As a pest management company, we routinely work with pesticides. Our clients’ responses to those pesticides range from fear and apprehension that the pesticides will cause further harm to asking us to use our strongest pesticides right out of the gate to make sure the pest problem is solved. This post will be a bit of a long one, but I want to explain how pesticides work, how they are regulated, why we use them, and why using the strongest insecticides is not always the answer to persistent pest problems.

What are pesticides and how do they work?

A pesticide is any chemical substance that is used to kill pests, which are living things that in some way cause harm to humans or human endeavors. Pesticides are further classified by the type of pest they kill; herbicides kill plants, insecticides kill insects, rodenticides kill rodents, etc. Because I am an entomologist and have the greatest familiarity with insect physiology, I am going to focus on insecticides for this discussion.

Insecticides work in a huge variety of ways. This variety in important, because insects are notoriously adept at developing resistance to specific modes of action, which forces pesticide manufacturers to develop new products that work in different ways. The Insecticide Resistance Action Committee (IRAC) has assembled an insecticide mode of action classification system that is a useful resource for figuring out how a specific insecticide works, as well as figuring out which products would be good alternatives if a resistance issue develops. Pesticide manufacturers also constantly research new modes of action to develop insecticides that are more specific, which translates into greater safety for people.

Many of the most commonly used insecticides affect the nerves, ultimately causing death by causing the nerves to fire uncontrollably, which exhausts the nerves and the muscles they control, as well as interfering with vital processes like breathing and heartbeat. At a basic level, insect nerves and human nerves work the same way. Ion channels along a nerve cell’s length cause sodium, potassium, and chloride ions to move in and out of the cell, which creates an electrical signal down the length of the nerve. When the electrical signal reaches the end of a nerve cell, that cell secretes neurotransmitters that bind to receptors on the next nerve cell, effectively relaying that signal. Insecticides can bind to ion channels, neurotransmitter receptors, or the enzymes that break down neurotransmitters to end a signal and cause these essential parts of the nerve to stop functioning.

The insecticide must fit into the part of the nerve it affects like a puzzle piece; if the fit is not quite right, the insecticide will not work. This specificity is both a good thing and a bad thing. On the positive side, many of the ion channels and neurotransmitter receptors in insects are shaped a little differently than they are in mammals (including humans). Because insecticides are specifically designed to bind well to insect nerves, they bind poorly to mammal nerves and have far lower toxicity to humans than they do to the insects they’re designed to control. However, other components of the nerves are virtually identical between insects and mammals, so some insecticide nerve poisons are relatively safe around people and pets, while others warrant greater caution.

On the negative side, if an insect has a mutation that gives it a slightly differently shaped ion channel or neurotransmitter receptor, the insecticide might not be able to bind to its nerves and mess them up, or it will bind less effectively. Mutations that change the shape of these parts of the nerves come about through random chance; every time an insect is born, it has a miniscule chance of having a mutation that makes it resist the insecticide. While the odds of any single insect having a mutation that makes it resistant are slim, insects produce huge quantities of offspring with a rapid generation turnover. If the odds of being born resistant to an insecticide are one in a billion and a fly infestation produces a trillion flies, you would expect a small portion of that population to be resistant. If you apply an insecticide that kills all the susceptible flies, the resistant flies are left to repopulate, and the new population will be mostly or entirely resistant. On the other hand, the mutations that create resistance may make the nerves not work as well or incur some other sort of cost, so without exposure to insecticides, the resistant individuals are outcompeted by their susceptible counterparts. Using the same insecticide over and over again can lead to an entire insect population becoming resistant, whereas switching between different insecticides and/or using other methods to control pests gives the susceptible population a chance to dominate the population, which keeps pesticide products working long-term. This is part of the reason why we do not have standard protocols for using certain products against certain pests all the time; if we did, we could reasonably expect our methods to fail within a few years of use. There is a fair bit more detail on the physiological mechanisms by which resistance evolves, but a blog post is not the place to attempt to recount the books’ worth of information on insecticide resistance beyond a general overview.

