Many eukaryotic hosts are associated with microbes that enhance their fitness. The establishment and maintenance of such relationships likely depends on the faithful transmission of beneficial microbes across generations. However, in some cases the microbes are not maternally transmitted, but are environmentally acquired. Thus, when hosts disperse to new habitats or switch to a new diet, they may not have access to the appropriate microbes, leading to reduced fitness. In such cases, can the host establish new microbial partnerships, and are they specific? More generally, how are microbial communities assembled, and (how) does host association influence this process? In this talk, I will discuss our efforts to address these questions in different natural and laboratory insect populations.