Secondary pests in genetically modified crops

"The phenomenon is as old as crop protection itself"

In spring 2010 there were a number of reports of secondary pests in Bt plants. The Western bean cutworm is reported to be spreading in the USA. This is a moth whose natural competitors have been decimated through cultivation of Bt maize. In China, certain species of bug are occurring in Bt cotton fields and attacking surrounding orchards. Does the cultivation of Bt crops entail risks that have been overlooked until now? GMO Safety spoke to Johannes Jehle.

Project Director Dr Johannes A. Jehle is head of the Institute for Biological Control at the Julius Kühn Institute in Darmstadt.

Secondary pests in the USA

The maize-growing regions of the USA have evidently seen a shift in the ecological balance of three species of moth. Cultivation of maize that expresses the Bt protein Cry1Ab harms the caterpillars of the European corn borer and those of the cotton bollworm. This has enabled a third species, the Western bean cutworm, to multiply. This species is not sensitive to Cry1Ab. Genetically modified maize varieties that produce several different Bt proteins, including Cry1F, which is toxic to the Western bean cutworm, have now been authorized in the USA.

Secondary pests in China

Studies in six Chinese provinces where Bt cotton is grown found that mirids have spread rapidly over the past ten years. They cause significant damage, both in cotton fields and in nearby orchards. According to the study, they were able to spread because less insecticide is sprayed when growing Bt cotton. Insecticide use in Bt cotton-growing areas is said to be increasing again now to control the mirids.

GMO Safety: Is the emergence of secondary pests a new phenomenon?

Johannes Jehle: The emergence of secondary pests is a phenomenon that is as old as crop protection itself. In crop protection, we differentiate between primary and secondary pests. Secondary pests are often disregarded when viewed against the damage caused by primary pests, or they are controlled a side-effect of managing the primary pests.

However, when the primary pest is removed, secondary pests often find a niche and multiply.

GMO Safety: Is this problem specific to the cultivation of genetically modified plants?

Johannes Jehle: In the long term, secondary pests often occur when pest control is non-specific. But in the short term, they can also be found in conjunction with selective methods, and even in association with biological and biotechnological crop protection – methods that are unrivalled in terms of selectivity and specificity. For instance, selective control of the cabbage moth (Mamestra brassicae) is very difficult because cabbages are attacked by a large number of other pests, including the cabbage white butterfly, the diamond-back moth and the cabbage flea. This means that crop protection should never focus on an individual pest, but always on the whole cultivation system.

The emergence of secondary pests is therefore not a problem specific to the cultivation of genetically modified crops. But it could be exacerbated in GM crops if the efficiency of the pest control leads farmers to neglect other measures of good farming practice, such as crop rotation, which help to prevent and limit pest pressure naturally.

GMO Safety: What circumstances favour the emergence of secondary pests?

Johannes Jehle: In general, we are aware of three main reasons for the emergence of secondary pests: firstly, the reduction of beneficial organisms and direct antagonists, secondly, the reduction of food or niche competitors, and thirdly, selective targeting of primary pests while sparing secondary pests.

However, the factors that promote the occurrence of secondary pests can be very diverse, sometimes even opposing. Broad-spectrum insecticides, most of which are no longer permitted, often kept both primary and secondary pests in check and farmers were initially very happy with their effectiveness. However, over the longer term, many broad-spectrum insecticides have a negative impact on the occurrence of beneficial organisms that play an important natural role in controlling potential pests. This means that the application of broad-spectrum insecticides frequently encourages certain secondary pests over the longer term.

By contrast, selective pest control methods help protect beneficial organisms, which means that it is easier to control secondary pests as well. However, if you are faced with a pest complex with a large number of secondary pests, selective control of the primary pest can sometimes increase the incidence of secondary pests. Here too, it is important to exploit and promote the natural antagonist potential.

GMO Safety: How can the occurrence of secondary pests be prevented?

Johannes Jehle: We don’t need to invent new methods for this; just make use of findings from good farming practice and sustainable agriculture. These include crop rotation, tillage, high biological diversity in the farming system and promotion of the pests’ natural antagonists. Using one particular method or one particular target always means selection in a particular direction: be that resistant target organisms, or increased incidence of secondary pests that were not a problem in the past. Therefore, it is important to develop pest control using as broad and multifaceted a spectrum of methods as possible.

This means that even cultivation of genetically modified plants will not be able to survive in the long term without accompanying biological or chemical measures. Whether this will make sense in economic terms is another matter entirely.

GMO Safety: From your point of view, is the use of stacked genes to combat secondary pests a sensible strategy? In other words, getting the crop plants to produce additional Bt proteins so that they target a broader spectrum of pests?

Johannes Jehle: This will certainly lead to short-term success, but will not, I believe, solve the long-term problem. For every pest you control, you open a new niche for another potential pest. This is why it is so important to develop farming systems that are as ecologically stable as possible. If you don’t tackle the root causes of the phenomenon, the battle – one that crop protection has been fighting for decades, if not centuries – will never be more than a race against time. In this respect, the emergence of secondary pests is not specific to the cultivation of genetically modified crops – but neither will the problem be solved by genetic engineering.

GMO Safety: Thank you for talking to us.