How Does RNA Interference Work in Insect Pest Management?

Since the discovery of insecticidal properties of DDT in 1939 opened the modern era of crop protection, synthetic pesticides remain the foundation of crop protection in the mainstream commercial farming around the world. Together with fertilizers and high-yielding crop varieties, they also made a considerable contribution to the “green revolution.”

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Ultimately, that allowed countless multitudes of people to quit farming, move to cities, and engage in pursuits that do not require getting up at 4:30 AM or performing rain dances in an attempt to stay in business for another year. Interestingly, these urban pursuits sometimes include political activism to ban farmers from using all pesticides; however, this is probably a topic for a different conversation.

For much of their history, pesticides were subjected to an often-merciless scrutiny of possible ill effects on human and non-human health. While the crop protection industry usually gets rather defensive about possible hazards of its products, it should not take much intellectual courage to accept that chemicals designed to kill certain living organisms may also harm some other living organisms.

If we think about it, organophosphates were developed to kill humans in the first place. As our understanding of such issues grows, some of the chemicals end up being withdrawn from the market, sometimes with the “what the heck were we thinking” kind of afterthought.

Pest populations do not stay idle either. Pesticide resistance is a persistent problem that leads to pest populations comprised of the mutants that literally eat previously deadly compounds for breakfast (as well as for lunch, dinner, and mid-afternoon snack). There are ways to delay the onset of resistance, but the process itself is relentless. As a result, some of the products that worked miracles in the past become next to useless in the present.

Despite all the turbulence and uncertainty, the chemical industry remains very much entrenched in the crop protection business. Insecticides that are no longer used because of environmental concerns or insect resistance are being invariably replaced by something new. For that, we should give ourselves a big pat on the back and acknowledge all the continuous innovation that is allowing this to happen. (Yes, I work in academia rather than for a private company, but when it comes to patting on the back, I might as well join in).

Insect control in particular, the area I have been working in for my entire career, benefits greatly from the relatively steady inflow of new insect ingredients that replace outdated older chemistries. Recent advances in molecular genetics open especially exciting opportunities in the development of new insecticides.

For all practical purposes, insecticides keep coming to an end user in jugs or cans supplied by a local friendly pesticide dealer, often complete with a baseball cap or a water bottle emblazoned by a company logo. There is a lot of work, however, that needs to be done before there is something new to put in a jug. Both historically and at present, much of that work has been rather mundane and uncreative. Existing chemicals, whether synthetic or natural in origin, are applied to a bunch of insects, often just because they are available. (By the way, about 65 years passed from the time when DDT was first synthesized and became available to the scientific community to the time when its insecticidal properties were first discovered).

If insects die, we are looking at a potential new active ingredient. If they do not, the procedure is repeated with something else. Just as with fishing using a drag net, a lot is left to a chance, but the approach works well when averaged across all the efforts.

A possible shortcut is to take a molecule that is known to kill insects, synthesize something similar (but different enough not to be protected by a patent), and then test whether it also kills the pests. This is a more targeted approach. On the downside, it is limited in how many similar compounds can be synthesized without violating the laws of physics.

How RNAi Works

Ribonucleic acid (RNA) interference, often abbreviated as RNAi, is a new technique that allows precise targeting and then turning off specific genes inside an organism’s cells. The mechanism behind RNAi was first described at the turn of the 21st century. Similar to the discovery of the insecticidal properties of DDT, the scientists responsible for this discovery received a Nobel Prize.

RNAi disrupts the process of extracting the information stored in an organism’s DNA and using it to produce proteins, thus interfering with physiological processes at a cellular level. Proteins are essential components of all living organisms on the planet Earth. They define the very state of being currently alive, as opposed to never being alive as most rocks (strictly speaking, some of the rocks are fossilized prehistoric creatures like dinosaurs) or being dead as a roadkill. Not only do proteins form building blocks of all tissues and organs from hearts to fingernails, but also, they catalyze biochemical reactions that are necessary to keep us going. Therefore, lacking sufficient amounts of certain proteins is incompatible with staying alive. This, of course, is exactly what we wish for the insect pests that affect our wellbeing.

DNA stores the information on what proteins to produce, and on where and when to produce them, as a sequence of small molecules called nucleotides that are strung together like beads on a wampum. This information is then transcribed by a cell from a certain segment of its DNA to a molecule of RNA that is called messenger RNA, or mRNA for short. This mRNA moves to ribosomes, which are cellular structures that assemble proteins based on the information that they read from the mRNA. The process is analogous to going to a library (DNA), taking a photocopy of a blueprint that is archived over there (mRNA), bringing it to a factory floor (ribosomes), and then using it to make something useful.

