Effects of Nutrient Availability on Pest Resistance of Trees

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The effects of nutrient availability on the pest resistance of trees has been studied by surveying damage occurrence and pest populations in fertilised field experiments (population-level studies), and by rearing herbivores and pathogens on fertilised trees or cuttings in the laboratory (individual-level studies). These experiments have clearly shown that fertilisers can have marked effects on herbivores (Stark 1965, Waring and Cobb 1992, Kyto et al. 1996b). However, the effects of nutrient availability on individual insects can differ from those at the population level, because the indirect effects on the population can be so strong that they override the effects at the individual level. Nitrogen fertilisation improves the nutritional quality of the foliage for insects by increasing the concentration of free amino acids and by decreasing the concentrations of defence compounds. In spite of this, fertilisation has had only a negligible effect at the insect population level. One explanation is that predators and parasitoids, as well as folivores, also benefit from fertilisation. Lush understorey vegetation and increased herbivore populations support larger predator and parasitoid populations, which restricts any increase in the folivore populations (Fig. 1). The response of folivores to a change in food quality depends on their degree of specialisation. Herbivores that are host-specific, but nitrogen generalists, respond less strongly to changes in food quality than host generalists that depend on specific forms of nitrogen (Prestidge and McNeill 1983).