Soil fertility as a mediator of interactions between an introduced specialist beetle and a native generalist nematode on an exotic invasive plant and its native congener

We conducted a common-garden experiment to investigate effects of the level of soil N and P availability (low, moderate, and high) on the performance of the specialist beetle Agasicles hygrophila and/or the nematode Meloidogyne incognita on the invasive plant Alligator weed, Alternanthera philoxeroides, and its native congener Alternanthera sessilis. When plants were harvested at the end of the experiment, the stem diameter, the stolon length, the number of leaves, and the number branches of each plant were measured. After the aboveground part of each plant had been cut off, the roots were carefully dug up and washed. The number of coarse roots (> 2 mm in diameter), fine roots (<2 mm in diameter), and the number of nematode root-knots on roots were counted. Finally, both the above-ground and below-ground parts of each plant were dried separately and weighed, and the total biomass was then calculated.