Density-dependent interactions between the nematode Meloidogyne incognita and the biological control agent Agasicles hygrophila on invasive Alternanthera philoxeroides and its native congener Alternantera sessilis

Biological invasions are a great threat to biodiversity. Invasive plants targeted for control with introduced biological control agents are also exposed to indigenous herbivores in the invaded range. The density of native herbivores may affect the interactions between biological control agents and those native herbivores. We conducted a common-garden experiment to examine the interactive effects between a native natural enemy, the root-knot nematode Meloidogyne incognita (Kofold & White), and the leaf beetle Agasicles hygrophila Selma & Vogt, an introduced biological control agent of the invasive alligator weed, Alternantera philoxeroides (Mart.) Griseb., and its native congener Alternantera sessilis (Linn.) DC. We also manipulated the density of the two herbivores to examine the role of herbivore density in modulating plant–herbivore interactions. We found that (1) the performance of A. hygrophila on A. philoxeroides was inhibited by the nematode regardless of nematode initial density, (2) A. philoxeroides plants subject to herbivory by two beetles had more leaves and fine roots under the high nematode density than those not exposed to nematodes, and (3) the stem diameter of A. philoxeroides subject to herbivory by one beetle was smaller under the moderate nematode density than for plants not exposed to nematodes. However, under the high initial nematode density, both the native and introduced plants had fewer root-knots when co-exposed to herbivory by one beetle than when co-exposed to plants with two beetles. Under the high initial nematode density, A. philoxeroides produced more aboveground biomass but less root biomass than did plants not exposed to nematodes. However, A. sessilis plants exposed to the high initial nematode density produced more root biomass but produced similar aboveground biomass than did plants not exposed to nematodes. These findings suggest that the biological control efficiency of biological control agents may be potentially affected by other herbivores such as nematodes from the soil, and herbivores’ densities may determine the nature of the interactive impacts among multiple herbivores on invasive plants.