Pesticides and pollinators: the good, the bad and the unknown

Pesticides are substances used to kill pests. Pests are living organisms that interfere with human activity. Any animal, plant, fungus or microbe can be a pest in the appropriate context. While admired in the wild, raccoons that raid your trash bins, weeds that invade your garden, wasps that nest above your front door and squirrels that take up residence in your chimney are all considered pests.

There are many ways to classify pesticides. Depending on their target, pesticides can be categorized as insecticides (for killing insects), herbicides (for killing weeds), fungicides (for killing fungi), and so on. Pesticides can also be labeled as organic or synthetic. Organic and natural pesticides are naturally occurring substances like plant oils or mineral formulations that are noxious to pests.1 Synthetic pesticides are manmade compounds, though many are inspired by natural, toxic substances.2 However, synthetic pesticides are generally modified to be more potent or environmentally persistent. For example, pyrethroids are a group of synthetic insecticides whose chemical formula is based on pyrethrin, a pesticides derived from chrysanthemums.3 Since all pesticides are ultimately designed to be lethal, a chemical's designation as synthetic or organic does not necessarily mean that it is more or less toxic.

A pesticide's specificity, its persistence and its ability to move through an environment following application all determine how it will affect target pests and nontarget organisms. Broad and indiscriminate pesticide use has negative environmental consequences.

Pesticides: the good

When used properly, pesticides are important Integrated Pest Management (IPM) tools.4 IPM is a cost-effective, science-based approach that relies on multiple strategies to deter, detect and control pests.5 Under an IPM framework, pesticides are used when other control measures have failed, and pesticides are only applied when the costs of pest damage are likely to exceed the costs of pesticide application. This minimizes the exposure of humans and other nontarget organisms. Furthermore, integrating pesticide applications with other management strategies reduces the chances of pests becoming resistant.

Pesticides are also key habitat conservation and restoration tools. For example, glyphosate is an herbicide used to kill weeds. The agricultural sector's extensive glyphosate use is linked to the loss of 'weedy habitat' in fields, especially milkweed, contributing to monarch butterfly declines.6 However, glyphosate is the most commonly applied herbicide used in restoration projects.7 Removing invasive or nonnative plants with herbicides combats weed competition in native plantings and contributes to restoration success.

Pesticides: the bad and the unknown

Beneficial, nontarget organisms like pollinators are exposed to pesticides, including insecticides, herbicides and fungicides, while foraging on agricultural or other chemically managed lands. Though herbicides and fungicides are not applied for insect control, these chemicals can still disrupt vital molecular processes in insects. Pesticides contain many chemicals: active ingredients and additives.8 Active ingredients are toxic and intended to kill target pests. Additives help pesticides evenly adhere to surfaces or penetrate insect cuticles. Additives are assumed to be “inert" and are not environmentally regulated like active ingredients. However, research shows that some additives alone can harm pollinators.

Ideally, pesticide applications are timed to limit the direct exposure of pollinators (e.g., when plants are not blooming). Infamous misapplications, like the Wilsonville Bee Kill, have resulted in extensive pollinator causalities.9 If not killed by pesticide exposure, pollinators can suffer sublethal harm when they encounter lingering chemicals in pollen and nectar.

Numerous sublethal effects of pesticide exposure have been reported in honey bees. These include and are not limited to shortened lifespan, compromised immunity, diminished foraging ability, and impaired navigation.10 The capacity for sublethal effects to interact with other stressors makes sublethal effects truly dangerous for pollinators. For example, immunocompromised bees are more susceptible to infection. Likewise, disoriented bees do not return to their nest, reducing the hive's productivity and chances of survival. The molecular structure of many pesticides makes them stable in fatty environments, like pollen and wax comb. Pesticides, and even beekeeper applied chemicals, can accumulate in colony comb over time, chronically exposing the hive to chemical cocktails with unknown effects.11,12 Though the available data about sublethal effects in pollinators is biased towards honey bees,13 other pollinators (e.g., bumble bees, solitary bees, butterflies, moths, etc.) show varying sensitivities.10

Pesticides are generally designed to persist on target plants, but pesticides can contaminate neighboring habitats through aerial drift, underground leaching and runoff. As these chemicals and their potentially toxic breakdown products accumulate in agricultural lands and adjacent habitats, scientists are documenting troubling repercussions, especially with the prophylactic use of a new class of insecticides, the neonicotinoids. Research and advocacy are needed to ensure that we use pesticides as part of a sustainable IPM plan to protect off-target habitats and organisms.

