Long-term research, individual studies and the Red Lists all tell the same story: the numbers and diversity of insects are heading downhill. Plugging the gaps in the data will do nothing to change this conclusion.
Entomological research – the study of insects – is not normally the stuff of headlines. But one ﬁnding hit the news around the world: in parts of Germany, more than 75 percent of the ﬂying insects have disappeared. The study, published in October 2017, is based on data from the Entomologischer Verein Krefeld (Krefeld Entomological Society).
Members of this society had studied the occurrence of ﬂying insects over a period of 27 years in over 60 locations, mostly in protected areas in the state of NorthRhine Westphalia. Although the study has been criticized for methodological shortcomings, it provides long-term data on the populations of whole groups of insects – data that had never before been collected anywhere. The data came from various parts of Germany and show a clear trend.
Apart from the Krefeld research,other long-term studies have been conducted in Germany on the populations of butterﬂies, wild bees and cicadas. They all show a decline in the number of species, and partly also a dramatic fall in the population density of the insects enumerated. In the case of butterﬂies, this mostly affects the specialist species.
These include butterﬂies whose caterpillars depend on particular food plants. Long-term counts in several parts of Germany reveal permanent losses in over 70 percent of such species. Almost half of the 561 wild bee species are in decline. Apart from habitat loss, the widespread use of highly effective neonicotinoid insecticides may have contributed to the marked drop in These bee populations. On the Swabian Jura, a range of hills in southern Germany, the number of nests of the common furrow-bee, Lasioglossum calceatum, shrank by 95 percent over a period of 46 years.
In the Isar River ﬂoodplains in Dingolﬁng, Bavaria, three-quarters of wild-bee species have disappeared in just 10 years. Other groups of insects have also been decimated. Populations of cicadas on dry grasslands in eastern Germany have declined by 54 percent over a timeframe of 40 to 60 years. In wet grasslands in Lower Saxony, losses were as high as 78 percent.
Overall, the research in Germany shows that losses are not conﬁned to particular localities but occur across the country. Species with widely differing lifestyles and habitats are affected. By far the highest insect losses are in open parts of the landscapes. Such areas include arable land and meadows. According to an international team of researchers led by the Technical University of Munich, the insect biomass in grassland areas fell by two-thirds between 2008 and 2017.
During the same period, forests lost 40 percent of their insect biomass. The Red Lists are the most comprehensive collections of information on the threat status of individual species. The German Federal Agency for Nature Conservation has compiled them for the last 40 years. Continually expanded, they portray the population developments of around 15,000 insect species over a period of 50 to 150 years. That means they cover the situation of just under half the 33,000 insect species in Germany.
The gaps that exist in the data are because many species are difﬁcult to identify, and there are not enough specialists to document their numbers on a continuous basis. The Red Lists show that one in every two species covered is in decline. Only a small fraction of species – around two percent – are on the increase. The past few decades have seen particularly striking losses among ant species. More than 90 percent of the 107 species found in Germany are decreasing.
A large number of beetles have now been placed under protection because they are threatened. The lack of dung beetles can be determined from the condition of cow dung on the ﬁelds. In many locations they no longer degrade, leading to the formation of “concrete cowpats”. One of the causes is the presence of insecticide residues in animal-feed concentrate, which the livestock excrete along with their dung. These residues go on to kill the beneﬁcial beetles that rely on the dung.
In a new study, experts from Krefeld analyse the decline in terms of individual insect groups and species. For hoverﬂies – the most important pollinators besides bees – the number of specimens in a protected area in North-Rhine Westphalia sank from nearly 17,300 to around 2,700 between 1989 and 2014: a loss of 84 percent. Of the former 143 species, only 104 were found 25 years later.
The project Diversity of Insects in Nature protected Areas, or DINA, launched in 2019, is looking at the causes: over a period of four years, the scientists will study as closely as possible the factors that give rise to insect mortality. They will try to determine how important these factors are. The results will help set priorities so as to reverse the current trends.