Declines in insect abundance and richness are now documented even from protected tropical forest ecosystems. Long-term studies from our lab, in lowland rainforest streams in Costa Rica, indicate that insect richness is decreasing. A decrease in insects could affect the rate of ecosystem processes, unless other organisms compensate for their decline. Fortunately, we have information on leaf litter decomposition and insect colonization from over 20-30 years ago that serves as a reference line to evaluate the effects on the decomposition process. The main objective of this study is to assess whether the observed regional decline in aquatic insects results in changes in insect colonization and decomposition rates of leaf litter. Our work was conducted at La Selva Research Station in Costa Rica. We ran a leaf decomposition study in seven streams along a natural phosphorus gradient. We placed replicated fine and coarse-mesh bags of Ficus insipida and Castilla elastica leaves, both used in studies during the 1990s and 2000s. We determined mass loss, decomposition rates, and quantified insect richness, abundance, and biomass. Although there has been an observed decline in aquatic insects at La Selva, decomposition rates have not changed from the 1990s or 2000s to our study results in 2025. The response to the P gradient was also similar to the one described in previous studies. For example, Ficus insipida leaves in coarse-mesh bags had decomposition rates 33.18% faster under high P, similar to previous reports. Insect assemblages colonizing leaves had lower richness than reports from the 1990s and 2000s and were dominated by larval Chironomidae. In contrast, previous studies report more diverse assemblages, dominated by chironomids, caddisflies, and other taxa. Our study provides critical insights into the consequences of insect diversity loss on a major, understudied ecosystem process in these globally significant habitats.