How can you help reduce pesticide use and pollinator exposure?

  1. Use IPM. Whether you are managing several acres of cropland or a raised-bed urban garden, use IPM to manage your pest problems. Try other pest management strategies before reaching for a pesticide bottle. If you have questions about the best IPM strategy for your pest problem, contact your local university extension program. To learn more, read these articles: What is integrated pest management? and Integrated Pest Management Tactics.
  2. Select pesticides carefully. If you must apply chemicals, determine if there are less persistent and more targeted options. To learn more, read these articles: Are you thinking about using pesticides?; Understanding synthetic, natural, organic and chemical pesticides; Organic insecticides; IPM Tactic Chemical Control; Less harmful pesticides; and Pesticides and pollinators
  3. Follow each pesticide's label instructions. Do not overapply chemicals. Time treatments to when plants are not blooming and thus less attractive to pollinators. If you must apply chemicals to blooming plants, apply treatments at night when pollinators are usually less active. Read more about applying chemicals to blooming fruit plantings.
  4. Elevate your environmental aesthetics. Recognize that “weedy" lawns or few bugs in your garden may represent a healthier habitat for pollinators than pristine landscapes devoid of biodiversity. You can install and certify habitat as pollinator friendly. Post educational signage to inform others.
  5. Participate in citizen science. Help scientists determine the effects of pesticides on pollinators by participating in The Great Sunflower Project.
  6. Lend pollinators your voice. Tell policy makers that research investigating off-target and sublethal effects in pollinators and other nontarget organisms is important.

References

  1. Guiser, S. Organic pesticides. Penn State Extension (2015). (Accessed: 29th September 2020)

  2. Knauss, N. Understanding synthetic, natural, organic and chemical pesticide designations. Penn State Extension (2014).

  3. United States Environmental Protection Agency. Pyrethrins and pyrethroids. Ingredients used in pesticides. (Accessed: 20th September 2020)

  4. Rajotte, E. What is Integrated Pest Management? Penn State Extension (2011). (Accessed: 29th September 2020)

  5. Rajotte, E. Integrated Pest Management (IPM) tactics. Penn State Extension (2011). (Accessed: 29th September 2020)

  6. M, P. J. & Oberhauser, K. S. Milkweed loss in agricultural fields because of herbicide use: effect on the monarch butterfly population. Insect Conserv. Divers. 6, 135–144 (2013).

  7. Kettenring, K. M. & Adams, C. R. Lessons learned from invasive plant control experiments: a systematic review and meta-analysis. J. Appl. Ecol. 48, 970–979 (2011).

  8. Mullin, C. A., Chen, J., Fine, J. D., Frazier, M. T. & Frazier, J. L. The formulation makes the honey bee poison. Pestic. Biochem. Physiol. 120, 27–35 (2015).

  9. Xerce. The Wilsonville bee kill. Latest news (2013).

  10. Wood, T. J. & Goulson, D. The environmental risks of neonicotinoid pesticides: a review of the evidence post 2013. Environ. Sci. Pollut. Res. 24, 17285–17325 (2017).

  11. Mullin, C. A. et al. High Levels of Miticides and Agrochemicals in North American Apiaries: Implications for Honey Bee Health. PLoS One 5, e9754 (2010).

  12. Grozinger, C. M., Fleischer, S. & Dupont, T. Pesticides and Pollinators. Penn State Extension (2016). (Accessed: 29th September 2020)

  13. Lundin, O., Rundlöf, M., Smith, H. G., Fries, I. & Bommarco, R. Neonicotinoid Insecticides and Their Impacts on Bees: A Systematic Review of Research Approaches and Identification of Knowledge Gaps. PLoS One 10, e0136928 (2015).