The biomass of flying insects has declined by a dramatic 76% in western German protected areas, according to a recent publication by Caspar A. Hallmann et al. (2017) PLoS ONE.
Over a 27 year period, from 1989 to 2016, biomass of insects captured in malaise traps at 96 unique location-year combinations across a wide variety of habitat types dropped precipitously through time (see their Fig 2 above). While the decline does not appear to be explained by weather, or by broad scale changes in land use categories or habitat characteristics, the authors speculate that it may be due to source-sink dynamics between the protected areas where they sampled and agricultural areas. For example, agricultural areas may serve as an ‘ecological trap’ for insects originating in the protected areas.
While the amount of arable land has has not increased in western Germany during the study period (in fact it has declined slightly), agricultural practices have changed considerably. Perhaps changes in insecticide use, decreases in weed biodiversity, or a decline in field margins are at least partially to blame.
Because the decline in insect biomass is seen across a variety of habitats and across seasons, the authors suggest that some large-scale factors must be driving the trend.
Unfortunately, the decline documented by Caspar et al. (2017) fits with other observations for invertebrates worldwide.