Effects of aphids and moth caterpillars on epiphytic microorganisms in canopies of forest trees

Different types of herbivores were investigated for their effects on microorganisms in the phyllosphere of forest trees during the growing season. Aphids on spruce, beech, and oak produced honeydew, which was readily consumed by microorganisms and resulted in two to three orders of magnitude higher densities (colony forming units) of bacteria, yeasts, and filamentous fungi on leaves of infested trees. The amounts of honeydew excreted by different aphid species and their mode of excretion (large droplets, tiny droplets scattered over leaves, production of wax wool) affected the degree to which honeydew could be processed by epiphytic microorganisms. All groups of microorganisms appeared to be energy limited. These results were consistent for different growth media offered to the microorganisms. Leaf-feeding moth caterpillars also positively affected the growth of microorganisms on leaves of beech and oak. The effects were more pronounced for bacteria and yeasts especially on oak. Thus, different functional groups of herbivores positively affected the growth of microorganisms in the phyllosphere of trees. It is suggested that the population dynamics of herbivores and their feeding characteristics are important features, which should be considered when the population dynamics of microorganisms in the canopies of trees is studied.