Microbiota can play critical roles in some host species, but may have facultative, less-obvious roles in other host taxa. A factor that can contribute to the impact of gut microbiota is host diet. Not only can host diet vary in nutritional content, but also in potent secondary metabolites. Moreover, for species such as Lepidoptera, diet plays a substantial role in structuring gut microbiomes and influences their establishment. There are several outstanding questions about the role of host-microbe interactions in lepidopteran species, especially related to how they may be benefiting their hosts. In this presentation, I discuss research conducted using Noctuid caterpillars, particularly focusing on larval fall armyworm (Spodotpera frugiperda) and beet armyworm (Spodoptera exigua). In a series of manipulative experiments using axenic hosts, the influence of diet on diet was a key contributor whether gut microbiota incurred costs or benefits. Where the establishment of gut microbiota was beneficial to one host insect, under identical conditions it was detrimental to the other. Unexpectedly, different bacterial isolates yielded unique responses to the hosts. Finally, the timing of host-microbe pairing was critical, as the either beneficial or detrimental impacts by bacteria were amplified in early larval instars and persisted across development. Potential mechanisms and ecological ramifications will be discussed.