Invasive plants have been found to adversely affect the ecosystems in habitats throughout the world, by reducing native species richness, altering water or fire regimes, changing the soil nutrient status and altering geomorphologic processes
(Macdonald et al., 1989). In addition to affecting ecosystems and contributing to the local extinction of native species, invasive species can even cause major socio-economic damage
(Pimentel et al., 2005).
There are about 250 species in the genus Mikania and it belongs to the tribe eupatorieae, within the family asteraceae. Most species of Mikania are native to America and
M.
micrantha is native to Central and South America. It has been reported as a weed in different Asian countries
viz. Bangladesh, Sri Lanka, Nepal, Mauritius, Thailand, Philippines, Malaysia, Indonesia, Papua New Guinea and India. This weed has rapid growing habit especially in humid tropical environments and reported to be one of the worst weed intentionally introduced to India.
As a fast-growing vine, it climbs over other vegetation blocking sunlight, smothering forests and preventing forest tree regeneration and suppress the underlying vegetation.
M.
micrantha covers the tree crown, affecting the photosynthesis of the trees and habitats of birds.
Li et al., 2006 and
Li et al., 2007 have reported that Mikania can alter soil microbial communities thereby strengthening it to successfully invade the natural ecosystem. The weed has the capacity to influence soil nitrogen availability and its transformation. In India, the most severely affected crops by this weed are coffee, coconut, cocoa, tea and banana. It causes enormous damage over a broad range of sectors.
Effect on crops
M.
micrantha causes a significant reduction in the yield of cash crops like tea, coffee, pineapple, banana and ginger. In India, it is a major problematic weed in tea gardens particularly in the north-east and south-west states. A high infestation of
M.
micrantha was noticed in young rubber and teak plantations of Karbi Anglong district, Assam. The infestation was also high in banana plantations and open forests
(Bora et al., 2019). Studies carried out in two tea gardens of Assam revealed that the yield of tea and revenue of the tea gardens were very adversely effected (Fig 1) by the presence of Mikania
(Puzari, 2010). In Nattika, Kerala, the weed created high risk for coconut homesteads and farmers had to spend around Rs. 4800 per acre towards mechanical weeding to control it
(Abhilash et al., 2013). In Fiji,
M.
micrantha was the most frequently occurring weed species in sugarcane and banana fields
(Macanawai, 2011). In the Yunan province of China, it has invaded sugarcane, lemon, banana, orange and caused over 60% yield loss
(Shen et al., 2013). A survey in Malaysia revealed that the weed problem of
M.
micrantha was associated with rubber and oil-palm plantations. Competition of Mikania was greatest in immature rubber and oil palms and declined as the crop matured
(Ahmad-Faiz, 1992). The weed was also found smothering papaya, young cocoa, banana, taro, young oil palms and ornamental plants in Papua New Guinea. It caused severe impacts on crop production and reduced the income of the farmers through declined yields and high weeding costs, particularly in subsistence mixed cropping systems
(Day et al., 2012).
Effect on soil
The presence of
M.
micrantha altered both the soil microbial community and soil chemical characteristics. The total population of bacteria, fungi and actinomycetes increased in
M.
micrantha infested soil. Due to smothering by
M.
micrantha, many plants die and their residues accumulate on the surface soil thus providing nutrients for microorganisms. There was a significant increase in aerobic bacteria but a decrease in anaerobic bacteria in the
M.
micrantha infested soil as compared to the area free from the weed. This was accompanied by changes in soil characteristics such as pH, Soil Organic Matter, total N, NO3-N, NH4-N and total P. There was a significant increase in pH, total N and total P with the increase of
M.
micrantha but decrease in soil organic matter. There were increased activities of soil enzymes such as b-glucosidase, invertase, protease, urease, phenol oxidase, acid phosphatase and alkaline phosphatise in
M.
micrantha invaded soil than non-invaded area. Enhanced activities of these enzymes involved in the cycling of carbon, nitrogen, phosphorus and soil organic matter greatly enhanced the nutrient catabolic capacity of the invaded soils. Higher activity of enzymes related to N cycling was found in the soil where the concentration of mineral N was low. Therefore, N availability could be a strong determinant for
M.
micrantha expansion
(Li et al., 2006). Soil treated with
M.
micrantha leaf leachate had higher amounts of water-soluble phenolics
(Kaur et al., 2012). As such, it may change the litter decomposition of native plants in
M.
micrantha invaded ecosystem. Aqueous extracts of
M.
micrantha increased the litter decomposition rates of
Ficus virens and
Acacia richii but decreased that of
Litsea glutinosa (Chen et al., 2007).
