Generalist predators are often applied in biological control of pests. Sine generalist predators often prey on herbivorous insects unselectively, they also influence biological control of weeds with herbivorous insects. The alligator weed flea beetle, Agasicles hygrophila (Coleoptera: Chrysomelidae) is acted as a specific biological control of the alligator weed Alternanthera philoxeroides (Amaranthacese:Alternanthera), and it had been introduced to China from Florida, USA. It has performed a good control effect on A. philoxeroides since the beetle was released in the areas invaded by A. philoxeroides. Although the population of A. hygrophila had been established and spread to adjacent regions from the release sites, the population abundance of the beetle maintains only a lower level. Thus, it can not suppress effectively the population expansion and spread of A. philoxeroides. We found that many generalist predator species such as spiders and predatory insects live in the habitat of A. philoxeroides. Whether the generalist predators are a biotic stress factor for suppressing the population expansion of the beetle? To demonstrate this problem, a predator-prey system including predators, i.e. lady beetle Propylaea japonica (Coleoptera: Coccinellidae), spider Oxyopes sertatus (Araneae: Oxyopidae) and Pirata subpiraticus (Araneae: Lycosidae), and host preys, i.e. egg, 1st-3rd instar larva and adult of A. hygrophila was built. Then the daily eating number of the above three predators on different immature stages and adults of A. hygrophila was observed in the laboratory. This aim to understand the biotic stress of predators on A. hygrophila in a natural ecosystem that may evaluate objectively the biological control efficiency of A. hygrophila on A. philoxeroides in the field. The results showed that P. japonica, O. sertatus and P. subpiraticus could feed on eggs as well as 1st-3rd instar larvae of A. hygrophila. Both O. sertatus and P. subpiraticus could feed on 3rd instar larvae of A. hygrophila. The predatory capacities of P. japonica, O. sertatus and P. subpiraticus to eggs and larvae of A. hygrophila increased, but searching efficiency of the predators decreased with the increasing densities of prey. However, the three predators did not prey on adult A. hygrophila in this experiment. The predatory function responses of P. japonica and P. subpiraticus on eggs, 1st-3rd instar larvae, and of O. sertatus on eggs, 1st-3rd instar larvae of A. hygrophila fitted to the disc equation of Holling II. With the exception of the predatory function response of P. subpiraticus on 3rd instar larvae of A. hygrophila, there were a significant correlation between predator and host prey that were fitted by the disc equation of Holling II. The maximum theoretical number of eggs, 1st insar and 2nd insar larvae of A. hygrophila captured by O. sertatus, P. subpiraticus and P. japonica per day was 10.9, 6.2 and 5.6 eggs, 17.1, 35.8 and 10.4 1st insar larvae, and 6.6, 11.2 and 2.9 2nd insar larvae, respectively. The maximum theoretical number of 3rd insar larvae of A. hygrophila captured by O. sertatus and P. subpiraticus was 12.3 and 1.1 larvae, respectively. The results of our present study suggest that the predation of predators can decrease the population density of A. hygrophila, which weakens the control efficiency of A. hygrophila on A. philoxeroides in the field. Therefore, the predators are an important biotic stress factor that affects survival and development of A. hygrophila in the field. Another further study should focus on how the biocontrol efficiency of A. philoxeroides are enhanced via increasing the population density of A. hygrophila in the field.