Systemic insecticides are often used to attack herbivorous insect species, but widespread use of these chemicals is linked to serious effects on ecosystems.

Photo: Nancy Adamson

As a group, insecticides are perilous for insect life, including bees and other beneficial insects. Those insecticides designed to permeate plants from within—systemic insecticides—move through plants and may be present in all tissues after application, including pollen and nectar, posing unique risks for pollinators.

Given their widespread use, Xerces decided to offer an easily accessible reference list of the insecticides currently registered  in the U.S. that are known to—or possess the potential to—exhibit systemic movement in plants. With this reference, you can search for and retrieve information about these chemicals, such as their toxicity to bees, their persistence, the strength of their systemic activity, and the sites and crops where systemic insecticides can be legally used.

GO TO SEARCHABLE SYSTEMIC INSECTICIDES LIST

 

What are Systemic Insecticides? 

Systemic pesticides (whether insecticides, fungicides, herbicides or other pesticides) are absorbed by and transported through plants. Systemic insecticides can render some or all of a plant toxic to insects that feed on plant tissue. Thus, they are commonly used to suppress herbivorous sucking or chewing insects like aphids, caterpillars, and root nematodes. Unfortunately, systemic insecticides harm both target insects and non-target beneficial insects.

Systemic insecticides include neonicotinoids, which have been widely recognized for their risk, in part because they are far more toxic to bees than most other insecticides, and are also very persistent. However, nearly 40 other systemic insecticides are in use in the U.S., including many newly approved chemicals that are not as commonly known.

 

What Risks do Systemic Insecticides Pose? 

Systemic insecticides contaminate plant tissues from the inside, potentially reaching pollen, nectar, leaves and stems. Therefore, the chemicals can be consumed not just by pests but also by bees, larval (juvenile) and adult butterflies, and the many beneficial predators and parasitoids that eat pollen or nectar as adults. 

Many systemic insecticides are toxic enough to kill adult or larval (juvenile) honey bees, bumble bees, and/or solitary bees at very low concentrations. In addition, researchers have documented effects including impaired reproductive capacity, flight, navigation, learning, immune response, and more in bees exposed to various systemic insecticides at concentrations frequently detected in crops or other treated plants (for example, see Cecala 2021, Siviter and Muth 2020, Pisa et al. 2017, Hopwood et al. 2016, and Smagghe et al. 2013). These subtle yet harmful effects, often termed “sublethal,” can render insects more vulnerable to disease and other stressors, weakening populations over time.

When toxicity tests don’t result in significant mortality to honey bees, people often assume a chemical is safe for all bees and other pollinators, such as butterflies and moths. But this assumption can be drastically incorrect, as made clear by researchers who tested monarch butterfly caterpillars with several neonicotinoids and the relatively new systemic, chlorantraniliprole. Chlorantraniliprole turned out to be deadly to monarch caterpillars at leaf tissue concentrations dramatically lower (up to 1000-fold) than the neonicotinoid concentrations causing mortality (Krishnan et al. 2020, Krishnan et al. 2021). This points to the need for more data on impacts to native bees, butterflies, and moths, since studies show some are even more sensitive to insecticides than honey bees, the standard test species (Arena and Sgolastra 2014).

Furthermore, when applied to the soil, systemic insecticides may migrate into plant tissue over time. Some systemic insecticides have even been detected inside plants years after application. This phenomenon appears to be especially common in woody plants, but has also been shown with milkweed grown near where fipronil was applied years earlier (Halsch et al. 2020).  Accordingly, use of systemic insecticides creates a potential for ongoing toxic exposure to bees and other beneficial insects long after an application.

Even insects that do not feed directly on treated plants may be affected. A recent study showed that the systemic insecticides flonicamid and pymetrozine can contaminate honeydew (a sugar-rich sticky liquid, secreted by some insects as they feed on plant sap), killing beneficial insects that feed on honeydew (Calvo-Agudo et al. 2020). An earlier study found similar results with the use of imidacloprid, thiamethoxam, and spirotetramat (Calvo-Agudo et al. 2019).

Another notable concern is that systemic insecticides tend to be water-soluble and prone to runoff and leaching from treated sites. Seed treatments on widely planted crops such as corn, soybean, wheat, and cotton are a major source of widespread contamination in streams and rivers, where systemic insecticides in concentrations harmful to aquatic life have been repeatedly documented. These and other soil treatments may also contaminate the nests of ground-nesting bees and put seed-eating birds in harm's way.

