In contrast to conventional organic insecticides, Bacillus thuringiensis israelensis (Bti) is a bacterium producing a toxin which is very effective against midges of the suborder Nematocera, i.e. mosquitoes. It is widely used in the Upper Rhine valley since 1970s to reduce nuisance caused by mosquitoes. The use of Bti is considered environmentally safe due to its specific mode of action. However, there have been contrasting results in the past aiming to evaluate the impact of Bti on the ecosystems.

Although Bti is not lethally toxic for non-target organisms, sublethal effects as well as indirect effects via the food web are not yet fully understood. Larvae of chironomidae, the non-biting midge, play a crucial role in aquatic food webs as they are the preferred food source of many predators, e.g. newt larvae. In this project in the DFG graduate school SystemLink we aim to uncover direct effects of Bti on frog larvae as well as indirect effects on frog and newt larvae via the food web. Twelve floodplain mesocosms at the Eußerthal Ecosystem Research Station (EERES) have been used to assess changes in the aquatic food web and dietary composition of newt larvae in a semi-field study approach.

As a part of this PhD project, Master student Priyanka Solanki aims to investigate the influence of different diets on the fitness of palmate newt larvae in a laboratory feeding experiment. The Palmate newts (Lissotriton helveticus) belong to the family Salamandridae, the more diverse family of European amphibians, inhabiting temperate small ponds as well as temporary ditches. Palmate newt larvae mostly feed on aquatic insect larvae or small crustaceans during their aquatic life stage. But the application of Bti may pose an issue for newt larvae since Bti reduces the abundance of non-biting midges, i.e. chironomids, of up to 50% which are an important food source for the amphibian larvae. Thus the use of Bti may induce a shift in the food web of carnivorous amphibian larvae. Since juvenile amphibians also act as a preferred food source for higher trophic levels and contribute to the link between aquatic-terrestrial food webs, the aim of the study is to determine the changes in food quality of newt larvae (fatty acid, protein content) as prey after a controlled diet.

After three Bti applications to the mesocosms, the benthos community was sampled to uncover possible changes in species diversity and abundance. Bachelor student Max Gerken will determine aquatic insect larvae and compare the benthos community composition in Bti-treated ponds with control ponds. At the same time, food web components in the ponds were sampled for stable isotope analysis (13C and 15N) in order to investigate changes in the food web after multiple Bti applications.

In a third experiment, direct sublethal effects of Bti on larvae of the European common frog under natural conditions will be determined using biochemical biomarkers. In scope of this experiment, Bachelor student Victoria Titze will investigate the effects on morphometrical changes and will discuss socio-economic aspects in a literature research.

More information on Bti and food web related effects can be found on the ecotoxblog.