Some of the insecticides we use have modes of action that affect physiological processes in insects that only occur in arthropods (i.e. arachnids, insects, crustaceans, centipedes, millipedes, etc.). Growth regulators are mostly chemical mimics of a hormone called juvenile hormone, which regulates the process of metamorphosis and controls whether a molting insect will remain in its current life stage (larva or pupa) or if it will progress to the next stage. Juvenile hormone also regulates egg production in many, if not all, adult insects. Dosing insects with even trace amounts of juvenile hormone can prevent young insects from becoming adults, and may sterilize adult insects, so it works sort of like bug birth control. Vertebrates have no analogous juvenile hormone system and no receptors for juvenile hormone or its chemical mimics, so we can metabolize juvenile hormone mimics in much the same way that we metabolize the fats and cholesterol in our food. Likewise, chitin synthesis inhibitors block the production of chitin, a crucial component of the arthropod exoskeleton. Blocking chitin production has no effect on mammals, because mammals do not produce chitin in the first place. Bt affects the digestive system while indoxacarb affect the nervous system (something that both insects and mammals possess), but each requires specific digestive enzymes that are only found in insects to transform the protein in the pesticide into its toxic form. To a mammal whose stomach pH and digestive enzymes are not the proper match for the pesticide, it is digested the same way as any other protein.

How are pesticides regulated?

Pesticides are regulated through the EPA under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA). Since its initial creation, FIFRA has had extensive amendments, additions, and revisions, and has been the primary guiding law on pesticide regulation in the US since the 1940s. FIFRA established a pesticide registration process whereby a pesticide manufacturer must submit a pesticide label to the EPA, along with controlled studies documenting evidence that the pesticide is effective and safe for humans and the environment if used according to label instructions. If the evidence that the product is safe and effective is satisfactory, then the EPA registers the product and it can be sold and used. If the evidence is lacking, then registration is denied and the product cannot be sold. The entire registration process takes several years, and product testing typically costs millions of dollars. Testing is not done exclusively by the manufacturer (that would have conflict of interest written all over it); rather, the manufacturer will contract out independent researchers, often at US accredited universities, to test products for them. In my experience at a university, I participated in a minor role on a product test; the manufacturer paid up-front for the study so that funding was not conditional on results, and the data we generated was combined with data from many other researchers scattered over the country to maximize the representation of the testing and minimize researcher bias. Because the evidence of safety is generated under the conditions of “used according to label instructions,” the label instructions are enforced by law. Any application of a pesticide that is inconsistent with the label instructions is a violation of federal law, and carries unknown risks because such use of the product has not been tested.

Perhaps surprisingly, not all pesticides are subject to extensive EPA scrutiny. FIFRA 25(b) exempt products are those whose ingredients are all generally regarded as safe, and typically include primarily products that are based on botanical essential oils. In the view of the EPA, these products are considered “minimum risk pesticides”. While these products are indeed likely to be safe, their exemption from registration means that they also do not have to demonstrate efficacy. As a result, a number of FIFRA 25(b) products lack efficacy testing and may not be effective, even if used according to label instructions. For example, a number of FIFRA 25(b) exempt mosquito repellents are available, with many performing abysmally in CDC mosquito repellent tests. Additionally, all snake repellents currently available on the market are FIFRA 25(b) exempt products, and to my knowledge none of them have been shown to be effective.

Why are pesticides used in pest control in the first place?

If you dig through some history books (specifically Silent Spring by Rachel Carson), you can find some remarkably irresponsible and dangerous uses of pesticides. These horror stories from the past can make it tempting to suggest that pest management should be done entirely without pesticides. Even current research on pesticides’ effects on pollinators and other beneficial nontarget organisms raises some concerns. However, we do not currently have the technology to effectively manage pests without using pesticides in many situations. Life is all about risks and balancing those risks. We are constantly developing newer, safer, more specific pesticides that create a relatively low risk to humans and/or the environment. Abandoning these products, and the pests they control, would almost certainly result in tremendous loss of crops and livestock that would undercut our food supply, while also allowing mosquitoes, ticks, and other major disease vectors to wreak havoc unabated. As a society, we have elected to take the risks of using pesticides, which are steadily shrinking as pesticides become safer and more heavily regulated, rather than the risks of cutting pesticides out of pest management and losing control of some truly catastrophic pests.

