Spotted lanternfly iNaturalist rates peaked 3-4 years after county establishment and then fell in 351 US counties, 2014-2025; agency density classes fell only within units

Question: do local spotted lanternfly (Lycorma delicatula) populations increase and then decrease after they establish (boom-bust)? Earlier support came mainly from iNaturalist photographs, which can decrease when observers lose interest. Approach: we used two independent data streams in the same US counties. Stream 1 is the GBIF iNaturalist research-grade SLF record count divided by all insect records (51,860 SLF records, 4,064 county-years). Stream 2 is the density class (Low, Medium, High) in 76,037 established agency field-survey records from lydemapr v4.1.0. Time is years since the agency-recorded county establishment (351 counties in 19 states, 2014-2025). We fitted Poisson and binomial event-time models with county fixed effects (within unit) or year fixed effects (within year), and used 1,000 county-cluster bootstrap replicates. Findings: the iNaturalist rate increased to a peak 3-4 years after establishment. Then it decreased by -1.17 log units within counties (95% CI -1.36 to -0.96; 51% of the rise) and by -0.61 within calendar years (-0.97 to -0.31). The decrease stayed with raw counts, with a Hemiptera denominator, in early cohorts and without 2025. A placebo (other Hemiptera) did not change. An agency abundance index also had a peak at 3-4 years and decreased within counties (-0.46, -0.85 to -0.06; 47% of its rise, the same share as iNaturalist) and within 10-km cells (-1.14, -1.54 to -0.69). Within agency-years the density decrease was not resolved (-0.21, -0.63 to 0.20), but it was clear without 2025 data. Within counties, the iNaturalist rate followed agency density only weakly (elasticity 0.10). Limitations: density classes are coarse and surveys are targeted and include control treatments. The late window has few early counties, mostly in Pennsylvania and New Jersey. Counted from the first SLF record of any type, the decrease was smaller and later. We do not identify the cause.

spotted lanternfly · Lycorma delicatula · biological invasion · boom-bust dynamics · iNaturalist · GBIF · citizen science · event study · lydemapr · invasive species · population dynamics · reporting bias