1. Protective symbionts can provide effective and specific protection to their hosts. This protection can differ between different symbiont strains with each strain providing protection against certain components of the parasite and pathogen community their host faces. Protective symbionts are especially well known from aphids where, among other functions, they provide protection against different parasitoid wasps. However, most of the evidence for this protection comes from laboratory experiments.

2. Our aim was to understand how consistent protection is across different symbiont strains under natural field conditions and whether symbiont diversity enhanced the species diversity of colonizing parasitoids, as could be expected from the specificity of their protection.

3. We used experimental colonies of the black bean aphid, Aphis fabae, to investigate symbiont conferred protection under natural field conditions over two seasons. Colonies differed only in their symbiont composition, carrying either no symbionts, a single strain of the protective symbiont Hamiltonella defensa, or a mixture of three H. defensa strains. These aphid colonies were exposed to natural parasitoid communities in the field. Subsequently, we determined the parasitoids hatched from each aphid colony.

4. The evidence for a protective effect of H. defensa was limited and inconsistent between years, and aphid colonies harboring multiple symbiont strains did not support a more diverse parasitoid community. Instead, parasitoid diversity tended to be highest in the absence of H. defensa.

5. Symbiont conferred protection, although a strong and repeatable effect under laboratory conditions, may not always cause the predicted bottom-up effects under natural conditions in the field.

We sampled plants which we had planted in the field with aphids at the end of each block in 2018 or weekly in 2019 (see paper for experimental set up). We measured the length of each plant and counted the number of aphids either by counting aphids in groups of roughly five individuals (2018) or counting their exact number (2019). We also counted the number of mummies, irrespective of whether or not the parasitoid had already hatched. Unhatched mummies were collected in insect dishes for hatching and subsequent identification. We additionally kept the plants in the cellophane bags to be able to catch and determine any parasitoids that would hatch from mummies that only formed after collection. Lastly, we counted all ants and predators. For predators we recorded which order they belonged to and which stage they were in (i.e. egg, larvae, pupae, or adult). After they had hatched, we determined parasitoids under a stereo microscope. We identified primary parasitoids to the species level and secondary parasitoids at least to the genus level.

See readme file.