 

Are Systemic Insecticides More Risky for Pollinators than Non-Systemic Insecticides?

Designed to kill insect pests, it is perhaps unsurprising that insecticides as a group are risky for pollinators. Systemic insecticides are intended to kill via ingestion of plant tissue contaminated from the inside over time, while non-systemics are designed to kill via contact with or ingestion of surface residues shortly after application. Some insecticides can be applied to cause either a contact or an ingestion (oral) exposure so there is overlap between these groups.

Both systemic and non-systemic insecticides can be lethal to pollinators, or cause sub-lethal deleterious effects. The differences are more about where and when the insecticide is present in or on plant tissue and how that affects exposure, as outlined below.

 

 

Because systemic chemicals remain in the plant tissue, sometimes for lengthy periods of time, application methods often recommended to minimize contact to bees, such as spraying at night or applying outside of the flowering season, could still allow harmful exposures.

 

Which Systemic Insecticides Are Riskiest?

Evaluating the risk that any individual pesticide poses to bees—and whether one pesticide is riskier than another—is complex. Considerations include: will bees die if exposed when the pesticide is applied at the label rate? If they don’t die, will exposure to the pesticide result in poorer health for individual adult bees, their offspring, or a colony as a whole? How robust is the science for any particular chemical?

The most common toxicity metric—testing the amount of chemical that causes 50% of the test subjects to die—is  often compared from one chemical to another. Theoretically then, the lower the amount of a chemical that causes 50% of test bees to die, the more toxic it is, and by inference, the more risky to all pollinators. Similarly, the longer it takes for a chemical to break down, the more persistent it is and the more likely pollinators and other insects are to come into contact with it (be exposed to it).

In our searchable systemic insecticides list, Xerces does not determine which are riskiest. However, we do include information on the toxicity and persistence for each listed insecticide with category levels (such as high and low) assigned to the displayed value, following systems developed by the EPA and by the National Pesticide Information Center. Category systems make it easier to compare chemicals, but by their nature are a rough cut. This should be kept in mind.

Toxicity and persistence values were drawn from studies submitted to pesticide regulatory agencies or published in the scientific literature. Different studies may result in a range of toxicity or persistence values. We chose to display the most conservative value reported for each insecticide that was deemed acceptable by the EPA or the University of Hertfordshire Pesticide Properties Database (PPDB). This is consistent with EPA’s risk assessment methodology where they model risk based on the most conservative values for toxicity and persistence.

• The most conservative value for toxicity is the lowest concentration found in studies to kill 50% of the test bees over a short exposure time (LD50).
• The most conservative value for persistence is the longest time reported or the value used by EPA in its risk assessment (for half-life in days) for the pesticide to dissipate or to break down under aerobic conditions.  

Keep in mind that these are imperfect measures.  Assessing toxicity by the LD50 is a very blunt measure that fails to take into account the numerous subtle concerns that are part of risk. Insecticides frequently adversely affect reproduction, growth, insect immune systems, learning, flying, or other attributes even at concentrations too low to cause death outright. These kinds of effects can weaken populations over time, even if death does not occur. Our dataset does not show which chemicals are most likely to result in these “sublethal” effects at typical environmental concentrations.

To understand toxicity it is also important to recognize that some native bee species have been demonstrated to be more sensitive to certain insecticides than honey bees in a number of studies, while others have been shown to be less sensitive. Furthermore, some insecticides may transform into compounds that are also toxic as they break down. Additionally, pesticides usually occur as mixtures (more than one chemical present) in the world. Exposure to more than one chemical at a time can amplify effects.

The soil half-lives we report are also not perfect or static measures of persistence. For example, the length of time it takes for a given insecticide to disappear from soil may differ depending on weather, temperature, soil texture, pH, and other conditions. In addition, persistence in the soil may not be an accurate measure of persistence inside plants. Unfortunately, we lack a robust data set on persistence in pollen, nectar, and even leaves, whereas soil persistence is a standard test required during the pesticide registration process. In the case of systemics applied via the soil, inferring plant persistence based on soil persistence is reasonable, since the soil may represent an ongoing source to leaf, pollen, and nectar tissue. It’s less clear how soil persistence predicts plant tissue persistence when systemics are applied to the foliage, via injection, or via bark spray. We recognize as well that native bees, most of whom nest in the ground, may also be exposed to soil residues in ways that honey bees are not.

