Over evolutionary time, repeated interactions between hosts and microbes within the environment can lead to stable symbiotic associations. Environmental microbes can profoundly influence host behavior, physiology, and evolutionary trajectories. These interactions can lead to intimate symbiotic relationships often fundamental to the ecological and evolutionary success of many organisms. Such partnerships can enable hosts to exploit new niches and adapt to diverse environments. How interactions with environmental microbes can influence the ecology and evolution of host and microbe is critical to our understanding of symbiosis and the dynamics that shape symbiotic outcomes. This research examines the evolutionary and ecological significance of interactions between hosts and microbes in two distinct insect systems: burying beetles (Nicrophorus spp.) and pea aphids (Acyrthosiphon pisum). First, I investigate how diverse fluorescent phyllosphere-inhabiting Pseudomonas affect the behavior of pea aphids. Second, I survey the fungi and bacteria associated with burying beetles, carrion feedings insects that rely on microbes to preserve carcasses. Third, I use phylogenetic approaches to understand the diversity and evolution of Yarrowia symbionts in various Nicrophorus species. Together, these investigations provide insights into how environmental microbes impact host behavior and evolutionary dynamics, from initial ecological interactions to symbiotic associations.