Abstract:  To understand if the feeding behavior and capacity of Agasicles hygrophila changed after the insect has been introduced to China for more than 20 years, the controlling effects of all stages (larvae and adults) at different infestation densities on alligator weed, Alternanthera philoxeroides, were quantitatively detected in the laboratory, and in the detection tests the experimental larvae and adults were obtained from adults collected in fields and reared with A. philoxeroides. The results showed that the 1st instar larvae of A. hygrophila preferred to feed the top buds and tender leaves. A. philoxeroides plants still produced new leaves and axillary buds, and its biomass, plant height and number of stem nodes increased at the infestation densities of both 0.2 and 1 1st instar larvae of A. hygrophila per plant. The biomass, numbers of leaves and axillary buds exhibited negative growth at the infestation density of 5 1st instar larvae of A. hygrophila per plant. All measured parameters including biomass, plant height, and the numbers of leaves, stem nodes and axillary buds exhibited negative growth at the infestation densities of 10 1st instar larvae of A. hygrophila per plant. The 2nd instar larvae of A. hygrophila preferred to feed on the top bud and tender leaves and also fed on tough foliages and stems. Forty percent of experimental A. philoxeroides plants died at 7 d after infestation at the density of 10 2nd instar larvae of A. hygrophila per plant. The 3rd instar larvae fed both leaves and stems, and then pupated in the stems at the later stage. Fifty-two percent of experimental A. philoxeroides plants died at 7 d after infestation at the density of 10 3rd instar larvae per plant,  and the number of stem nodes of surviving plants markedly decreased. The adults could continuously eat any tissues of the weed for 24 h. At the infestation density of 0.2 adults per plant, the numbers of leaves and axillary buds of the experimental plants revealed negative growth. At the infestation density of 5 adults per plant, the biomass, plant height, and the numbers of leaves, stem nodes and axillary buds of the experimental plants exhibited remarkably negative growth. A. hygrophila at the infestation density of 10 adults per plant had a better controlling effect on the alligator weed than that at the infestation density of 5 adults per plant.

Key words:  Agasicles hygrophila, Alternanthera philoxeroides, controlling effect, quantitative detection, natural enemy evaluation