Seasonal variation in leaf characteristics and food selectionby larval noctuids on an evergreen Mediterranean shrub

Abstract

Despite year round availability of foliage, abundance of generalist noctuid larvae (Lepidoptera: Noctuidae) in evergreen-dominated Mediterranean forests has a narrow, distinct spring peak. This restricted larval period has been suggested to result in part from avoidance of the nutritionally poor mature foliage, and preference for nutritionally superior spring-produced young leaves. This study examines this hypothesis by (i) documenting differences in nutritional characteristics between expanding (April) and mature (June) young leaves of the evergreen Mediterranean shrub Daphne laureola L. (Thymelaeaceae), and (ii) experimentally studying the feeding preferences of noctuid larvae for young leaves, old leaves (≥ 1 yr old), and developing fruits of this species in one south-eastern Spanish locality. Young leaves of D. laureola declined in nutrient concentration and specific dry mass from April to June. The responses of noctuid larvae, in terms of both relative preference and total consumption, to this seasonal variation in chemical and physical features of young leaves were also investigated. When noctuid larvae were simultaneously offered young leaves, old leaves and developing fruits, they exhibited similar preferences for young leaves and developing fruits, and rejected old leaves developed during the previous year. Noctuid larvae did not modify their consumption of young leaves relative to old leaves and developing fruits in response to seasonal changes. Food selection patterns exhibited by D. laureola noctuid herbivores, notably the rejection of old leaves in favour of young ones, are consistent with the hypothesis relating restricted larval periods of these generalist consumers with the low food value of the previous season leaves of evergreen Mediterranean plants.