Insect Herbivory on European Tall-Shrub Species: The Need to Distinguish Leaves before and after Unfolding or Unrolling, and the Advantage of Longitudinal Sampling

We investigated whether or not leaves of tall-shrub species show lower losses to herbivorous insects in the folded or rolled (FR) stage than in the unfolded or unrolled expanding (UE) stage, and support lower rates of growth and survival of relevant insects. In a season-long field study on Cornus sanguinea, Euonymus europaeus, Ligustrum vulgare, Prunus spinosa and Rosa canina in SW-Germany we found that rates of area loss were markedly lower on FR than UE leaves, and also lower on fully expanded, mature (M) leaves. The prevalent herbivores were lepidopteran larvae (Croesia bergmanniana, Ectropis bistortata, Yponomeuta padellus). A longitudinal method of recording revealed 2-3 times higher leaf-area losses than discrete sampling toward the end of the season; even the ranking of the shrub species differed between longitudinal and discrete sampling. In a laboratory trial larvae of the polyphagous moth Ectropis bistortata (the commonest herbivore) strongly preferred UE leaves of Rosa canina over FR or M leaves. Also rates of survival, growth and pupal weight were greatest on UE leaves, and lowest on FR leaves. In a field study on eleven shrub species in SE-England we sought to find reasons why FR leaves are less attractive and less nutritious. Leaves in the FR stage had a significantly higher nitrogen concentration than the UE stage, and their water content was similar. Only in five of eleven species were the leaves hairy at the FR stage, and glabrous at the M stage. Specific leaf area was lower in the FR stage than in the UE stage. Avoidance of FR leaves could possibly be caused by difficulties in eating through the edges of double-thickness laminae in folded leaves, gaining access to the leaf edge in rolled leaves, and maintaining attachment.