, technology providers and regulators pursue strategies to delay resistance development, because they see large benefits of Bt crops in farming.

One strategy mandated for years in several countries with Bt crops are refuge areas. Typically, a field is not exclusively sown with Bt seeds, but has a certain share of conventional plants on strips along its borders or inside the field. The rationale is that resistant insects selected under Bt exposure will mate with susceptible individuals found on nearby conventional plants. Consequently, resistant individuals may not take over the population as fast as they would without refuge, especially if resistance is a recessive trait.

Another strategy relies on Bt crops with multiple Bt genes that produce a variety of Bt toxins (called “pyramiding” or “stacking“). Insects resistant to one toxin may not be resistant to others, and it is less likely for simultaneous resistance to emerge. Like refuge areas, pyramiding is widely used and now mostly based on two toxins. Under certain assumptions pyramiding could dramatically cut the need for refuge. In 2007, the EPA approved “natural refuge” for a pyramided cotton, arguing that sufficient refuge is provided by other crops on neighboring fields. In Australia farmers may use pigeonpea, sorghum and corn as natural refuge.

Meanwhile, entomologists are working on improving their incomplete understanding the complex mechanisms involved in resistance evolution. Recently published research suggests that pyramiding might not work as well in delaying resistance as previously thought. In the laboratory, scientists selected cotton bollworm (Helicoverpa zea) for resistance against Cry1Ac. They exposed the resistant insects and a susceptible control group to Bt cotton expressing Cry1Ac/Cry2Ab and found that the group resistant to Cry1Ac exhibited a much higher survival rate than the control group, violating the assumption of redundant killing that is crucial to this strategy. So far, despite multiple reported instances of resistant insects, large-scale failure of Bt crops due to evolved resistance has not occurred, but it may come sooner than expected.

Should refuge requirements be expanded?

This research finding is bad news because the potential of pyramided Bt crops might be lower than believed. (Actually, some scientists have been positively surprised at the long delays observed in resistance development.) Let’s assume the results also apply to other Bt pyramids and insect species (there is evidence to the contrary). What should be made of such a scenario? Should larger refuge areas be required?

Before answering that question, it must be recognized that the sustainable application of a particular technology is not a primary goal of farming. A much more important goal is efficiency. Efficiency means getting the most output (e.g. food) from a set of scarce inputs (natural resources, labor, capital). The technologies transforming inputs into outputs, be they biological, chemical, or mechanical, are valuable only insofar as they contribute towards efficiency.

Cotton field, by Kimberlykv

When deciding whether to expand refuge requirements, policymakers must take into account that there is a tradeoff between the size of the refuge area and productivity. If refuge area increases, more plants will get damaged by pests and hence reduce effective yield. The crucial question is whether the benefits of delaying resistance outweigh the costs of these yield losses and other potential drawbacks of refuges such as the need for additional land, sprays, separation costs, and sowing and harvest times. Costs of monitoring compliance with refuge requirements must also be considered, while pyramiding will incur more R&D expenditures. (In some developing countries with larger monitoring costs, refuge requirements may be less efficient also because of natural refuge in small-scale cropping systems.) The point here is not to question whether the optimal refuge requirement is 0%, 20% or 40%, but to realize that there are costs that have to be weighed against benefits. It is possible that an arms race based on adding more Bt genes is more efficient than slowing resistance development by expanding mandated refuges.

Besides Bt crops, there is a host of other pest management options including chemical control, biological control and cultural control such as ploughing and crop rotation. Like Bt, they all have their particular drawbacks, be it risk of resistance development, low effectiveness, or environmental and economic cost. The most efficient pest management strategy depends on local context, but will involve multiple instruments. For breeders, genes producing insect toxins, whether introduced using conventional or GM techniques, are not the only route towards pest protection. There are exciting possibilities on the horizon, including transgenic plants that emit volatile organic compounds to repel herbivores or attract their natural enemies. The use of nano-silica that kill pests by purely physical means are just one example of potential applications of nanotechnology in pest management. New approaches will have benefits and costs to be assessed against existing alternatives. As of today, there are no magic bullets protecting crops from pests. But there are excellent reasons that we should keep looking for them. Bt will not be the end of the road.

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Jonas Kathage is a research fellow at the Joint Research Centre (JRC) of the European Commission. The views expressed are his own.