Citation: Kasalo, N.; Tvrtkovi´ c, N.; Bogi´ c, D.; Kokan, B.; Vukovi´ c, M.; Kuˇ cini´ c, M.; Skejo, J. An Overview of Orthoptera Mass Occurrences in Croatia from 1900 to 2023. Insects 2024, 15, 82. https://doi.org/ 10.3390/insects15020082 Academic Editors: Michael G. Sergeev and Alexandre V. Latchininsky Received: 30 November 2023 Revised: 22 December 2023 Accepted: 15 January 2024 Published: 23 January 2024 Copyright: © 2024 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). insects Article An Overview of Orthoptera Mass Occurrences in Croatia from 1900 to 2023 Niko Kasalo 1, * ,† , Nikola Tvrtkovi´ c 2 , Domagoj Bogi´ c 3 , Bože Kokan 4 , Marijana Vukovi´ c 5 , Mladen Kuˇ cini´ c 3 and Josip Skejo 3, * ,† 1 Independent Researcher, Matice Hrvatske 11, BA-80101 Livno, Bosnia and Herzegovina 2 Independent Researcher, Alagovi´ ceva 21, HR-10000 Zagreb, Croatia; nikolatvrtkovic71@gmail.com 3 Division of Zoology, Evolution Lab, Department of Biology, Faculty of Science, University of Zagreb, Rooseveltov trg 6, HR-10000 Zagreb, Croatia; mascer2@gmail.com (D.B.); mladen.kucinic@biol.pmf.hr (M.K.) 4 Natural History Museum Split, Kolombatovi´ cevo Šetalište 2, HR-21000 Split, Croatia; boze@prirodoslovni.hr 5 Croatian Natural History Museum, Demetrova 1, HR-10000 Zagreb, Croatia; mvukovic@hpm.hr * Correspondence: niko.kasalo5@gmail.com (N.K.); skejo.josip@gmail.com (J.S.) † These authors contributed equally to this work. Simple Summary: Grasshopper and cricket (Orthoptera) mass occurrences represent an unusually large increase in populations of certain species. Grasshopper species known as locusts are well known to cause severe damage to many crops. The severity of each outbreak depends on the species and on a wide range of ecological factors, including temperature and rainfall regime. In Croatia, a small country in SE Europe, there has never been systematic research on these Orthoptera species, and this phenomenon is regarded as a rare event despite several huge outbreaks being reported in the past. For the first time, all known mass occurrences of grasshoppers, locusts, crickets, bush-crickets, and mole crickets (of the order Orthoptera) from Croatia are summarized. The study represents a historical overview of all Orthoptera mass occurrences known based on the literature but also provides many new records. It seems that Orthoptera mass occurrences are not rare in the country at all, especially in the Mediterranean part. The present situation suggests that thorough research into locusts as agricultural pests in Croatia should be conducted to determine the potential threat to the local economy and with special emphasis on the Adriatic region. The trends of climate change in the Mediterranean imply a rise in locust destructiveness risk in the near future and invites further research. Abstract: During the last century, well-known locust species, such as Calliptamus italicus and Do- ciostaurus maroccanus, have produced outbreaks of varying degrees in the Balkans. The literature data on outbreaks in the region are scarce, and Croatia is not an exception. This study summarized the data on 23 Orthoptera mass occurrences in Croatia from 1900 to 2023 from 28 localities, representing 12 species. This is a low level of outbreak activity compared with other locust and pest grasshopper species in other parts of the world. The species with the most reporting is C. italicus with altogether six mass occurrences, while second is Barbitistes ocskayi and Miramella irena with three records, and in the third, place D. maroccanus and Gryllotalpa sp., each with two mass occurrences having been reported. One of the most notable swarms is that of Anacridium aegyptium which occurred around Šibenik in 1998, and this paper provides the first account of it, 25 years after it took place. The most recent outbreaks took place in 2022, and the most notable one was that of D. maroccanus swarm in Štikovo. The 2022 and 2023 reports were brief and muted, despite the affected agriculturists claiming significant damages. Keywords: Mediterranean region; locusts; grasshoppers; katydids; mole crickets; pest species; abundance; outbreaks (hrv. najezda) Insects 2024, 15, 82. https://doi.org/10.3390/insects15020082 https://www.mdpi.com/journal/insects