These are just a few of the limitations on the state of the science in understanding the risk of systemic insecticides to pollinators. As science progresses, new data is often published on chemicals that sheds new light on their toxicity or persistence. As a result, we are always learning more. Therefore, the data in our systemic insecticides reference is intended to inform, but stops short of conclusions about risk.   

How Does Translocation Work?

Just as humans have arteries and veins that circulate nutrients and waste products around the body in the bloodstream, plants also move water and nutrients through a system of vessels. You've seen these before; the linear patterns in wood grain consist of old hardened xylem vessels.

Xylem vessels transport water upwards from the roots to the leaf canopy. Phloem vessels transport sugars (made during photosynthesis) to where they are needed, including to young leaves, nectar, and seeds. To be transported inside the plant (translocated), a systemic pesticide must first be absorbed, then cross into the xylem or the phloem to be distributed elsewhere in the plant. Exchange between the xylem and phloem also occurs but is poorly understood. 

Some insecticides (for example the neonicotinoid dinotefuran and the organophosphate fosthiazate) are reliably systemic, with high percentages of the applied chemical consistently translocated (Namiki et al. 2018). This can be measured by comparing shoot to root concentrations under a soil application scenario. Other insecticides, such as spinosad, exhibit modest systemic activity, with relatively low percentages of the applied chemical translocated. 

Therefore, systemic potential should be understood as occurring along a continuum, depending on a variety of factors including plant species (Orita 2012, Gierer et al. 2019) and physical properties, such as how water soluble, acidic, and lipophilic (fat-loving) the pesticide is (Orita 2012, Bonmatin et al. 2015, Mineau 2021). These physical properties were used by Mineau (2021) to develop an index (“Relative Index of Systemic Activity”) which predicts the relative strength of systemic transport for several hundred active ingredients. Active ingredients with index scores higher than 1.0 would be predicted to be more likely to translocate than three well-known neonics; active ingredients with scores lower than 1.0 would be expected to show lesser potential for movement. See field definition for more detail. 

The searchable systemic insecticides list that Xerces has compiled includes all currently U.S. registered insecticides for which translocation is well-documented. Also included are a handful of insecticides that are not identified as systemic by the US Environmental Protection Agency (EPA) or in pesticide manufacturer marketing materials. However, these have been included where there appears to be strong potential for translocation based on the chemical’s properties. A few others are also not widely regarded as systemic but are included if one or more studies have demonstrated at least modest systemic activity; this is the case with chlorpyrifos, for example. “Translaminar” or locally systemic insecticides, which penetrate leaves but generally do not move to the rest of the plant, are not included in the table. Other insecticides may show limited translocation in some crops under some conditions, but if the degree of translocation in available studies was very slight, they were not included. The use of surfactants or co-formulants in pesticide products or mixes may increase absorption and subsequent translocation in some instances.

 

Where and How Are Systemic Insecticides Applied?

Systemic insecticides are used against a wide variety of insects, mites, and nematodes. Many of the active ingredients are approved for use on hundreds of crops. Nursery and greenhouse plants, landscape plantings, trees, and turf, and non-crop sites (such as animal feeds and Christmas trees), are also commonly treated with systemic insecticides. 

Some systemic insecticides are applied so that they are absorbed through the roots; from there they spread into above-ground plant tissues through the xylem vessels. Seed treatments, chemigation, soil granules, soil drenches, and soil injection are typical application methods for such xylem-mobile chemicals. Trunk injections can also send insecticides directly into the xylem, and are commonly used on orchard crops and woody plants produced in nurseries or grown for landscaping. Some systemics are also applied to trees through basal bark sprays. Xylem-mobile insecticides can be applied to plant foliage, but this method may result in less translocation, due partly to barriers to uptake through the leaves as well as to the removal of the insecticide from leaves by rain, dew, and mist. 

Phloem-mobile systemics are typically applied as foliar sprays; after absorption, they move through the phloem vessels, potentially reaching young leaves, roots, nectar and/or seeds.

 

Additional Resources

Take a deeper dive into the ecological impacts of neonicotinoids at Xerces’ Understanding Neonicotinoids webpage.

Learn simple steps for Buying Bee-Safe Plants. 

Read about how Insecticide Seed Treatments Threaten Midwestern Waterways.

Browse research summaries featuring systemic insecticides and other pesticides at Xerces Impacts of Pesticides to Invertebrates database. 

Learn more about the Effects of Neonicotinoid Insecticides on Agriculturally Important Beneficial Insects, by the Xerces Society.  

