The life history of Anax junius (Drury) in Minnesota: Determining instars, growth development pathways, emergence phenology, and the effect of temperature on development (Odonata: Aeshnidae)

Abstract

This dissertation is a report based on careful observations of Anax junius, the common green darner dragonfly. Odonatology, the study of dragonflies and damselflies, is presently at a delightful stage of maturity. The basics of odonate life history are well understood but the details of how different species express variations in their development and reproduction, in response to different evolutionary drivers, is ripe for exploration. These variations can only be discovered through field observations executed with thoughtful experimental design and then the observational data collected must be interpreted with appropriate statistical and analytical tools. For a naturalist with a patient and inquisitive mind this kind of research is exceptionally rewarding. Chapters 1, 2, and 3 focus on these goals. Observational data on A. junius growth and emergence were collected over a period of two years at Crow-Hassan Park Reserve, near Minneapolis, Minnesota. Severe winter conditions are the largest constraint influencing dragonfly life histories in Minnesota. Odonata must both survive the winter, and time their emergence and reproduction so that they occur during the short growing season. This research was designed to explore how northern A. junius have adapted to survive winter. Chapter 1 demonstrates the application of a statistical tool that identifies the number of instars in a field-collected sample of nymphs: a need for interpreting observed growth and development data. This mixed distribution analysis has been used in other fields of entomology, but this is the first time (to the author’s knowledge) that it has been applied to dragonflies. Chapter 2 maps nymph growth over time and identifies different A. junius growth pathways that are associated with two different overwintering strategies. Chapter 3 summarizes observations of A. junius emergence phenology via exhaustive exuviae collection, and reveals that cold temperatures and stochastic events are the greatest constraints on emergence duration. Northern ectoderms, like dragonflies, with life histories that are constrained by severe winters are impacted by climate change. The implications of the changing climate on dragonfly natural history are of increasing conservation interest. Rare and threatened niche species of Odonata could be at risk of extinction if their habitat conditions are altered beyond to what they can adapt to. However, A. junius is a common and abundant dragonfly and is of conservation interest for different reasons. Extremely common species are the skeletons of ecosystems; they make up most of the biomass and provide structure and support for all the other components. A change in the geography or abundance of A. junius could have wide-ranging and cascading effects. Models based on known temperature thresholds are required to predict ectoderm response to the changing climate. Chapter 4 describes an experiment that defined these threshold temperatures for the northern winter growth pathway of A. junius. Development duration and rate were measured for nymphs reared in different temperature-controlled chambers, and the base and optimum growth temperatures were calculated for the last two nymph development stages. These values can now be used in models to predict the impact of climate change on the phenology of A. junius development and emergence. Note: Some materials and methods descriptions, figures, and tables are repeated in this dissertation because each chapter was written with the intent to be published individually.