Proceedings of the 21st Asian Pacific Weed Science Society Conference (Editors: B. Marambe, U.R. Sangakkara, W.A.J.M. De Costa, and A.S.K. Abeysekara), 2-6 October 2007, Colombo, Sri Lanka.

CONTROLLING NOXIOUS INVASIVE WEED micrantha H.B.K. IN CHINA

F. Li1, M. Li1, Q. Zan2, W. Zhang3, B. Wang1 and Y. Wang4 1State Key Laboratory of Biocontrol, Sun Yat-sen University, Guangzhou, China [email protected] 2Office of Greening Committee of Shenzhen City, Shenzhen, China 3Institute of Forest Ecology, Environment and Protection, Chinese Academy of Forestry, Beijing, China 4Guangdong Neilingding Futian National Nature Reserve, Shenzhen, China

Abstract: Mikania micrantha H.B.K., a fast-growing vine native to tropical South and Central America, is one of the world’s worst weeds. It was cultivated in Hong Kong in 1884 and now expanded widely in South China, becoming a noxious invasive weed in disturbed forests and plantation crops, causing direct economic loss in farming, wood and fruit industry and serious damage to the natural ecosystem. In order to curb the invasion and to resume the destroyed natural ecosystem, many Chinese researchers have been working on this weed and hoping to find effective ways to manage it. This paper reported managing methods of mechanical removal, spraying, biological control, using parasitic species and vegetation-ecological control in China. Among these methods, it was concluded that large scale mechanical removal is unsustainable, selected was effective but the possibility of causing undesired impact to environment, crops and human could not be neglected, practical use of biological control was still a long way to go, parasitic plant spp. was a new sustainable control agent, and vegetation-ecological control had a promising future to control the invader entirely. In practice, combination of multiple methods was recommended.

Key words: Mikania micrantha, invasive species, control

Introduction

Mikania micrantha H.B.K., a climbing perennial vine, native to Central and South America, referred to as “the weed” in this paper, is one of the first ten worst weeds in the world (Holm et al. 1977), although it is only of minor importance in its native ranges. The first specimen collected in China was from cultivation in Hong Kong Botanical and Zoological Garden in 1884 (Wang et al. 2003). This record was the earliest one in the old world. More specimens were collected in the vicinity of the Garden in the early 20th century. Currently, the weed had spread widely in mainland South China and Taiwan Island. In the mainland, it is distributed most abundantly in Hong Kong, Macao, and Pearl River delta (Zhang et al. 2004). Controlling the weed was extremely difficult. The weed was photophilic and able to climb up on top of shrubs and small trees, forming a mantel suffocating the supporting plant underneath. The weed grew very fast in open and wet habitat. The total length of branching from one inter-node reached 1007 m in 12 months (Wang et al. 2003) in open and wet habitats. The weed produced large amount of tiny seeds with pappus. The weight was only 0.0892 g per 1000 seeds (Hu and Paul, 1994; Zhang et al. 2003; Yang et al. 2003) and spread by wind. It could root from stems and stipites and thus a broken part of stem could re-grow easily. The weed could not tolerate deep shade. A vast area survey in China indicated that very few individuals could be found in close forest and no damaging effects were noticed (also see Huang et al. 2000). This finding was consistent with other reports (see Bogidarmanti, 1989; Ipor, 1991). The weed most damaged shrubs, secondary forest and forest at the early succession stage (Zan et al. 2003), and degrading natural ecological systems (Lan and Wang, 2001; Zhong et al. 2004; Zan and Wang, 2000). It also cost more clearing labor in affected agricultural lands and plantations.

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Proceedings of the 21st Asian Pacific Weed Science Society Conference (Editors: B. Marambe, U.R. Sangakkara, W.A.J.M. De Costa, and A.S.K. Abeysekara), 2-6 October 2007, Colombo, Sri Lanka.

Control

Physical removal An experiment to use physical removal methods in a vast area was conducted in July 1998 and proved to be futile. A site of 2000 m2 densely grown weed was selected in Neilingding Island, an island close to Hong Kong. By using a total of 70 days manpower, the above ground portions were all cut and the roots were dug out as much as possible. The slashed weed was laid under full sunlight. Unfortunately, the weed regenerated from remnants and re-colonized 80% of the ground by 3 months and 100% by 6 months (Zan and Wang, 2000). The fast re-colonization of the weed was the result of fast sprouting from the remnant of the root system and the cut stem fragments developed adventitious roots from nodes and started to re-grow in a short time. This indicated that for effective physical removal, the measure should be repeated within several months. Although this could be done in plantations under intense care, where the weed were regularly cleared before noticeable damage appeared, over a vast area physical removal of the weed in the field was not practical.