Read about some of the general concerns posed by the use of systemics in this 2013 open-access paper by Sanchez-Bayo, Tennekes and Goka.

 

References

Arena, M. and F. Sgolastra. 2014. A meta-analysis comparing the sensitivity of bees to pesticides. Ecotoxicology 23(3):324–334.

Bonmatin, J-M., C. Giorio, V. Girolami, D. Goulson, D. P. Kreutzweiser, C. Krupke, M. Liess, E. Long, M. Marzaro, E. A. D. Mitchell, D. A. Noome, N. Simon-Delso and A. Tapparo. 2015. Environmental Fate and Exposure; Neonicotinoids and Fipronil. Environmental Science and Pollution Research International 22(1):35–67. https://doi.org/10.1007/s11356-014-3332-7.

Calvo-Agudo, M., J. González-Cabrera, Y. Picó, P. Calatayud-Vernich, A. Urbaneja, M. Dicke, and A. Tena. 2019. Neonicotinoids in Excretion Product of Phloem-Feeding Insects Kill Beneficial Insects. Proceedings of the National Academy of Sciences of the United States of America 116(34):16817–16822. https://doi.org/10.1073/pnas.1904298116.

Calvo-Agudo, M., J. González-Cabrera, D. Sadutto, Y. Picó, A. Urbaneja, M. Dicke, and A. Tena. 2020. IPM-Recommended Insecticides Harm Beneficial Insects through Contaminated Honeydew. Environmental Pollution 267:115581. https://doi.org/10.1016/j.envpol.2020.115581.

Cecala, J. M., and E.E. Wilson Rankin. 2021. Pollinators and Plant Nurseries: How Irrigation and Pesticide Treatment of Native Ornamental Plants Impact Solitary Bees. Proceedings. Biological Sciences / The Royal Society 288(1955):20211287. https://doi.org/10.1098/rspb.2021.1287.

Faske, T., J. Mueller, and K. Bissonnette. How seed-applied nematicides work. Crop Protection Network. https://cropprotectionnetwork.org/publications/how-seed-applied-nematicides-work 

Gierer, F., S. Vaughan, M. Slater, H.M. Thompson, J. S. Elmore, and R.D. Girling. 2019. A Review of the Factors That Influence Pesticide Residues in Pollen and Nectar: Future Research Requirements for Optimising the Estimation of Pollinator Exposure. Environmental Pollution 249:236–247. https://doi.org/10.1016/j.envpol.2019.03.025. 

Hopwood, J., A. Code, M. Vaughn, D. Biddinger, M. Shepherd, S. Hoffman Black, E. Lee-Mader, and C. Mazzacano. 2016. “How Neonicotinoids Can Kill Bees.” 2nd Edition. The Xerces Society. https://www.xerces.org/wp-content/uploads/2016/10/HowNeonicsCanKillBees_XercesSociety_Nov2016.pdf.

Mineau, P. 2021. “Analyses around the Issue of Systemic Activity and Hazard to Pollinators.” Report to the Xerces Society. 32 pp.

Namiki, S., T. Otani, Y. Motoki, N. Seike, and T. Iwafune. 2018. Differential Uptake and Translocation of Organic Chemicals by Several Plant Species from Soil. Journal of Pesticide Science 43(2):96–107. https://doi.org/10.1584/jpestics.D17-088.

Orita, N. 2012. “Root Uptake of Organic Contaminants into Plants: Species Differences.” Edited by William J. Doucette. M.S., Utah State University.

Pisa, L., D. Goulson, E. Yang, D. Gibbons, F. Sánchez-Bayo, E. Mitchell, A. Aebi, J. van der Sluijs, C.J.K. MacQuarrie, C. Giorio, E.Y. Long, M. McField, M.B. van Lexmond & J. Bonmatin. 2017. An Update of the Worldwide Integrated Assessment (WIA) on Systemic Insecticides. Part 2: Impacts on Organisms and Ecosystems. Environmental Science and Pollution Research International. https://doi.org/10.1007/s11356-017-0341-3.

Siviter, H., and F. Muth. 2020. Do Novel Insecticides Pose a Threat to Beneficial Insects? Proceedings. Biological Sciences / The Royal Society 287(1935):20201265. https://doi.org/10.1098/rspb.2020.1265.

Smagghe, G., J. Deknopper, I. Meeus, and V. Mommaerts. 2013. Dietary Chlorantraniliprole Suppresses Reproduction in Worker Bumblebees. Pest Management Science 69(7):787–791. https://doi.org/10.1002/ps.3504.