Shifts in Insect Herbivory in the Canopy of Black Locust, Robinia pseudacacia, after Fertilization

Black locust trees (Robinia pseudacacia L.) fertilized with N, P, and K initially incurred higher losses to chewing insects, but subsequently gained some protection from herbivory. This protection consisted of two distinct tactics: (1) a tolerance response, in which greater photosynthetic area was produced, and (2) a resistance response, in which insect preference shifted to foliage of non-fertilized trees. Consumption damage accrued in the foliage of control trees in a linear fashion throughout the season. Relative concentrations of five major macronutrients were statistically the same or greater in foliage of fertilized trees than in controls from mid-June through August. A small secondary leaf flush was observed exclusively in fertilized trees. That relative foliar nutrient levels were higher than controls as insect herbivory was decreasing was counter to the nitrogen-correlation hypothesis of Onuf and others, and suggests that some mechanism of resistance or protection from herbivory was operating in fertilized trees. Artificial additions of nutrients may temporarily disrupt an adjustment between the plant and its associated herbivores. These results may provide an explanation for some conflicting reports in the literature about the impact of fertilization on insect herbivory, since the effect changes with respect to the interval of observation after fertilization.