Neonicotinoids: a dangerous harvest

Neonicotinoids (Neonics) are a class of synthetic insecticides that were first marketed in the mid 1990s.1 The adoption of neonics by the agricultural industry was so rapid and pervasive that neonics are the most widely applied pesticides today. Use in North American agriculture is substaintial.2,3 For example, the US plants ~90 million acres of corn annually.4 Between 79-100% of this cropland was treated with neonics in 2011, with neonicotinoid use projected to increase in corn and other crops.3

Two factors strongly contribute to the popularity of neonics: 1. Their selective mode of action against insects; and 2. Their systemic protection of plants. Neonics poison the insect nervous system by binding to nicotinic acetylcholine receptors, causing paralysis and death.1 Structural differences in analogous vertebrate receptors make neonics far less toxic to vertebrates. Neonics are also water soluble. Plants absorb neonics through their vascular system into every root, shoot, leaf and flower. Neonics are even present in pollen and nectar. This universal absorption protects plants against herbivorous insects. Because neonics are such powerful insect neurotoxins, very small concentrations (5-10 parts per billion) in plant tissues confer protection.1

Neonics are applied to plants in many ways (soil drenches, irrigation water, foliar sprays, trunk injections). However, neonics are most commonly applied as powdered seed coatings.3 Germinating plants absorb neonics from the coating. Seed coatings violate science-based Integrated Pest Management (IPM) principles.1,3,5 IPM guidelines recommend applying pesticides only when pests are present at damaging levels and other control methods have failed. Seed coatings preemptively assume a pest problem. Thus, seed treatments can be an unnecessary expense that increase environmental chemical loads and selection pressures for resistant pests.1,3

Pollinators can be exposed to neonics in many ways. The high neonic levels in dust that is generated during seed planting can kill flying pollinators. Modifications to seed drilling machines reduce but do not eliminate dust emission. Pollinators may also be exposed by eating contaminated pollen and nectar. The lower neonic concentrations in pollen and nectar usually do not kill pollinators outright, but a consuming a diet laced with neurotoxins has sublethal effects for pollinators.6

Tractor and Field

Most of the studies examining sublethal effects in pollinators use honey bees as a model organism.6 However, newer studies describe effects in several wild bee species. Preliminary data suggest that the larger average size of bumble bees may confer some protection, while smaller solitary species may suffer greater harm from exposure. Sublethal effects in bees vary by species and study, and examples include lower brood production, failure to build nests or building fewer nests, reduced foraging and impaired immunity. Sublethal effects can be amplified by other stressors, especially disease, poor nutrition and exposure to other pesticides. Ultimately, neonic exposure decreases bee survival and reproduction. Less is known about sublethal effects in other pollinators, including butterflies, moths and flies.

Neonics are environmentally persistent, and drifting applications may contaminate adjacent terrestrial and aquatic habitats.6 Only 1.6-20% of neonics in seed coatings are absorbed by plants. A small percent of the coating is released as dust, and the remainder is leached into soil or water runoff. Neonic movement and persistence in soil or water are modified by many factors such as pH, UV exposure and temperature. Half-life estimates of neonics in soil range from ~200-1,000 days. This long breakdown period allows neonics to accumulate in agricultural soils with each application until levels plateau after 2-6 years. Of concern, neonics have sublethal effects on soil-dwelling organisms like earthworms7 and can even shift the soil microbiome composition.8 Neonic persistence in water is considerably shorter, with half-life estimates ranging from minutes to weeks. Numerous surface water surveys have detected neonics in water bodies adjacent to fields. Contamination of adjacent habitats has negative implications for the health of associated insects, other invertebrates and the animals that rely them as a food source.

Neonics have many uses outside of agriculture.1,6 Neonics are used in residential and business landscaping (e.g., yards, golf courses). Neonics are also applied to pets to control fleas and to treat buildings with insect infestations. However, agriculture's massive footprint on North America and associated neonic usage eclipses other applications. Worryingly, recent history has seen huge decreases in insect numbers across North America and Europe, with local declines and extinctions reported in multiple insect taxa.9,10 Global warming, habitat loss, invasive species and agricultural chemicals all contribute to these declines. Insects are integral and vital components in complex ecosystems, and their disappearance will have cascading effects. Scientists have already linked declines in insectivorous bird species to neonicotinoid use.11 Environmental stewardship is necessary not only for the preservation of the natural world, but for our continued survival. Neonics and other pesticides have a role to play in pest management, but humans must exercise strategic caution in their use.

How can you help reduce neonicotinoid use and pollinator exposure?

  1. Create safe pollinator habitat. If you are creating pollinator habitat, ensure that the seeds or plants your purchase have not been treated with neonicotinoids. If you are unsure, ask an employee. Local nurseries are often better able to track plant origins and growing conditions than chain stores.
  2. Listen to an expert. Read an article by or watch an interview of Dr. David Goulson, a leading expert on the environmental risks of neonicotinoids to pollinators and other nontarget organisms.
  3. 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.
  4. 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
  5. 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.
  6. 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.
  7. Participate in citizen science. Help scientists determine the effects of pesticides on pollinators by participating in The Great Sunflower Project.
  8. 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. Goulson, D. An overview of the environmental risks posed by neonicotinoid insecticides. J. Appl. Ecol. 50, 977–987 (2013).

  2. Main, A. R. et al. Widespread Use and Frequent Detection of Neonicotinoid Insecticides in Wetlands of Canada's Prairie Pothole Region. PLoS One 9, e92821 (2014).

  3. Douglas, M. R. & Tooker, J. F. Large-Scale Deployment of Seed Treatments Has Driven Rapid Increase in Use of Neonicotinoid Insecticides and Preemptive Pest Management in U.S. Field Crops. Environ. Sci. Technol. 49, 5088–5097 (2015).

  4. United States Department of Agriculture. National statistics for corn. National Agricultural Statistics Service (2020). (Accessed: 30th September 2020)

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

  6. 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).

  7. Basley, K. & Goulson, D. Effects of chronic exposure to clothianidin on the earthworm Lumbricus terrestris. PeerJ 5, e3177–e3177 (2017).

  8. Zhang, P., Ren, C., Sun, H. & Min, L. Sorption, desorption and degradation of neonicotinoids in four agricultural soils and their effects on soil microorganisms. Sci. Total Environ. 615, 59–69 (2018).

  9. Sánchez-Bayo, F. & Wyckhuys, K. A. G. Worldwide decline of the entomofauna: A review of its drivers. Biol. Conserv. 232, 8–27 (2019).

  10. Simmons, B. I. et al. Worldwide insect declines: An important message, but interpret with caution. Ecol. Evol. 9, 3678–3680 (2019).

  11. Li, Y., Miao, R. & Khanna, M. Neonicotinoids and decline in bird biodiversity in the United States. Nat. Sustain. (2020). doi:10.1038/s41893-020-0582-x