| Abstrakt: |
Since the 1990s Sympetrum fonscolombii increasingly became a regular immigrant. It was annually present at many different places in Lower Saxony from 2005 onwards. A separation of the data into immigrants or their progeny was problematic because most observers did not report their data on imagines with a sufficient age specification. However, by means of special requests the following picture arose: Immigrants were on the wing from early May until August. They arrived in almost or completely mature colouration and oviposited from mid-May until mid-July. At one lake, in 2019, the immigrants stayed for at least 68 days. At many places the migrantsʼ offspring emerged in the same year. The long emergence period started presumably in mid-July and continued until early November, demonstrating a short development cycle, even in the North. Emerged individuals left their breeding site in an immature stage. Six cases of spring emergence were documented. We therefore conclude that, in a proportion of the eggs laid, larvae were unable to develop and reach emergence within the same year. Those individuals hibernated in the larval stage and emerged between mid-May and early July in the subsequent year, thus representing a univoltine life cycle. Nothing is known about their destiny. The model presented here is based on the evidence-based assumption that immigrants establish only one subsequent generation, which predominantly emerges within the same year, but does not mature in the invasion area. Larvae which do not reach the ultimate stage until the autumn hibernate and emerge in the following spring, just in the period when the next immigrants appear. This model is broadly supported by further data from northwestern Europe, but there are controversial interpretations which are discussed. The contradictory issue could be solved if in future more observers pay attention to the age of imagines seen. [ABSTRACT FROM AUTHOR] |
