On the Role of Plant Defences in the Fluctuation of Herbivore Populations

The role of plant defences in creating cyclic and less regular fluctuations in herbivore density is evaluated on the basis of a case study concerning the birch Betula pubescens and the autumnal moth Oporinia autumnata. Birch strains from the outbreak area of the moth had basically similar but less intense immediate defensive responses when compared with birches from outside the outbreak area. After defoliation, however, birches from the outbreak area had strong intensification of defence mechanisms lasting at least three years while a birch strain originating from outside the outbreak area did not have any such response. Hence, birch defences offer a causal explanation why the moth population will probably not start a new increase immediately after the previous one in the outbreak area. Several density-induced responses of the moth could be explained as adaptations when the defensive tactics of the host tree were known. For example, aggregated larvae grew better than solitary ones on poor birch leaves with strong induced defence (probable diet at high larval densities) while solitary larvae grew better than crowded ones on high quality birch leaves (probable at low densities). Assumptions underlying the plant defence theory on herbivore fluctuations are discussed. The true importance of plant defences is easily masked by increased susceptibility to predation, parasitism, etc. Hence experiments are needed to reveal causal relations. Some experiments able to falsify the theory are proposed, as well as suggestions for avoiding obvious errors in the design of experiments. The evolutionary basis of herbivore fluctuation, as well as the evolution of resource-saving adaptations, is discussed.