Field and laboratory studies were conducted to evaluate the use of bacterial growth on aquatic insects as a metric for determining the existence of nutrient impacts in wetlands. Results from field investigations indicated that elevated concentrations of nitrate and phosphate were associated with growth of filamentous bacteria on insect body surfaces and that there were significantly fewer mayflies (Ephemeroptera) in the nutrient-enriched wetland. Laboratory investigations confirmed a strong linkage between bacterial growth and reduced survival of mayflies. Survival was examined for individuals with bacterial infestation ranging from 0% to 60% body coverage. A threshold for catastrophic mortality was present at about the 25% level of coverage; there were very few survivors above that level. Based on these findings, the diagnostic endpoint for the bioindicator is 25% body coverage by bacterial growth, a level that signifies major differences in insect populations in the field and is also easy to detect visually. This study provides evidence that the insect-bacteria bioindicator is a reliable tool for assessing nutrient impacts on wetland macroinvertebrate communities. The bioindicator could be useful in the development of a Wetland Bioassessment Protocol.