The Battle of Resistance

By Dion Garrett (Waitrose CTP Student)

A consent battle for dominance is being fought between grower and agricultural pest every growing season. This fight has been waged ever since civilisations required food to feed the masses. It was inevitable that various pests would take advantage of this veritable feast. To fully understand how the pieces fit together, a brief introduction to the laws that underpin this conflict is necessary:

Once ridiculed and subject to much controversy, evolution is now regarded as fact due to the overwhelming evidence in its favour. It was once thought that evolution was a very slow process, taking thousands of years for changes to visibly occur. This made it extremely difficult to provide any substantial evidence to support the initial theory.

Charles Darwin documented beak variation in finches during a particularly famous expedition to the Galapagos Islands (Ecuador) in 1835. Beak variation was observed on different islands where these finches ate different foods, suggesting that a relationship between their ecology and evolution existed1. Darwin would have never have imagined that it was possible to witness evolution first-hand because he thought that this process occurs over a long period of time. This consequently would make it near-impossible for someone to witness these changes over their lifetime.

Due to huge advances in scientific understanding and technology, it is now possible to witness evolution on different levels. Ecological examples of evolution by natural selection are spread throughout human history. During the nineteenth century, industrialisation and coal fires polluted the environment, which led to a rapid change in the proportion of two colour-forms of peppered moth (Biston betularia). Where once the lighter colour-form was much more common, these changes to the environment resulted in the darker colour-form to be favoured4. This selection was simply because the darker colour-form blended into its environment much better than its lighter counterpart and subsequently resulted in more successful breeding of the darker colour-form. With the knowledge that these ecological processes are not as predictable as once thought, ecologists needed to reassess how they approached fundamental ecological questions.

So now armed with this information, where do we go from here? And where does resistance and evolution come into the picture?

Farmers are faced with a worthy adversary when it comes to resistance development in agricultural pests. Not surprisingly the best candidates to demonstrate ‘evolution in action’ are organisms which have multiple generations per year. There are many species of insects which are known pests of agriculture3. Thus, there is considerable financial investment in how to control and reduce pest outbreaks. When a population of agricultural pests are exposed to chemical controls to reduce their numbers within a crop, can result in a high selection pressure in favour of resistance. This essentially means that most individuals of that population will be killed by the chemical control, which is good news for the farmers and consumers. Unfortunately, the few individuals that do manage to survive and reproduce will now be more likely to have characteristics (traits) which can resist the control (known as resistant-breaking populations)6. Being subjected to such high selective pressures and, combined with short successive generations, can lead to rapid resistance development.

As government legislation is constantly tightening on chemical control, research into alternative methods are being explored. This restriction on chemical use is problematic for the industry due to an increased risk of the development of resistant populations evolving much faster. This consequently can greatly reduce the effectiveness of chemical controls and leave the industry vulnerable.

An entomologist at Rothamsted Research recently reported that a sample of willow-carrot aphid (Cavariella aegopodii) has been found to be resistant to pyrethroid, a widely-used plant-derived insecticide in agriculture2,7. When a population develops resistance due to strong selection pressure (insecticide in this instance), there is usually an accompanying fitness cost associated with it. This fitness cost could potentially make that population more susceptible to other conditions such as overwintering survival or increased mortality to diseases3,5.

Figure 1: Willow-carrot aphid (Cavariella aegopodii) on a carrot stem. Photo credit: Rothamsted Research VCU.

In theory, removing this selection pressure from the environment you remove the genetic drive towards keeping the resistance. In this instance, however, the willow-carrot aphid population remained resistant to pyrethroid after 3 years in glasshouse conditions, without the presence of pyrethroids. This could spell potential disaster for growers and there ever shrinking supply of available treatments. One of the Waitrose CTP students, Hannah McGrath, is investigating some of these complex interactions in willow-carrot aphid for her PhD project.
So why is that even though this selective pressure was removed from this population they were still resistant? To understand the genetic mechanisms and drivers that underpin these such resistances is a fascinating area of research. This is an area I hope to explore further during my PhD project.

Like the red queen hypothesis, growers are constantly running just to keep pace with their arch-pest nemesis. Food security is an ongoing battle of resistance supremacy between cutting edge science and evolution. Is it time to put gloves down and search for more sustainable and alternative methods?

References
1. Abzhanov, A., Kuo, W.P., Hartmann, C., Grant, B.R., Grant, P.R. and Tabin, C.J., 2006. The calmodulin pathway and evolution of elongated beak morphology in Darwin’s finches. Nature, 442(7102), p.563.
2. AHDB Horticulture. (2018). Pyrethroid resistance in willow-carrot aphid. Available: https://horticulture.ahdb.org.uk/news-item/pyrethroid-resistance-willow-carrot-aphid. Last accessed 23rd Feb 2018.
3. Bates, S.L., Zhao, J.Z., Roush, R.T. and Shelton, A.M., 2005. Insect resistance management in GM crops: past, present and future. Nature biotechnology, 23(1), p.57.
4. Cook, L.M. and Saccheri, I.J., 2013. The peppered moth and industrial melanism: evolution of a natural selection case study. Heredity, 110(3), p.207.
5. Gassmann, A.J., Carrière, Y. and Tabashnik, B.E., 2009. Fitness costs of insect resistance to Bacillus thuringiensis. Annual review of entomology, 54.
6. Tolmay, V.L., Lindeque, R.C. and Prinsloo, G.J., 2007. Preliminary evidence of a resistance-breaking biotype of the Russian wheat aphid, Diuraphis noxia (Kurdjumov)(Homoptera: Aphididae), in South Africa. African Entomology, 15(1), pp.228-230.
7. Tyler D B MacKenzie, Irin Arju, René Poirier, Mathuresh Singh. (2018). A Genetic Survey of Pyrethroid Insecticide Resistance in Aphids in New Brunswick, Canada, with Particular Emphasis on Aphids as Vectors of Potato virus Y. Journal of Economic Entomology. 1 (35).

Photo credit: Rothamsted Research VCU image database.