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

1. Invasive plants are often attacked by both introduced specialist and native generalist natural enemies in new ranges. Soil fertility can potentially alter the interactions of these natural enemies with native versus invasive plants in ways that have largely unexplored implications for biological invasions and biological control. 

2. Common garden experiment was conducted to compare the performance of an introduced specialist flea beetle, Agasicles hygrophila, and / or a native generalist nematode, Meloidogyne incognita, on invasive alligator weed, Alternanthera philoxeroides, and its native congener sessile joyweed, Alternanthera sessilis, under different levels of soil nitrogen (N) and phosphorus (P). 

3. At a relatively low or moderate N and P level, the flea beetle and the nematodes were not significantly affected by each other. Under these conditions, alligator weed responded plastically by producing more branches and biomass, as well as longer stolons, in response to attack by the flea beetle and / or the nematode, compared to sessile joyweed responses to similar levels of damage. However, under a relatively high N and P level, nematode infestations significantly reduced flea beetle damage on alligator weed, resulting in significantly greater above- and below-ground biomass and longer stolons than plants without herbivory. In contrast, beetle herbivory significantly increased the level of nematode infestations on sessile joyweed, resulting in significantly fewer fine and coarse roots, and lower above- and below-ground biomass compared to plants without herbivory. 

4. Synthesis and applications. Our findings illustrate the importance of soil fertility in mediating interactions between specialist biocontrol agents and native generalists on native versus invasive plants. High phenotypic plasticity seems to be an important attribute that contributes to the success of invasive plants like alligator weed in relatively nutrient-poor environments. However, nutrient-rich environments could potentially confer greater growth benefits on invasive plants than on native ones by changing herbivore-herbivore interactions on plants differently. There is a great need to fully investigate the direct and indirect interactions between biocontrol agents and generalists across food webs following classical biocontrol releases. Nutrient measurements of both soil and water bodies should also be incorporated into all stages of biocontrol programs.

Methods
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.