RNAi prevents mRNA from getting to ribosomes in one piece. This is done by applying another molecule, called a double stranded RNA, or dsRNA for short, that matches the target mRNA in its structure. (I know that there are perhaps too many RNAs to keep track of in this text, but there is not much that I can do about it). This molecule enters the cell, reacts with a few special proteins that are already present in the cell, and then literally shreds the target mRNA. As a result, no information is delivered to the ribosomes and no new proteins are produced. To continue the analogy from the previous paragraph, this is similar to intercepting the photocopy of a blueprint and running it through a paper shredder. As a result, machinery on the factory floor remains idle because nobody knows what to do.

How RNAi Works in Insect Control

RNAi is a powerful technique that has many potential uses in biotechnology and medicine. One of the promising applications is developing new insecticides. We can identify a specific gene that is unique to an organism belonging to a given pest species and is essential for its survival. We can then synthesize dsRNA that matches this gene, get it inside the target organisms, and then kill them by turning off this gene. Non-target organisms that do not have the same gene should not be affected in the process.

Unlike a fishing expedition of screening thousands of compounds in hopes to find the one with insecticidal properties, RNAi allows designing an active ingredient that aims at a specific pest in a specific manner. In the field, such a dsRNA can be delivered to a target insect in two ways. The first one is to genetically modify crop plants to make them produce dsRNA in their foliage. SmartStax PRO is the first transgenic corn trait that employs dsRNA against the Western corn rootworm by targeting a gene involved in sorting and transporting receptors in cell membranes. It was originally developed by Monsanto and then acquired by Bayer CropScience together with Monsanto. The trait has been approved by the U.S. EPA and is currently making its way towards the market.

The second way of deploying insecticidal dsRNA is to produce it at a factory and then spray on the foliage. There are several such products at different stages of development, including Monsanto’s BioDirect and GreenLight Bioscience’s yet-to-be-named insecticide that target Colorado potato beetle. I expect that the first dsRNA-based sprayable insecticide will be on the market within the next couple of years.

What RNAi Can’t Do in Insect Control

RNAi may well open a new chapter in insect control, complementing and maybe even replacing to some degree synthetic chemicals that currently dominate this field. However, it will not provide a silver-bullet solution that will once and for all protect crops from insect damage. So far, it appears that insecticides containing dsRNA as their active ingredients will have the same problems as their predecessors.

Western Corn Rootworm_photo courtesy Syngenta

Beetles, such as Western corn rootworm (pictured) and Colorado potato beetle, are generally susceptible to RNAi. Photo courtesy of Syngenta

First, some groups of insects are not particularly responsive to RNAi. Beetles, such as Western corn rootworm and Colorado potato beetle mentioned above, are generally susceptible. However, caterpillars, as well as aphids, leafhoppers, scales, whiteflies, and their relatives are rather recalcitrant. We do not yet know why this happens, but it may have something to do with their ability to digest dsRNA in their guts before they make into the cells. As we learn more about RNAi mechanisms, we may find ways to overcome this resistance. However, we are not there yet.

Secondly, while dsRNA appears to be generally safe to non-target organisms, this safety should not be taken for granted. Some of the genes are similar or even identical among different species. This is not surprising, taking to consideration that all life on Earth evolved from the same common ancestor (or was placed here by the same Creator, whichever explanation you find more convincing). Therefore, accidentally targeting one of these genes may have far-reaching unintended consequences. We have an increasingly good information on what species has which genes, but it is still very far from being complete.

Thirdly, insecticide resistance is likely to be an issue for the dsRNA just like it is an issue for other active ingredients. Mutants will arise that will be able to somehow destroy or avoid dsRNA molecules. These mutants will survive, proliferate, and take over the world (or at least the field). Resistant strains of both Western corn rootworm and Colorado potato beetle have been already successfully developed in the laboratory, even though RNAi has not been used against them in the field yet. Furthermore, it appears that once one type of dsRNA fails against a particular beetle strain, so will the rest of them. As a result, we will be back to the circle one of controlling these pests with previously existing active ingredients.

Development of RNAi-based insecticides is an important example of bringing recent scientific advances to the service of the crop protection industry. Resulting products will provide valuable tools for managing insect pests. However, they are not going to provide a complete and ultimate solution to pest problems, just as previous synthetic chemicals failed to do that. These are simply tools, which should be used carefully and thoughtfully together with other tools, including non-chemicals means of insect control. In my experience, this is the only way to sustain effective pest management.

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