Objective Clonal integration of alligator weed (Alternanthera philoxeroid) affected by the density of flea beetles (Agasicles hygrophila) population on site was studied to understand how the plants alleviate feeding pressure of the natural enemy.

Methods In a field study on gardens of similar properties, A. philoxeroides was exposed to A. hygrophila in varying population densities to examine the clonal integration of the weed in response. Plant and root growth, apical and basal ramets biomasses, and clonal fragment of the weed were monitored.

Results  The aboveground, belowground, and total biomasses, the counts of leaves and thick roots of apical ramets, and the belowground biomass of entire clonal fragment of the alligator weed on the field under herbivory of A. hygrophila were significantly lower than that free of the beetles. The numbers of leaves, thick roots, and total roots and the above-, belowground, and total biomasses of apical ramets, the diameters of basal ramets and belowground stem, the above-, belowground, and total biomasses of entire clonal fragment of the alligator weed were significantly higher with the existence of the clonal integration than without. In the presence of one beetle per weed plant, the number of thick apical ramet roots, belowground stem girth of basal ramets, and belowground stem girth, thick root count, and aboveground biomass of entire clonal fragment of the weed were significantly higher with clonal integration than without. However, when the density of the beetles increased to two on a plant with clonal integration, even though no significant differences in the belowground stem diameter, thick root count, and aboveground biomass of apical and basal ramets, the leaf count, stem length, and belowground stem girth of the entire clonal fragment were significantly greater than the weed with no clonal integration.

Conclusion The density of A. hygrophila presented in an alligator weed growing area significantly affected the capacity of the weed clonal integration to curtail the feeding by the beetles. The weed could significantly mitigate the herbivorous act with the clone integration when the A. hygrophila population remained relatively low, e.g., one beetle per plant. But as the beetle density increased to two per plant, the effectiveness of the clonal integration of the weed diminished significantly, thereby effective biological control of the alligator weed proliferation could be achieved.