Effects of nitrogen fertilization on host plant and insect herbivore fitness

Identifying tools that can be used in conjunction with biological control insects to control invasive weeds is vital to optimizing biological control programs. Nitrogen fertilization causes plants to reallocate resources to aboveground growth and increases plant susceptibility to secondary stress. Thus, the effects of nitrogen deposition on insect herbivores are twofold: increasing the palatability of plant tissues and reducing plant defenses. The invasive plant Russian knapweed (Rhaponticum repens) is attacked by a midge (Jaapiella ivannikovi) biological control insect that lays eggs in galls it forms in meristematic tissues. Gall formation stunts plant growth and juvenile midges feed on plant resources until emerging as adults. To assess the impact of nitrogen fertilization on host plant and insect herbivore fitness, a two by four factorial greenhouse experiment will be performed with two insect levels (control and midge) and four nitrogen levels (control: 0 kg N ha-1, low: 25 kg N ha-1, intermediate: 50 kg N ha-1, and high: 100 kg N ha-1). Increased nitrogen fertilization is predicted to result in better control of Russian knapweed by 1) causing the plant to reallocate resources to aboveground growth which can then be attacked by biocontrol agents, and 2) improving the fitness of biological control insects by reducing development time and altering insect oviposition preferences. Results will inform management of Russian knapweed in the field as well as contribute to our knowledge of how nitrogen deposition alters plant-insect interactions.