The phenological window of opportunity for early‐season birch sawflies

Abstract

Abstract 1. The phenological window of opportunity hypothesis posits that an insect herbivore cannot survive outside a specific leaf age interval of its host plant. 2. Previous studies have shown that mountain birch displays extensive seasonal changes in leaf biochemical characteristics. Young, expanding leaves are full of water and amino acids but they also contain large amounts of protein‐precipitating gallotannins as well as flavonoid‐glycosides. Concentrations of these compounds decline during leaf growth whereas concentrations of sugars and proanthocyanidins and leaf toughness increase. 3. Adult birch sawflies hatch in early summer and oviposit on growing leaves, which compels larvae to feed on leaves that are already well developed. The purpose of this study was to test whether leaves that were younger and biochemically different from the leaves available under natural conditions are within the phenological window of opportunity for the larvae of two early‐season birch sawfly species, Amauronematus amplus Konow and Pristiphora alpestris (Konow). 4. Amauronematus amplus larvae survived better and developed faster, and P. alpestris larvae developed faster and became bigger, on atypically young leaves compared with larvae reared on leaves encountered normally. Therefore, these species can exploit the putative nutritional superiority of very young leaves, which probably outweighs the potential impact of the new set of secondary metabolites offered to them. In conclusion, young leaves that are consumed rarely by sawfly larvae are within their phenological window of opportunity, even though the timing of sawfly life cycles constrains their utilisation.