Field Experiments On Replacement Control Of Exotic Eupatorium Adenophorum And Native Parasitoids Of Procecidochares Utilis

The invasive exotic weed Eupatorium adenophorum Spreng (Compositae) has been invading the most part of southwest China. The replacement control, using a competitive plant to replace a weed after it is eradicated, is one of multiple approaches to sustainable management of invasive exotic weeds. This research was to investigate in field plots the relative competition between E. adenophorum and replacement plant L. perenne under different planting plans. We also conducted a field trial to evaluate effects of the replacement control practice. The fruit fly, Procecidochares utilis Stone (Tephritidae, Diptera), is an introduced biocontrol agent against E. adenophorum in southwest China in 1980'. We made a survey to investigate the impact of native parasitoids on the fruit fly so as to evaluate the classical biological control.1. The relative competition between the replacement species Lolium perenne and the invasive exotic Eupatorium adenophorum under different planting plansIn the field plots, an additive series design was used to test the ability of replacement ryegrass Lolium perenne to suppress the invasive exotic weed Eupatorium adenophorum, a severe spreading weed species in the southwestern China. Seedlings of the ryegrass were planted at three different times (same time as the weed species, 15 and 30 days later than the weed species) and three mixtures (E. adenophorum : L. perenne = 2:5, 3:4, and 4:3) at four different densities (30, 56, 120, and 460 plants-m^-2). The biomass of the whole plant and shoots was significantly affected by the planting time of ryegrass, density, and proportional mixture, whereas root biomass was only affected by density and proportional mixture. The relative competitive ability of E. adenophorum increased with delay of planting times of ryegrass. which was even greater than the ryegrass at the mixture ratio of 3 : 4 for the delay of 30 days of planting of the ryegrass. The relative competitive ability of E. adenophorum was not correlated with the proportion in mixtures.2. Field trials on the replacement control of E. adenophorum by L. perenne To investigate the effect of planting L perenne for replacement of E. adenophorum, field trials were performed both in open field and under story in forest. In field plots E. adenophorum was excavated by manually uprooting and ploughing, and then ryegrass was sowed. We measured the growth performances of both weed and ryegrass, and also the plant diversity. The result shows that the covers of E. adenophorum in the forest were significant greater than in open field. However, the cover of E. adenophorum in replacement plots is significant greater than in the controls. The cover of L perenne was greatest in the forest plots where the weed was cut aboveground. The richness and diversity index in forest plots varied greater than in open field plots, where in forest plots the speices richness was 14 and Shannon-Wiener diversity index 2.32, much higher than the control (4 and 1.27 respectively). Fewer plant species were shared by different treatment plots. 3. Parasitism of native parasitoids on Procecidochares utilis Stone (Tephritidae, Diptera), a biocontrol agent against the invasive exotic weed Eupatorium adenophorum Spreng (Compositae) in southwest ChinaTo investigate the native parasitoid guild attacking the fruit fly, Procecidochares utilis Stone (Tephritidae, Diptera), an introduced biocontrol agent against the exotic crofton weed Eupatorium adenophorum (Compositae) in southwest China, we took samples of galls induced by the fruit fly from two habitats, the open field and forest, and reared half of the samples for emergence of the fruit fly or parasitoids and dissected another half for parasitism. Six species of solitary parasitoids emerged from galls with a combined parasitism of over 70%, among which three parasitoids, Torymoides kiesenwetteri (Mayr). Bracon sp. and Eurytoma trypeicola Zerova, accounted for most of the parasitism. Even though E. adenophorum grew better (as measured by plant height and number of branches) and the infestation rate of galls was higher in the open field than in the forest, the parasitism of each parasitoid and total parasitism did not significantly differ between two habitats. The number of emerged parasitoids per gall increased significantly with the gall size as measured by the horizontal diameter of galls.