Interaction of insecticides and fungicides on honeybees and wild bees : Effects of sublethal concentrations of insecticides and fungicides on sucrose responsiveness, learning performance and foraging activity of honeybees and bumblebees

In recent decades, a global decline in insects has been observed, which includes many bee species, whose pollination services are essential for wild and cultivated plants. In contrast to the domesticated honeybee (Apis mellifera), which is maintained by beekeepers, it is mainly wild bees that have been suffering from this decline. Nevertheless, honeybees, just like wild bees, are exposed to many threats during their lifetime. Research focuses primarily on honeybees, while wild bees receive less attention. However, as there are several differences between honeybees and wild bees, it is not possible to simply transfer research results from one species to another. In my thesis, the honeybee (A. mellifera) was investigated as a model organism, while the bumblebee (Bombus terrestris) was studied as a representative of wild bees. There are many threats for pollinators such as climate change, the occurrence of parasites or the loss of food sources and nesting opportunities. But the use of plant protection products (PPPs) is one of the most important ones. Bees do not usually ingest a single PPP in the field but are exposed to a variety of substances. The toxicity of such mixtures is difficult to predict. It is already known that mixtures can lead to enhanced (synergistic) or weakened (antagonistic) effects compared to the individual PPPs. However, a summation of the individual effects (additive) is also possible as an effect of the mixture. In my thesis, the neonicotinoid Mospilan® (insecticide), the SBI (sterol biosynthesis inhibiting) fungicide Difcor® and the non-SBI fungicide Cantus® Gold were investigated in their individual effects and in combination. While a harmful effect is directly visible through increased mortality, sublethal effects are often underestimated. However, these can have serious consequences for the individual or the entire colony. For this reason, in addition to mortality, sucrose responsiveness, learning performance and flight activity of honeybees and bumblebees were investigated in my thesis. The honeybee was affected by the PPPs, as the insecticide led to effects on sucrose responsiveness and learning performance. However, in combination with an SBI fungicide, the treatment resulted in an antagonistic effect. In contrast, the same mixture led to a synergistic effect on honeybee flight activity. These effects illustrate that the impact of PPPs or their mixtures can be highly dependent on various factors. Looking at different behaviours, varying effects can occur because the responsible receptors in the brain, which often serve as a target for PPPs, can be differently expressed. Furthermore, additional stressors that occur in laboratory or field studies can have a further influence on the effects. To investigate possible effects on wild bees (here: B. terrestris), a new method of dividing one bumblebee colony into four quarters was developed, which enabled simultaneous treatment of a colony with four different solutions. The same behaviours were studied as in the experiments with honeybees. The bumblebees proved to be quite robust. Neither insecticide or fungicide alone nor the mixture had any effect on sucrose responsiveness, learning performance or flight activity. Bumblebees, like honeybees, are social insects and form colonies. In contrast, many other wild bees display a solitary lifestyle. Body size, food preference or activity time are also factors that differ between various wild bee species and make a direct comparison with the bumblebee difficult. My thesis showed that insecticides and PPP mixtures can affect honeybees. Additionally, it became clear that the research on the effects of PPPs is a complex field that can be additionally influenced by a variety of factors. Bumblebees seem to be more resilient to PPP exposure than honeybees. However, even if the bumblebee was handled as a representative of wild bees, the effects on solitary wild bees could differ significantly from the results of social bumblebees. Due to the observed effects and the possible interaction patterns of insecticides and fungicides, it remains essential to optimize risk assessment methods in order to minimize the hazard to honeybees and wild bees.