Controlling Effects Of Agasicles Hygrophila On Alternanthera Philoxeroides

Agasicles hygrophila Selman & Vogt is an important natural enemy widely used to control Alternanthera philoxeroides worldwide. It has different controlling effects on various ecotypes of A. philoxeroides. In order to utilize A. hygrophila to control terrestrial A. philoxeroide effectively, this paper states investigation on the control effect of A. hygrophila to terrestrial-type grass, the population dynamics, expansion, and structure, and the impact to regenerative capacity of A. philoxeroides both in laboratory and field. The main results are as follows:I. Various controlling effects on A.philoxeroides by A. hygrophilaThe controlling effects on A. hygrophila various stages to the weed in laboratory experiment indicated that: As imago and larval of A. hygrophila population density increased, the speed of grass control effect is faster and more effective. It was 100% effective that 100 imagoes in one pot need 15 d and 50 imagoes in one pot need 18 d to control A. philoxeroides, which is more efficient than the release of 10 imagoes, 20 imagoes and 200 imagoes which need 33d. In the same way, released 50 larvae in one pot and 40 larvae in one pot all need 30 d, which is better than the release of 10 larvae in one pot which need 39 d. Release 200 imagoes, due to relatively small space, large initial density of A. hygrophila, exacerbated site-specific competition, leading to population crashes, weaken the effects on grass control. On the same base of initial population, with released 50 imagoes and larvae respectively, the imago's control rate is faster than larva's.Experiments on different growth stages with different A. hygrophila showed that: The interaction of growth stage of plant and releasing density affect the control effect. With higher density of A. hygrophila and younger plant, the control effect was much better. At the seedling stage,20 pairs of imagoes need 42 d to make the control 100% effective, which is better than 5 pairs of imagoes and 105 d get only 59.2% controlling rate at later stages.II. Effects on the population dynamics of A. hygrophila by field testingThe fields testing of the population dynamics of A. hygrophila from June to December,2008 showed that, the population dynamics crest are both in June to July and October to November, and the releasing population density is in proportion to the peak.A. hygrophila with 20 pairs as initial population couldn't reproduce at any growth stage of A. philoxeroides because of the large population base and fast reduction of food. At seedling stage and maturity stage of A. philoxeroides, A. hygrophila with 5 pairs and 10 pairs as initial.population can both reproduce. At the seedling stage of A. philoxeroides, the population dynamics with 10 pairs of adult as initial population crested at 35 d,5 pairs at 49 d. At the maturity stage of A. philoxeroides, the population dynamics with 10 pairs of imagoes as initial population crested at 42 d, but due to the low base of initial population, the population dynamics with 5 pairs of imagoes as initial population didn't show its population dynamics peak until 70 d.Comparing dispersal ability and population changes of A. hygrophila in different habitats, dry land was suitable for A. hygrophila to dispersal than marsh and paddy field. After releasing A. hygrophila 18 d, the population number 6 m apart from releasing center in dry land was greater than marsh and paddy field.The age/fecundity specific life table of the field experimental population of A. hygrophila was constructed by investigating. The results showed that the trend index of population (I) was 140.44, the intrinsic capacity for increase (rm) was 0.25, and the major factor of population inhibiting is the survival rate of egg.III. Wormy A. philoxeroides's regenerated abilityThe experiment of wormy A. philoxeroides's regenerated ability indicated that: Under different cultivation methods, there were significant differences in plant survival rate, diameter, rooting rate and height between wormy plant and healthy plant, however, plant weight ratio was not obvious. After 21 days of planting, the healthy" vertical planting" plants had the best condition, but the wormy ones appeared the worst growth status for the highest inhibiting rate.