Costa Rica caterpillar - Danielle Salcido

Moths are the most taxonomically and ecologically diverse insect taxon for which considerable abundance and distribution data exist in various parts of the world. A new paper by the universities of Connecticut and Nevada, in collaboration with Butterfly Conservation, presents new analyses of caterpillar abundance data for Arizona (USA), Costa Rica and Ecuador, and reviews the global evidence for moth biodiversity change1.

The world is facing a biodiversity crisis with an estimated one million species threatened with extinction2. However, the trends of insects, which make up at least 60% of all species on Earth3 and contribute vital and valuable ecosystem services, such as decomposition, pest control and pollination, remain poorly understood.

In the past few years, media reports of impending ‘insect Armageddon’, the catastrophic loss of insects across the whole world, has caused great public concern. Much of this media coverage stemmed from studies that have themselves been heavily criticised by the scientific community4. Global and continental assessments for insects (and other invertebrates) have generally shown overall declines in the abundance of terrestrial species5,6, but the available evidence is heavily biased towards Europe and North America. Thus, while there is considerable reason for concern about insect declines, the data are insufficient to support extrapolated conclusions about the scale of the worldwide decreases across all insect groups7.

The new paper, part of a special issue of Proceedings of the National Academy of Sciences of the USA, addresses the question of what studies of moths can tell us about the reality of insect decline. The key finding is that patterns of moth biodiversity change are highly heterogeneous, both spatially and taxonomically, yielding a complex overall picture. For example, while many European studies have reported overall declines, trends can vary considerably, even within countries, and summary results conceal a multitude of different responses by species. Assessments carried out in the Netherlands8 and Great Britain9, for example, both documented a minority of moth species that had increased in abundance against a general backdrop of decline. Such ‘winners’ are ecologically and taxonomically diverse and include long-term resident moth species as well as recent colonists10. The new paper found no studies in which all moth taxa were declining. The few time-series of moth population data outside of Europe reflect the same heterogeneity, with major decreases in moth caterpillar abundance and diversity at a site in Costa Rica1, but no overall decline at monitored sites in Ecuador1 or in the US states of Arizona1 and Missouri11.

In addition, sometimes there are conflicting signals from different measures of change (such as abundance and distribution). In Finland, for example, moth species’ richness has increased but abundance has declined12, while in Hungary, moth diversity has decreased but total abundance has not13. The latest assessment of macro-moths in Britain found that while four times as many species had decreased in abundance as had increased over a 50-year period, moths had, on average, increased in distribution9.

Based on the evidence of moth trends, the new paper considers the relative importance of drivers of change and the potential knock-on effects on other organisms, ranging from the hyper-diverse Hymenoptera parasitising the immature stages of moths to Grizzly Bears feeding on aestivating Army Cutworm (Euxoa auxiliaris) moths. However, the authors conclude that current knowledge about moth biodiversity change is insufficient to dissect the impacts of numerous potential drivers or to generalize trends across communities, countries, and biomes. The evidence we do have for moths suggests highly complex and heterogenous patterns of change, rather than cataclysmic decline, but more data are needed, especially from the tropics, where the majority of moth diversity resides.

The lack of data needed to make global assessments of biodiversity change for moths should not delay action, however. Large-scale habitat destruction, diffuse pollution and accelerating climate change pose major threats to the richness and abundance of moth communities and require an immediate response from individuals, organisations and governments to safeguard biodiversity.

Richard Fox

Associate Director Recording and Monitoring, Butterfly Conservation


  1. Wagner, D.L., Fox, R., Salcido, D.M. & Dyer, L.A. (2021) A window to the world of global insect declines: moth biodiversity trends are complex and heterogeneous. Proceedings of the National Academy of Sciences of the USA 118:e2002549117. doi:10.1073/pnas.2002549117
  2. Díaz, S., et al. (2019) Pervasive human-driven decline of life on Earth points to the need for transformative change. Science 366:eaax3100. doi:10.1126/science.aax3100
  3. Eggleton, P. (2020) The state of the World's insects. Annual Review of Environment and Resources 45:61–82. doi:10.1146/annurev-environ-012420-050035
  4. Saunders, M.E., Janes, J.K., & O’Hanlon, J.C. (2020) Moving on from the insect apocalypse narrative: Engaging with evidence-based insect conservation. BioScience 70:80–89. doi:10.1093/biosci/biz143
  5. Dirzo, R., Young, H.S., Galetti, M., Ceballos, G., Isaac, N.J. & Collen, B. (2014) Defaunation in the Anthropocene. Science 345:401–406. doi:10.1126/science.1251817
  6. Van Klink, R., Bowler, D.E., Gongalsky, K.B., Swengel, A.B., Gentile, A. & Chase, J.M. (2020) Meta-analysis reveals declines in terrestrial but increases in freshwater insect abundances. Science 368:417–420. doi:10.1126/science.aax9931
  7. Montgomery, G.A., Dunn, R.R., Fox, R., Jongejans, E., Leather, S.R., Saunders, M.E., Shortall, C.R., Tingley, M.W. & Wagner, D.L. (2020) Is the insect apocalypse upon us? How to find out. Biological Conservation 241:108327. doi:10.1016/j.biocon.2019.108327
  8. Groenendijk, D. & Ellis, W.N. (2011) The state of the Dutch larger moth fauna. Journal of Insect Conservation 15:95–101. doi:10.1007/s10841-010-9326-y
  9. Fox, R., et al. (2021) The State of Britain’s Larger Moths 2021. Butterfly Conservation, Rothamsted Research and UK Centre for Ecology & Hydrology, Wareham, Dorset, UK.
  10. Boyes, D.H., Fox, R., Shortall, C.R. & Whittaker R.J. (2019) Bucking the trend: the diversity of Anthropocene ‘winners’ among British moths. Frontiers of Biogeography 11:e43862. doi:10.21425/F5FBG43862
  11.  Marquis, R.J., Lill, J.T., Forkner, R.E., Le Corff, J., Landosky, J.M. & Whitfield, J.B. (2019) Declines and resilience of communities of leaf chewing insects on Missouri oaks following spring frost and summer drought. Frontiers in Ecology and Evolution 7:396. doi:10.3389/fevo.2019.00396
  12. Antão, L.H., Pöyry, J., Leinonen, R. & Roslin, T. (2020) Contrasting latitudinal patterns in diversity and stability in a high‐latitude species‐rich moth community. Global Ecology and Biogeography 29:896–907. doi:10.1111/geb.13073
  13. Valtonen, A., Hirka, A., Szőcs, L., Ayres, M.P., Roininen, H. & Csóka, G. (2017) Long‐term species loss and homogenization of moth communities in Central Europe. Journal of Animal Ecology 86:730–738. doi:10.1111/1365-2656.12687