The Effects of Leaf Damage on the Performance of Insect Herbivores on Birch, Betula pubescens

Publisher Website
Google Scholar
Abstract
(1) Branches of birch trees with all their leaves punched with one hole, diameter 6 mm, and matched controls were individually caged completely, partial caged allowing access to flying insects, or left uncaged. The intention was to distinguish between effects on herbivores due to damage-induced changes in leaves and effects due to the attraction of natural enemies to damaged foliage. (2) First and second instar larvae of a geometrid moth, Apocheima pilosaria (D and S) were introduced onto the branches after 4 weeks. There was a lower mean larval weight, higher larval mortality and fewer emerging adults in cages containing damaged foliage than with undamaged controls. Larval losses on uncaged branches were the highest, probably due to dispersion, and were not significantly different between damaged and undamaged branches. Levels of parasitism in the introduced larvae were < 10% and showed no differences between treatments. (3) Three other herbivores taxa (summed phloem feeders, adult Phyllobius spp. weevils and larvae of the other common geometrid moth species), all present at natural levels of abundance, showed reduced numbers on damaged branches in some of the caging treatments. These herbivores could conceivably suffer from interspecific competition with an early season leaf chewer that removed about 10% of the leaf area. (4) Despite effects on the population dynamics of individual herbivore species, total leaf damage by herbivorous insects was not significantly different between previously damaged and undamaged branches. Damage by larvae of the case-bearing moth species was significantly higher on previously damaged foliage, suggesting that these larvae compensate for damaged foliage by increasing consumption. The apparent damage-induced deterioration in birch foliage does not appear to benefit the tree via decreased subsequent consumption by herbivorous insects, as might be expected if such changes were a true defence.