Do invasive clonal plants always benefit from clonal integration? Exploring interactions between biocontrol agents and indigenous herbivores

Clonal integration can help plants counter environmental stresses, but its effects on plant responses to herbivory remain unclear. We investigated how clonal integration in the invasive Alternanthera philoxeroides and the native Alternanthera sessilis affects herbivory by the foliar insect Agasicles hygrophila and/or the root-feeding nematode Meloidogyne incognita. Plant fragments were cultivated, assigned to connected or severed treatments, and subjected to herbivory by A. hygrophila, M. incognita, both, or neither. Under clonal integration, A. philoxeroides showed significantly reduced aboveground biomass under A. hygrophila herbivory and fewer taproots and belowground biomass under M. incognita herbivory, while A. sessilis showed significantly fewer taproots and reduced aboveground biomass under M. incognita herbivory compared to no herbivory. However, A. philoxeroides exhibited significantly more ramets and fine roots, along with greater aboveground biomass under M. incognita herbivory, and greater belowground biomass under A. hygrophila herbivory, when clonal integration was maintained. Conversely, these traits in A. sessilis were either significantly lower or remained unchanged with clonal integration. Under combined herbivory by both herbivores, the stolon length, number of taproots, and aboveground and belowground biomass were significantly lower in A. philoxeroides, while these traits in A. sessilis were significantly higher with clonal integration than without. These results suggest that although clonal integration helps A. philoxeroides cope with individual herbivory by either A. hygrophila or M. incognita, it confers no benefit when two herbivores act together. Since M. incognita survives only in terrestrial environments, combining A. hygrophila and M. incognita may improve the management of terrestrial A. philoxeroides.