Environmental control of emergence patterns: Case study of changes in hourly and daily emergence of aquatic insects at constant and variable water temperatures

The aim of this study was to determine which environmental factors influence emergence of insects at two contrasting habitats: one with constant and one with variable water temperature. We hypothesized that emergence of holometabolous insects is triggered by water temperature where temperature variations occur, while light is the main stimulus for emergence at site with constant water temperature. We expected that for the emergence of hemimetabolous insects, some additional environmental stimuli might be required. We also expected weather conditions to be more important at sites that lack variations in water temperature. To test our hypotheses we placed six pyramid‐type emergence traps at the two sites. Emergent aquatic insects were collected at 8‐h intervals over a 13‐day period, during peak emergence for most of the target species. Most taxa emerged during the afternoon at both sites. Only Hydropsyche saxonica / instabilis emerged nocturnally. At the site with constant water temperature, emergence of Drusus croaticus was stimulated by length of sunlight period. Emergence of Protonemura auberti was promoted by higher air temperature and humidity of the day before. Brachyptera tristis emerged in higher numbers when humidity and cloudiness were high the preceding day. At site with variable water temperature, an increase in water temperature, with a threshold at 16°C, was a significant factor for the emergence of Hemerodromia unilineata . This study gives new insight into the complexity of relationships between aquatic insect emergence patterns and environmental drivers, and show that light and weather conditions trigger emergence of most insects under constant water temperature conditions, while temperature is a dominant trigger at variable water temperature habitat.