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

Despite year round availability of foliage, abundance of generalist noctuid larvae (Lepidoptera: Noctuidae) in evergreen-dominated Mediterranean fores ts has a narrow, distinct spring peak. This restricted larval period has be en suggested to result in part from avoidance of the nutritionally poor mat ure foliage, and preference for nutritionally superior spring-produced youn g leaves. This study examines this hypothesis by (i) documenting difference s in nutritional characteristics between expanding (April) and mature (June ) young leaves of the evergreen Mediterranean shrub Daphne laureola L. (Thy melaeaceae), and (ii) experimentally studying the feeding preferences of no ctuid larvae for young leaves, old leaves (greater than or equal to 1 yr ol d), and developing fruits of this species in one south-eastern Spanish loca lity. Young leaves of D. laureola declined in nutrient concentration and sp ecific dry mass from April to June. The responses of noctuid larvae, in ter ms of both relative preference and total consumption, to this seasonal vari ation in chemical and physical features of young leaves were also investiga ted. When noctuid larvae were simultaneously offered young leaves, old leav es and developing fruits, they exhibited similar preferences for young leav es and developing fruits, and rejected old leaves developed during the prev ious year. Noctuid larvae did not modify their consumption of young leaves relative to old leaves and developing fruits in response to seasonal change s. Food selection patterns exhibited by D. laureola noctuid herbivores, not ably the rejection of old leaves in favour of young ones, are consistent wi th the hypothesis relating restricted larval periods of these generalist co nsumers with the low food value of the previous season leaves of evergreen Mediterranean plants. (C) 2000 Editions scientifiques et medicales Elsevier SAS.