Posted: November 10, 2020
By definition and design, insecticides kill insects. The term “pest” is generally defined by a human-driven need and desire to eliminate an unwanted organism within a range of contexts. In agriculture, there will always be a need to protect crops from insect pest pressures.
Image: https://www.bayer.com/en/innovation/nauen
Today, we widely rely on the application of man-made insecticides, which has led to gradual and significant improvements to the production and quality of domesticated crops since about the 1940's [1]. However, widespread use of insecticides can frequently lead to the accumulation of contaminants in non-pest insects, including beneficial pollinators. While we are still learning about what the long-term, behavioral impacts of such exposure has on insect communities, studies suggest that plant systemic insecticides – although they drastically reduce direct exposure to sucking pests - can adversely affect pollinator health and reproduction [2].
Plant systemic insecticides are a common type of pesticide where insecticidal chemicals are taken up by and translocated to the shoots and leaves of plants, which makes their tissues lethal or damaging to agricultural pests [3]. Some systemic insecticides you might recognize are the neonicotinoid insecticides, so-called because their mode of action is similar to nicotine, which is a naturally insecticidal product, but highly toxic to vertebrates, whereas neonicotinoids are 10,000-fold more selective to insects [4]. Research has shown that systemic insecticides can be found not just in the leaf tissue of plants, but also in the pollen and nectar of their flowers [5]. As such, many have posed that pollinating insects like bees, flies and some beetles could be inadvertently harmed by the use of these insecticides [2].
One thing we have learned over the years is that not all insecticides are equally toxic, and importantly, not all insects are equally susceptible to exposure. For example, honey bees respond differently to imidacloprid and thiacloprid [6], which are two commonly used and chemically similar neonicotinoids [7]. Additionally, the combined effects of multiple synthetic chemicals in the agricultural industry may lead to unexpected or synergistic outcomes on insect health and reproduction [8]. Efforts to understand insect metabolic systems, by evaluating their underlying molecular framework, might help us to predict how toxicity and susceptibility to chemical exposure varies within and between species. By using molecular approaches to learn and predict insect resistance/tolerance to different chemicals, scientists hope to inform and advise on pest management strategies that sustain and promote beneficial pollinators in agroecosystems, while keeping pests at bay.
Dr. Ralf Nauen is a Fellow of the Entomological Society of America, a Bayer Distinguished Science Fellow, and an insect toxicologist and biochemist in research and development at the Crop Science Division of Bayer AG in Monheim, Germany. His research is dedicated to identifying the molecular pathways responsible for differential sensitivities of pollinators to insecticides. One 2018 study led by Nauen and others discovered marked differences in sensitivity to imidacloprid and thiacloprid by honey bees and bumble bees [9]. His team found that a family of genes called the cytochrome P450 monooxygenases, or 'P450s', are responsible for the variability they saw. P450s comprise part of an evolutionarily conserved molecular defense mechanism that fights potentially harmful compounds, including plant secondary compounds (such as nicotine) and agrochemicals [6]. While all insect species have P450s, the combination of P450's that they have vary [10]. This means that each bee species might respond differently to the same dose and/or combination of chemicals based on the P450s they have.
Another priority of Nauen's work is to understand and test the risk of insecticide resistance developing in various pest insects. Over many generations, some pest populations will slowly evolve a tolerance to pesticide formulations and doses that used to kill them [11]. When that happens, it prevents growers from using that chemical to protect their crop from pest damage. In a recent publication [12], Nauen and colleagues shared that it can be challenging to develop cost-effective insecticides which maintain efficacy against the target pest insect, while also being environmentally safe [13]. To address these challenges, he and his collaborators developed a decision support tool called 'Fly-Tox', which is a panel of manipulated live fruit fly lines that contain a variety of P450 genes in them. By testing candidate insecticides active against these different lines, researchers can now rapidly screen novel insecticides for sensitivity, toxicity, and resilience across multiple species. Additionally, it may serve as a platform to test for potentially harmful pesticide-pesticide interactions.
Undeniably, systemic insecticides such as neonicotinoids can and do affect pollinator health and are under discussion to have even contributed to the decline of some species [14]. However, the need to protect one's crops from pests necessitates effective pest management applications. The significant increases in crop yield provided by synthetic pesticides renders them invaluable to agriculture, thus it is the work of scientists like Ralf Nauen that enable companies which market these chemicals to ensure that they are also invaluable to pollinator management.
By Sean Bresnahan (MCIBS)
References
[1] Popp, J, Petö, K., Nagy, J. (2012). Pesticide productivity and food security. A review. Agronomy for Sustainable Development. 33, 243-255
[2] Sponsler, D. B., Grozinger, C. M., Hitaj, C., Rundlöf, M., Botías, C. et al. (2019). Pesticides and pollinators: A socioecological synthesis. Science of The Total Environment. 662: 1012-1027
[3] Vryzas, Z. (2016). The plant as metaorganism and research on next-generation systemic pesticides – prospects and challenges. Front. Microbiol. fmicb.2016.01968
[4] Jeschke, P., Nauen, R., Schindler, M., Elbert, A. (2011). Overview of the status and global strategy for neonicotinoids. J. Agric. Food Chem. 59(7): 2897-2908
[5] Mörtl, M., Vehovszy, Á, Klátyik, S., Takács, E., Györi, J., Székács, A. (2020). Neonicotinoids: spreading, translocation and aquatic toxicity. Int. J. Environ. Res. Public Health. 17, 2006
[6] Iwasa, T., Motoyama, N., Ambrose, J. T., Roe, M. (2004). Mechanism for the differential toxicity of neonicotinoid insecticides in the honey bee, Apis mellifera. Crop Prot. 23, 371-378
[7] Cressey, D. (2017). The bitter battle over the world's most popular insecticides. Nature. 551, 156-158
[8] Samantsidis, G-R., Panteleri, R., Denecke, S., Kounadi, S., Christou, I., Nauen, R., Douris, V., Vontas, J. (2020). 'What I cannot create, I do not understand': functionally validated synergism of metabolic and target site insecticide resistance. Proc. Royal. Soc. B. 287(1927)
[9] Manjon, C., Troczka, B. J., Zaworra, M., Beadle, K., Randall, E. et al. (2018). Unravelling the molecular determinants of bee sensitivity to neonicotinoid insecticides. Curr. Biol. 28(7): 1137-1143
[10] Feyereisen, R. (2018). Toxicology: bee P450's take the sting out of cyanoamide neonicotinoids. Curr. Biol. 28(9): R560-R562
[11] Li, X., Schuler, M. A., Berenbaum, M. R. (2007). Molecular mechanisms of metabolic resistance to synthetic and natural xenobiotics. Annu. Rev. Entomol. 52: 231-253
[12] McLeman, A., Troczka, B. J., Homem, R. A., Duarte, A., Zimmer, C. et al. (2020). Fly-Tox: a panel of transgenic flies expressing pest and pollinator cytochrome P450s. Pesticide Biochem and Phys. 169: 104674
[13] Sparks, T. C. (2013). Insecticide discovery: an evaluation and analysis. Pesticide Biochem and Phys. 107(1): 8-17
[14] Sánches-Bayo, F., Wyckhuys, K. A. G. (2019). Worldwide decline of the entomofauna: a review of its drivers. Biological Conservation. 232: 8-27
