Fertilizer-mediated bottom-up effects on cranberry-Phytoplasma-herbivore interactions

Understanding the bottom-up effects of nutrient availability on plant-pathogen-herbivore interactions is important for pest management in agroecosystems. Although many studies have documented the effects of pathogen-infected plants on insect herbivores, only a few have investigated whether these effects are influenced by nutrient availability. Thus, I tested the effects of fertilizer on the interactions between a phytoplasma that causes false blossom disease of cranberries and two insect herbivores: the vector of the disease (blunt-nosed leafhopper, Limotettix vaccinii) and a non-vector insect herbivore (spongy moth, Lymantria dispar). In a greenhouse, I conducted a (4×2) factorial experiment that tested 4 fertilizer regimes (0, 1/4x, 1x, and 2x of the standard rate) and 2 disease levels (non-infected and infected). I then assessed the effects on plants (i.e., plant size) and the performance (i.e., mortality and weights) of L. vaccinii nymphs and L. dispar larvae. The results indicate that phytoplasma infection decreases plant size while fertilizer increases plant size but not at extreme fertilizer levels (significant phytoplasma x fertilizer effect). In general, both increased fertilizer application and phytoplasma infection had a positive effect on the performance of the two insect herbivores. These results will not only provide novel theoretical predictions on the bottom-up effects of nutrient availability on plant-pathogen-herbivore interactions but can also offer guidelines to manage insect pests and diseases in agroecosystems.