Neonicotinoids not only act as an insecticide, but also disrupt the embryonic development of frogs. This has been documented in several studies by a research team at the Institute of Biochemistry and Molecular Biology (iBMB) at Ulm University. Tadpoles that were exposed to various neonicotinoids in pure form in laboratory experiments had smaller bodies and eyes, malformed cranial nerves and altered heart structures. The research results show that insecticides authorised in the EU also have a harmful effect on amphibians and provide evidence of a possible link between the poisons and global amphibian extinction. In earlier studies, the research team had already demonstrated similar effects of the herbicide glyphosate.
Neonicotinoids are used worldwide as pesticides. The active ingredients attack the nervous system of insects and are sprayed in agriculture to control pests. Over-the-counter products such as anti-insect sprays and collars against fleas and ticks for pets may also contain neonicotinoids. In three consecutive studies, the most recent of which has now been published in the Journal of Current Research in Toxicology, the researchers from Ulm investigated the extent to which the active substances have negative effects on amphibians.
In laboratory experiments, the scientists exposed embryos of the South African clawed frog to five neonicotinoids at an early stage of development: acetamiprid, thiacloprid, imidacloprid, thiamethoxam and clothianidin. The neonicotinoids were each used as a single substance and in different concentrations. Low concentrations corresponded to those already measured in natural waters; high concentrations simulated extreme cases, such as those that could arise from illegal discharges into water bodies. After several days, the researchers checked the embryos for damage to tissue and organs and analysed mobility and heartbeat as well as certain genes in order to identify the molecular causes of defects.
Diverse negative effects
"Compared to the control group, all five neonicotinoids showed negative effects, albeit to varying degrees," reports one of the first authors, Dr. Hannah Flach from the iBMB. All neonicotinoids caused smaller eyes and altered head cartilage. In solutions containing acetamiprid and thiacloprid, the tadpoles exhibited reduced body length and head edema. A high concentration of thiacloprid led to smaller brains and altered cranial nerves; the latter were also observed with a high concentration of imidacloprid and clothianidin. Cardiac oedema and altered heart rates were also common. In addition, there were changes in mobility, including complete immobility of the tadpoles. The embryos also showed disturbed gene activity very early in development.
"We have thus demonstrated that the neonicotinoids we analysed can cause significant damage not only to insects, but also to amphibian embryos. Damage to the eyes in particular occurred even at relatively low concentrations, which have already been measured in nature," says Professor Susanne Kühl. The head of the study points out that only a few measured values exist for the typical ponds in which amphibians grow up. "In order to be able to assess the potential risks of neonicotinoids in nature in real terms, many more measurements would be needed in small bodies of water close to agricultural fields."
The research team's work stands out from other studies on the effects of pesticides in its level of detail. "Our results also show that not only the pesticides themselves, but also their degradation products can cause damage. We are observing very complex processes that should be better taken into account in authorisation procedures," says Kühl.
Risks also possible for other vertebrates
"By focusing on the embryos of the South African clawed frog, an established model organism in developmental biology, we can also link the observed effects to global amphibian extinction," emphasises the study leader. Thiacloprid is no longer authorised in the EU; imidacloprid, thiamethoxam and clothianidin may be used in closed greenhouses, although emergency authorisations are still possible in open areas within the EU. Acetamiprid, on the other hand, has EU authorisation until 2033 and is widely used as an active ingredient in products such as sprays for houseplants and roses as well as in products against flies, ants and fleas. Neonicotinoids have already been detected in foods such as honey and fruit as well as in the human body, for example in sperm. "In view of the proven harmful effects, risks to other vertebrates cannot be ruled out," emphasises Kühl, who advocates taking a global view of the problem, as the use of neonicotinoids is not restricted outside the EU. "Pesticides jeopardise biodiversity through their global use. The extinction of amphibians is the greatest extinction of vertebrate species."
Further information:
Prof. Dr Susanne Kühl, Institute of Biochemistry and Molecular Biology, University of Ulm, e-mail: susanne.kuehl(at)uni-ulm.de
Publication references:
Flach, H., Brendler, C., Schöpf, M., Xu, L., Schneider, J., Dewald, K., Dietmann, P., Kühl, M., Kühl, S. (2024). Comparing the effects of three neonicotinoids on embryogenesis of the South African clawed frog Xenopus laevis. Current Research in Toxicology 6/2024
https://doi.org/10.1016/j.crtox.2024.100169
Kerner, M., Flach, H., Dietmann, P., Kühl, M., Kühl, S. (2023). The impact of the insecticide acetamiprid on the embryogenesis of the aquatic model organism Xenopus laevis. Environmental Toxicology and Pharmacology Volume 103, October 2023
https://doi.org/10.1016/j.etap.2023.104278
Flach, H., Geiß, K., Lohse, K., Feikert, M., Dietmann, P., Pfeffer, S., Kühl, M., Kühl, S. (2023). The neonicotinoid thiacloprid leads to multiple defects during early embryogenesis of the South African clawed frog (Xenopus laevis). Food and Chemical Toxicology, Volume 176, June 2023
https://doi.org/10.1016/j.fct.2023.113761
Text: Christoph Karcher
Translated with DeepL