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Throwing a Wrench in the Pesticide Treadmill

FFAR Fellow, Swati Mishra

University of Tennessee

University of Tennessee

  • Next Generation Crops

Adorned in beautiful black and golden stripes, the Colorado potato beetle (CPB) is a perfect example of the saying, “all that glitters isn’t gold.” These charming and seemingly innocuous beetles cost potato farmers tens of millions of dollars annually. A single young beetle can eat up to 40 centimeters of leaf surface every day, which is about half the size of a sheet of copy paper. Combine that with a reproduction rate of about 500 eggs per female, leaving a potato field with nothing but plant skeletons.

The struggle between potato farmers and CPB has been ongoing for centuries. In fact, CPB can largely take credit for creating our modern-day insecticide industry; in 1864 U.S. farmers applied the first manufactured insecticide, Paris Green (copper acetoarsenite), to mitigate the spread of CPB. Since then, many different types of insecticides have been created and used. With the discovery of each new insecticide, more potent than the last one, scientists believed they had solved the problem once and for all. Yet they were proven wrong every single time. Every time a new insecticide would be introduced, CPB would develop resistance against it; evolution being the reason. We think of evolution as a super slow process taking place over millions of years. However, in cases when the selection pressure is very high– meaning the force to select a desirable characteristic is exceedingly important for survival—evolution can occur over a few generations. For CPB, a generation is just about a month.

Potato plant

Selection pressure in this case is the high concentration of insecticide intended to kill insects; however, no insecticide can ensure 100 percent mortality, especially if the insect population is in the millions-billions. For example, if only two beetles survive a particular insecticide concentration, they will naturally breed with each other since there are no other beetles available. The next generation resulting from these two beetles will now have a large proportion of beetles that can survive the high insecticide concentration. Needless to say, a farmer will spray an even higher concentration of insecticide trying to control these beetles, further increasing the selection pressure and eventually resulting in beetles that can survive an even higher concentration. At the same time, higher concentrations of insecticide can be damaging to other insects, such as pollinators. This process continues resulting in more and more resistant beetles until a point where the whole population is now resistant to the insecticide. This then requires discovery of a new pesticide lethal to the resistant beetles and the process continues.

This repeated use of higher and higher concentrations of pesticide until failure and the subsequent use of new pesticide is known as the “pesticide treadmill.” In the case of CPB, this has led to the failure of all known pesticides, earning CPB recognition as a “super pest”. To effectively control CPB growers need new pesticides, and they need them fast! Speed is a challenge; however, since decades of research and millions of dollars are typically needed to develop safe and effective insecticides.

Fortunately, in the last decade scientists discovered a new class of insecticides that use a process called RNA-interference (RNAi) to cause insect mortality. To understand how the RNAi process works we need to dig a little deeper into molecular biology. Every living organism contains a set of permanent instructions required to produce all the proteins that make up and preserve functionality of that particular organism. These instructions are DNA. When an organism requires a particular protein to be made, instead of copying the whole DNA, a temporary copy is created that contains instructions to produce only the required protein. This temporary set of instructions for a particular protein is called messenger RNA (mRNA). If this mRNA is somehow destroyed the corresponding protein cannot be made. This is exactly what RNAi does. It destroys the mRNA for vital insect proteins, causing insect death.

RNAi insecticides can be designed to destroy the mRNA of a specific insect species such that it does not affect any other insect species, especially the beneficial ones such as honeybees, making RNAi not only highly effective but also environmentally sound. RNAi insecticides also do not adversely affect soil or water systems, making them even more desirable as an insect control method. A commercial RNAi-based insecticide has already been developed to fight the Colorado potato beetle and is expected to be approved for use by the US Environmental Protection Agency (EPA) in the next year. However, this is only the initial step. Learning from past mistakes, scientists now understand that no single pesticide can prove to be a panacea and that we need to focus on delaying resistance and slowing the pesticide treadmill as much as possible.

Colorado potato beetle

As a Ph.D. student at the University of Tennessee-Knoxville I am investigating how CPB develops resistance to RNAi insecticides. My research is working to determine how to optimize these pesticides to help delay resistance and identify other methods of control that can be used along with RNAi pesticides so that we don’t continue running the pesticide treadmill or at least slow it down. I hope to help create a framework for efficient long-term use of RNAi insecticides to control CPB and other relevant agricultural and medical pests.

Gratitude

I am thankful to Bayer Crop Science for sponsoring my FFAR fellowship. This program has immensely impacted my professional career and personal growth. I have become a much more confident and competent individual thanks to all the training provided by the program.