There is mounting evidence of widespread declines in the diversity and abundance of insects from
across the globe (Sánchez-Bayo and Wyckhuys 2019, Seibold et al. 2019, van Klink et al. 2020,
Wagner 2020). This gives a stark warning for the perilous state of biodiversity (Díaz et al. 2019), and
demonstrates that addressing the gap in knowledge of the status of insects is vital (Cardoso et al.
2020, Samways et al. 2020). Insects are estimated to comprise more than half of all described species
and are a dominant component of biodiversity in most ecosystems (Bar-On et al. 2018). Insects also
provide a crucial role in the functioning of ecosystems. They are not only related to the supply of
many ecosystem services such as pollination, biological control, soil fertility regulation and diverse
cultural ecosystem services but also to disservices such as damage to crops and spread of diseases to
livestock and humans (Gutierrez-Arellano and Mulligan 2018, Noriega et al. 2018). There is a
pressing need to assess the status of insects to set and evaluate conservation targets.

At the Convention on Biological Diversity (CBD) meeting in Nagoya (Japan), the Strategic Plan for
Biodiversity 2011–2020 was adopted. It proposed five goals and 20 “Aichi” biodiversity targets. In
line with this plan, a new EU biodiversity strategy was adopted by the European Commission in May
2011. This strategy provided a framework for the EU to meet its biodiversity targets and global
commitments as a party to the CBD. The Headline Target in the existing EU Biodiversity Strategy 2020
is to halt the loss of biodiversity and the degradation of ecosystem services in the EU by 2020, and
restore them, in so far as feasible, while stepping up the EU contribution to averting global
biodiversity loss. Under Target 3A the EU is committed to increasing the contribution of agriculture
to biodiversity recovery. Further, the EU Biodiversity Strategy 2030 includes the development of a
coherent framework for monitoring, assessing and reporting on progress in implementing actions.
Such a framework is needed to link existing biodiversity data and knowledge systems with the
strategy, to help assess achievement of the goals and to streamline EU and global monitoring,
reporting and review obligations.

Some of the EU biodiversity indicators provide specific measurements and trends on genetic, species
and ecosystem/landscape diversity, but many have a more indirect link to biodiversity. Very few have
been explicitly established to assess biodiversity. The status indicators on species only cover birds,
bats and butterflies, since these are the only taxa/species groups for which reasonably harmonized
European monitoring data are available (EEA, 2012). This technical report builds upon previous
technical reports for the EU Grassland Butterfly Indicator (e.g. van Swaay et al., 2019) to:

1. Describe a new approach for assessing butterfly trends and developing indicators of
European butterflies
2. Give an overview of the main results, and present a range of butterfly indicators
3. Discuss the next steps to improve butterfly indicators for Europe

Butterflies are ideal biological indicators: they are well-documented, measurable, sensitive to
environmental change, occur in a wide range of habitat types, represent many other insects, and are
popular with the public because of their beauty. Field monitoring is essential to assess changes in
their abundance. Indicators based on butterfly monitoring data are valuable to understand the state
of the environment and help evaluate policy and implementation. Trained volunteers are a cost-
effective way of gathering robust data on butterflies, more so when supported by informative
materials and efficient online recording.