Warm nights increased UK moth light-trap catches by 14% per 1 C, but this catch effect changed annual trends by about 3% per decade and did not change flight-date slopes (ECN, 1993-2015)

Question: do warmer sampling nights bias insect trap indices and phenology slopes? Trap counts depend on abundance and on activity, and climate change makes sampling nights warmer. Data: UK Environmental Change Network nightly light-trap moth catches (48,763 single nights, 11 sites), fortnightly pitfall carabid catches (6,833 samples, 12 sites) and hourly on-site weather, 1992-2015. Method: Poisson models with site x trap x year x week fixed effects isolate night-to-night weather effects. We then removed this catch effect from annual indices and from catch-weighted mean flight dates. Results: moth catch increased by 14.4% per +1 C of night temperature (95% CI 13.3 to 15.6%), at all 10 sites with enough data, in both 1993-2003 and 2004-2015 and in 39 of 40 species. Wind, rain, clear skies and moonlight decreased catches. Carabid pitfall catch increased by 20% per +1 C of interval temperature. Sampled nights warmed by 0.42 C per decade, but the temperature artefact in annual moth trends was only +2.8% per decade (95% CI -1.0 to +6.9%; raw +15.3%, adjusted +12.2%) and explained about 8% of year-to-year index variance. The carabid artefact was +0.3% per decade. A warmer pre-season moved mean flight dates 4.5 days earlier per +1 C (35 species); the catch adjustment changed this by -0.12 days per +1 C (95% CI -0.46 to +0.14). Conclusion: night weather is a large source of noise in single samples but a small bias in annual trends and phenology slopes at these sites. Limitations: 10 moth series, sensor steps in wind and net radiation, within-week effects can include short-term emergence, and raw ECN moth trends are not national trends.

moths · light trap · carabid beetles · pitfall trap · observation bias · detectability · catchability · night temperature · phenology · insect decline · Environmental Change Network · weather · climate change · fixed-effects Poisson · Rothamsted light trap