The more insects there are, the more food there is for insectivores and the higher the likelihood of insect-associated ecosystem services. Yet, we lack insights into the drivers of insect biomass over space and seasons, both for tropical and temperate zones. We used 245 Malaise traps, managed by 191 volunteers and park guards, to characterise year-round flying insect biomass in a temperate (Sweden) and a tropical (Madagascar) country. Surprisingly, we found that local insect biomass was similar across zones. In Sweden, local insect biomass increased with accumulated heat and varied across habitats, while biomass in Madagascar was unrelated to the environmental predictors measured. Drivers behind seasonality partly converged: In both countries, the seasonality of insect biomass differed between warmer and colder sites and wetter and drier sites. In Sweden, short-term deviations from expected season-specific biomass were explained by week-to-week fluctuations in accumulated heat, rainfall, and soil moisture, whereas in Madagascar, weeks with higher soil moisture had higher insect biomass. Overall, our study identifies key drivers of the seasonal distribution of flying insect biomass in a temperate and tropical climate. This knowledge is key to understanding the spatial and seasonal availability of insects — as well as predicting future scenarios of insect biomass change.

To describe the large-scale distribution of flying insect biomass, we sampled insects during one year in a temperate (Sweden; latitude 55.3 to 69.1) and a tropical (Madagascar; latitude -25.6 to -12.0) country, across the full latitudinal and longitudinal gradient of each country. Sweden and Madagascar are comparable in their surface area, covering ~450,000 and ~590,000 km² respectively. The climate in Sweden ranges from oceanic to sub-Arctic, and in Madagascar from tropical humid to dry tropical.

Insects were collected with Malaise traps, and preserved in 95% ethanol. In Sweden, 195 Malaise traps were set out between January and December 2019. Samples from Malaise traps were collected monthly or bi-weekly during winter and autumn (approx. October to February, depending on latitude) and weekly during spring and summer (approx. March to September, depending on latitude). In Madagascar, 50 Malaise traps were set out from August 2019 to July 2020. Samples from Malaise traps were collected every week during the entire year, and about 75% of the catch was processed further. The collection dates of the selected samples for each trap were evenly distributed across the year. Hence, the selected samples provided us with weekly to biweekly data on insect biomass in Madagascar. 

Insect biomass from each sample was obtained following the protocol described in Iwaszkiewicz-Eggebrecht et al. (in revision). All samples were drained to remove excess ethanol. The insect biomass in the sample was then weighed to the nearest 0.001 gram, and the average bottle weight was subtracted from this value.