Until now, it was known that deworming agents can affect individual species: seeds of typical grassland plants, for example, change their germination behavior when they come into contact with deworming products. Beetle and fly larvae that live in the dung of treated grazing animals show lower survival rates. How these effects play out in the interaction between multiple species had not previously been studied.
Livestock farmers use deworming agents to keep their animals healthy and to farm economically, so doing without these products is generally not an option. The product studied contains the active ingredient moxidectin, which belongs to the group of macrocyclic lactones — long-acting antiparasitic agents. Animals' bodies break the substance down only to a small extent. As a result, a large proportion of it ends up in the soil via the dung of treated animals — or when farmers fertilize their fields with slurry or solid manure.
Fewer fruits, more aphids
For their study, the researchers built a simplified food chain model, combining three species: the cultivated strawberry, the green peach aphid, and the marmalade hoverfly. The strawberry plant served as food for both insects. The aphid fed on plant sap, while the hoverfly used pollen and nectar from the flowers as an adult and, in its larval stage, preyed on the aphids. The team placed the animals and plants in small, insect-proof cages. Half of the cages contained strawberry pots with soil contaminated with the deworming agent, while the other half contained uncontaminated control plants. The researchers then recorded how many fruits the plants produced, how many aphids appeared on the plants, and how many eggs the hoverflies laid.
All three species responded to the deworming agent: under its influence, the strawberry plants produced roughly a quarter fewer fruits, and the aphids multiplied tenfold more — but only when no hoverfly larvae were present. One possible explanation is that the deworming agent weakens the plant's defenses against aphids. The hoverflies also responded: females that visited flowers of treated plants laid four times as many eggs. However, these eggs were more variable in size and appeared visually less viable. On balance, this could reduce the reproductive success of the hoverflies.
"We were surprised by how far-reaching the effects of a single product can be on the interplay between plant and insect," says Dr. Andrés García. "This shows that when assessing veterinary medicines, we need to look not just at individual species, but at entire ecological relationships."
It remains unclear exactly how the deworming agent triggers these effects. Further studies are intended to determine what role the active ingredient moxidectin, the other ingredients, and their breakdown products play in these effects — and how they are passed along the food chain. The study's authors call for complex ecological relationships between species to be given greater consideration in the future when examining the environmental effects of veterinary medicines.
Andrés García, Tim Diekötter, Jitin Sabu, Laura Olivia Greene, Lilli Kuhtz, Tobias W. Donath, and Carsten Eichberg recently published their findings in the journal Science of the Total Environment. Lead author Andrés García conducted the study as part of a Georg Forster Research Fellowship from the Alexander von Humboldt Foundation.
Contact:
Dr. Andrés García
Abt. Landschaftsökologie
Institut für Natur- und Ressourcenschutz
Christian-Albrechts-Universität zu Kiel
Mail: agarcia@ecology.uni-kiel.de
Dr. Carsten Eichberg
Abt. Geobotanik
Raum- und Umweltwissenschaften
Universität Trier
Mail: eichberg@uni-trier.de