Allelopathic effect
M.
micrantha had allelopathic effects on crops, weeds, co-occurring plant species, insects and pathogens
(Ismail and Mah, 1993; Ismail and Chong, 2002; Kong and Xu, 2002; Lan and Wang, 2001). Different phytochemicals such as seven phenolics, five flavonoids, one alkaloid and sixty six terpenes and their derivatives have been identified in
M.
micrantha (Table 1). However, the presence of the compounds varies depending upon the geographic area where
M.
micrantha grows. These allelochemicals are released by volatilization or decomposition of plant debris.
Allelopathic potential of
M.
micrantha was evaluated through bioassays of volatile oil and extracts of roots, stems and leaves
(Ni et al., 2007). Results of bioassay showed that the volatile oils inhibited the growth of large number of plants (
Lolium multiflorum,
Echinochloa crusgalli,
Oryza sativa,
Cucumis sativus,
Raphanus sativus,
Brassica chinensis,
Brassica parachinensis and
Brassica alboglabra) and pathogenic fungi (
Fusarium oxysporum,
Pyricularia grisea and
Phytophthora nicotiacae)
(Ni et al., 2007). Inhibitory potential of volatile oil increased with an increase in concentration on plant and phytopathogenic fungi but had little effect on plant pathogenic bacteria (
Ralstonia solanacearum). The volatile oil bioassays on phytophagous insects showed a significant deterrent effect on the oviposition of
Plutella xylostella,
Phyllotreta striolata and
Phaedon brassicae (Zhang et al., 2002).
Seed germination and seedling growth of
Oryza sativa,
Raphanus sativus and
Triticum aestivum were inhibited by aqueous leaf extract of
M.
micrantha.
(Sahu and Devkota, 2013; Adhikari et al., 2013). Aqueous leaf extracts of
M.
micrantha under high extract concentration suppressed the germination and growth of
Zea mays L. and
Oryza sativa L. There was a slight decrease in 10% concentration in the case of both the test crops. As the concentration increased from 30-100% the aqueous leaf extract of
M.
micrantha had a detrimental effect on the root and shoot growth of both the test crops
(Lalmuanpuii and Sahoo, 2011). In a greenhouse experiment, the incorporation of
M.
micrantha debris into the soil reduced the growth of
Brassica chinensis and
Lycopersicon esculentum whereas the emergence of
Zea mays and
Vigna sesquipedalis seedlings was not affected
(Ismail and Chong, 2002). The aqueous leachates of
M.
micrantha had allelopathic potentials on
Raphanus sativus,
Lolium multiforum,
Lactuca sativa and
Trifolium repens (Shao et al., 2003). It also inhibits the growth of Chinese
Brassica parachinensis and
Raphanus sativus (Liang et al., 2006).
The shoot length and fresh weight of
Asystasia intrusa,
Chrysopogon aciculatus and
Paspalum conjugatum seedlings were reduced by
M.
Micrantha.
(Ismail and Mah, 1993). Aqueous extracts from
M.
micrantha roots and leaves inhibited the seed germination and seedling growth of two woody plants
i.
e.
Lagerstroemia indica L. and
Robinia pseudoacacia L in southern China. Allelopathic activity of the weed varied based on the concentration of the extracts, target species and the extract sources (
i.
e., leaves vs. roots of Mikania)
(Wu et al., 2009). Similarly, the seedling growth of
Acacia mangium,
Pinus massoniana and
Eucalyptus robusta was inhibited by the aqueous and ethyl acetate extracts of aerial parts of
M.
micrantha (Shao et al., 2003).
