Insects on trees: Population and individual response to fertilization

We review the literature dealing with the effect of soil fertilization on phytophagous insects and mites on trees. Most published studies focus on nitrogen fertilization. The results of different experiments are often contradictory. In this review we attempt to facilitate the interpretation of the experimental results by distinguishing between direct effects on insect individuals and population level effects. and by grouping insects according to their feeding mode. By making these distinctions it is possible to explain at least some of the ostensibly contradictory results, such as where a fertilization-induced improvement in food quality from an individual point of view does not positively affect population growth. Enhanced nitrogen availability usually benefits individual herbivores by improving the nutritional quality of the host plant. However, except for aphids where a positive response to nitrogen fertilization is usually found, the effects of nitrogen fertilization on insects at the population level are often nonsignificant or even negative. The reason for this might be that fertilization also affects higher trophic levels in the ecosystem, i.e. predators and parasitoids, which control herbivore populations, and the effects mediated by community structure override the effects mediated by the improved quality of the host tree.