Chemical control The first chemical control study conducted in China was in 1994. Four herbicides, i.e., Roundup, Bentazon, Torton and Ronstar, were found to be able to suppress seed germination and seedling growth, and among which Bentazon and Torton had more inhibitory effects (Hu and But, 1994). Later, in 1999~2000, four other herbicides, i.e., 24-D, Monsanto, 25% Hexazinone and 75% sulbometuron-methyl, were tested (Zan et al. 2001). Considering both the killing ability and the negative effect on non target species, it was found that sulbometuron-methyl being the best among them. Further tests indicated that 0.01~0.1g m-2 of sulbometuron-methyl could kill the weed totally (Wang et al. 2003; see also Xu, 2001). The best time of spraying was after mass seed produced in winter germinate in spring and early summer and grow considerably. There would be no more seeds from the mass seed production which is still viable at this time. A good practice was to perform a second spraying about 15 days after the first spraying. This was necessary because it was impossible to spray all individuals in the vast area once. The second spraying was done when leaves of sprayed weed were turned brown and dried, leaving un-sprayed ones obviously green and could be spotted easily. Based on our experience, two sprays ensured no more living clump left. From November 2001 to October 2002, sulfometuron-methyl at concentrations mentioned above were sprayed on a total area of 510 hm2 heavily covered by the weed in Shenzhen and Neilingding Islands. Except on swampy areas, the weed was completely killed after spraying and there were only minor negative effect on other , vertebrates and insects. Within 5 months of monitoring after the second spray, neither sprout nor new individuals were discovered except on swampy areas. Considering the fact that devastating effects of the weed on the plant community usually appeared after several years of growth, it was reasonable to anticipate that spraying herbicide could relieve plant community from the suppression of the weed at least for several years. The backdrop of chemical spraying was that the weed would come back several years later. Furthermore, even though selected herbicides were powerful in controlling the weed, the use of herbicide must be carried out wisely to avoid the possible negative effects to other organism and environment (Li et al. 2000).

Biological control Biological control was the most attracting method to control exotic species. Many researchers had done a lot of work looking for natural enemies of the weed. In China, the weed samples with some lesions were collected in Shenzhen and Zhuhai (Han et al. 2001). The microbe was

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Proceedings of the 21st Asian Pacific Weed Science Society Conference (Editors: B. Marambe, U.R. Sangakkara, W.A.J.M. De Costa, and A.S.K. Abeysekara), 2-6 October 2007, Colombo, Sri Lanka. then identified as Pesudocercospora sp. The microbe could infect the new leaves of M. micrantha and made them turn yellow till they perished. The fungus Sclerotium rolfsii which has a lethal effect on M. micrantha was separated for the first time (Fu et al. 2003). The host range tests with 86 plant species from 42 families showed that Actinote anteas only completed its life cycle on Chromolaena odorata and M. micrantha (Li et al. 2004). However, larvae of A. anteas consumed Brassica spp. This eventually hindered the release of A. anteas (Han, 2007 - personal communication).

Parasitic infection During a vast area investigation in early 2001, it was discovered that parasitic plants Cuscuta campestris, C. chinensis and C. australis could infect M. micrantha and cause death. C. campestris was found to be the most effective one among them (Liao et al. 2002; Zan et al. 2003). Eleven infection plots established during 2000-2001 were revisited in May 2002. Results showed that M. micrantha was suppressed in all plots. However, it should be noted that the growing of Cuscuta sp. always lagged behind that of M. micrantha. Investigation on the ecological safety indicated that C. campestris did not pose a threat on other locally available species.

Vegetation-ecological control The weed neither existed in primary closed forest nor climbed on top of closed forest, indicating the closed forest environment is unsuitable for the weed to run wild. Hence, two pilot projects of vegetation reconstruction was established in 2001 and 2002, respectively, to create environments not favoring the weed to grow with least human intervention. Some regional tree species were selected. An idealized preference was fast growing, wide and dense canopy, high ability to reproduce, and the ability to expand easily. In addition, seedling should be available in the local nursery. 18 species were used although no single species met all preferred criteria. All caring was terminated at the end of 2002 until summer 2005 when climbers on the canopy were cut at the base. Thus the reconstruct forest almost grew by itself and it was predicted that the weed would never damage the reconstructed forest in the future. The reconstruction of the forest proved to be successful. Further, it was anticipated that the reconstructed forest would gradually expand as some trees start to fruit.

Conclusions

Physical removal could be done in small area but it is impractical for large area management. Certain herbicides could be used after careful selection. Emphasis should be placed to avoid damage to non-target plants and environments. Biological control was possible but there is a long way to go before agents could be safely and successfully released. Using certain parasitic Cuscuta sp. to suppress the weed was useful and found to be ecologically safe, but the growth of dodder always lagged behind the weed. Creating and environment that was unsuitable for the weed to grow would not only terminate the invasion but also reverse the invasion. Integrated management was highly recommended to combat the weed.

Acknowledgements

This research was supported by the National Natural Science Foundation of China (30370243, 30570330), Foundation of Chinese Academy of Forestry (2002ZD01 (2003)), Key Task Project of Forestry Department of Guangdong Province, China (2000-1), and Science and Technology Department of Shenzhen, China (200001050).

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Proceedings of the 21st Asian Pacific Weed Science Society Conference (Editors: B. Marambe, U.R. Sangakkara, W.A.J.M. De Costa, and A.S.K. Abeysekara), 2-6 October 2007, Colombo, Sri Lanka.

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