Insects 2022, 13, 911. https://doi.org/10.3390/insects13100911 www.mdpi.com/journal/insects Article Response of Chironomids to Key Environmental Factors: Perspective for Biomonitoring Bruno Rossaro 1, *, Laura Marziali 2, * and Angela Boggero 3 1 Dipartimento Ambiente Territorio, Università degli Studi di Milano, via Celoria 2, 20133 Milano, Italy 2 National Research Council—Water Research Institute (CNR-IRSA), via del Mulino 19, 20861 Brugherio, Italy 3 National Research Council—Water Research Institute (CNR-IRSA), corso Tonolli 50, 28922 Verbania Pallanza, Italy * Correspondence: bruno.rossaro@unimi.it (B.R.); laura.marziali@irsa.cnr.it (L.M.) Simple Summary: Benthic macroinvertebrates of inland waters, including running waters and lakes, are frequently used in biomonitoring. Sometimes, environmental data associated with species lists are not available; in this situation traits or functional adaptations of species to environment can be considered as a tool to translate the list of species into a useful index to evaluate the environmen- tal quality a body of water. Abstract: Chironomids are the species-richest family among macroinvertebrates and are often used as indicators of ecological conditions in inland waters. High taxonomic expertise is needed for iden- tification and new species are still being described even in the well-known West Palearctic region. Our Microsoft Access relational database comprises data on Chironomid species collected in rivers and lakes in Italy and some other European countries over a period of about 50 years, often associ- ated with physical-chemical data, but in some cases, only data on Chironomids are available with no associated environmental data. The aim of the present paper was to propose the calculation of ecological traits of Chironomid species as a tool to derive information on water quality, when only data on Chironomid species composition are present, while environmental data are lacking. Traits summarizing the species’ response to environmental variables were evaluated, with emphasis on natural and man-influenced factors: current velocity, water temperature, conductivity, dissolved oxygen, and nutrients. Traits calculations were carried out in the R environment using a subset of our data, including both environmental data and Chironomid abundances. The relations between sites, Chironomid, species and traits were evaluated using correspondence analysis and other mul- tivariate methods. The response of species showed an interaction among different factors, with the possibility of ordering species along a single environmental gradient, extending from cold running waters to warm standing waters, with few exceptions. Keywords: chironomidae; freshwaters; macroinvertebrates; ecological indicators 1. Introduction The analysis of environmental factors responsible for macroinvertebrate assemblage structure has a long history. Among macroinvertebrates, Chironomids are considered a hard-to-identify group, therefore studies concerning macroinvertebrates as bioindicators have been often limited to Ephemeroptera, Plecoptera, and Trichoptera (EPT) [1]. Taxo- nomic problems were brought forward to justify this choice, but even though Chirono- mids are a difficult group, this is not a valid reason to disregard them. Chironomids in- clude species living in almost all water bodies, sometimes present with a very large num- ber of species, so their exclusion can lead to a serious misjudgment in water quality as- sessment [2]. Citation: Rossaro, B.; Marziali, L.; Boggero, A. Response of Chironomids to Key Environmental Factors: Perspective for Biomonitoring. Insects 2022, 13, 911. https://doi.org/10.3390/ insects13100911 Academic Editors: Xiao-Long Lin, Trond Andersen, Susan E. Gresens and Bei-Xin Wang Received: 23 August 2022 Accepted: 3 October 2022 Published: 7 October 2022 Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional claims in published maps and institu- tional affiliations. Copyright: © 2022 by the authors. Li- censee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and con- ditions of the Creative Commons At- tribution (CC BY) license (https://cre- ativecommons.org/licenses/by/4.0/).