This is an opportune point to mention organic certification; many people erroneously believe that “certified organic” means “pesticide free.” However, certified organic actually indicates that something is synthetic pesticide free, which is an important distinction. Many pesticides are based off of natural products, especially extracts from plants, fungi, and bacteria. The form that is extracted from a plant, fungus, or bacterium or that is mined from the earth may be used as a pesticide without violating an organic certification. However, chemists often make molecular tweaks to these products to make them more suitable for their use as a pesticide, which may include a longer or shorter life in the environment, increasing or decreasing repellency to the pest, making the pesticide more or less potent, or more specific, then synthesize the product in a lab rather than extracting it from a living thing. A synthetic version may even be chemically identical to the substance produced in nature and synthesized in a lab to reduce costs or environmental impact. The point is, if a pesticide is synthetic it is incompatible with organic certification; if it is harvested, it may be used in certified organic. There is no consistent correlation between whether a pesticide meets organic criteria and the toxicity of the pesticide. However, from following the work of a colleague, Joe Ballenger

While pesticides have a firmly entrenched position in pest control, the past half-century has seen a tremendous boom in integrated pest management (IPM), which uses pesticides as just one part of more comprehensive pest management. By combining many techniques into a comprehensive pest management strategy, pest management professionals can achieve more effective and more long-term solutions to pests, usually with long-term financial savings compared to a pesticide-centric pest management strategy.

The greatest barrier to IPM is its complexity. To devise an effective IPM plan, a pest manager needs to really know the pest and use its biology against it. In many cases, a pest manager also does not have complete control over what needs to be done to effectively manage a pest. We require client cooperation to correct the conditions that invite the pests in and allow them to flourish. The more a client works with a pest controller to get an infestation under control, the faster the pest will be suppressed, the less return visits the pest controller will need to make, and the longer the pests will stay controlled. If a client refuses to cooperate (for example, by cleaning up trash that attracts flies and provides a breeding site), then the pest manager may be able to knock down some of the pests with insecticides, but the pests will return – often quickly, and potentially with resistance to future pesticide treatments.

If pesticides are unavoidable, how worried should we be?

Thanks to the heavy regulation the EPA puts on pesticides, we generally do not need to be too concerned about pesticides as long as they are applied according to label rates. That being said, some pesticides have a little more potential to cause harm than others if they are misapplied or if an accidental exposure occurs. As a general rule of thumb, the closer the relationship between the pest and the nontarget life-form you are worried about, the greater the risk. Herbicides used to control weeds may be able to harm desirable plants, miticides used to control parasitic mites in beehives are hard on the bees, and rodenticides have a mode of action that works on humans similarly to target rodents. That being said, the dose makes the poison; a rodenticide might have enough toxicant to kill a mouse, but its effects would be far less on a human that is 2000 times as big. Pesticides can also be applied in ways that minimize non-target exposures. Continuing with the rodenticide example, properly applied rodenticide baits are applied so that children, pets, etc. cannot access them. At most, these non-targets can come in contact with a bait station that came in contact with a gloved hand that handled the rodent bait. By comparison, the mouse that the bait is controlling walks into the bait station and eats a full meal of the rodenticide. In this case, the much smaller pest is exposed to a much greater amount of pesticide in a manner that easily allows the pesticide to enter the bloodstream, whereas the much larger person or pet is exposed to traces of pesticide, most likely on the skin which provides some degree of a barrier.

A pesticide label also provides clues on how dangerous a pesticide is. Unless a pesticide meets standards of extremely low mammalian toxicity, it will have a signal word listed on the label. The signal words “Danger-Poison,” “Warning,” and “Caution” each directly reflect the amount of the pesticide that it would take to kill a person, with “Danger-Poison” requiring little to kill, “Warning” requiring an intermediate amount to kill, and “Caution” requiring a large exposure to kill. A signal word of “Danger” indicates a killing power comparable to “Caution” or “Warning,” but with an added risk of strong skin or eye irritation. All pesticides are potentially dangerous and should be treated with respect, but the pests they are used to control often threaten health, property, or food, so the use of pesticides is a weighed risk of controlled pesticide application against uncontrolled pest pressure.

Take-home messages:

Pesticides are a firmly entrenched component of pest management, and current technology would be incapable of providing satisfactory pest control without them. They are one of many valuable tools in the toolbox of IPM, and are applied as needed. Pesticides have been misused in the past, but between developing much safer pesticides and the testing and regulations imposed on pesticides, they are far safer and more specific now than they once were. If an appropriate pesticide is used according to label directions and treated with respect, there is very little reason to fear it.