Agricultural: Ageratum conyzoides is a weed in many annual and perennial crops and has been reported as host of many crop diseases (Ekeleme et al. 2005). Weeds interfere with growth and production of crops and therefore exert significant ecological and economic impacts (Singh et al. 2001, Batish et al. 2009). For example, in Asia rice yield Asia has been negatively associated with A. conyzoides density (Roder et al. 1998). Some studies have demonstrated allelopathy in the weed, however, shoot competition for light appears to be a major mode of interference in crops (Ekeleme et al. 2005).
Allelopathy: Allelopathy is a type of biotic interference wherein a plant releases bioactive metabolites into the surrounding environment. Growth of nearby vegetation is negatively affected and a selective advantage to the donor plant is provided (Batish et al. 2009a). Volatile components of A. conyzoides that contribute to phytoinhibition/allelopathy include precocenes and their derivatives monoterpenes and sesquiterpenes. There is much evidence that A. conyzoides inhibits germination and growth of other plants through chemicals produced by its root and shoot systems. Fresh leaves and volatile oils of A. conyzoides inhibit seedling growth of various crops (Kong et al. 1999) including peanut, redroot amaranth, cucumber and ryegrass (Kong Hu & Xu 2002). Studies have shown that shoot extracts of A. conyzoides inhibit germination of Amaranthus caudatus, Digitaria sanguinalis and lettuce (Lactuca sativa). Extracts of A. conyzoides inhibit germination of wheat and rice (Oryza sativa) seeds (Jha & Dhakal 1990, in Ming 1999). The phenolics present in leaf extracts and residues of A. conyzoides negatively interfere with the growth and development of wheat (Singh et al. 2003, in Batish et al. 2009a). Root and shoot length and biomass accumulation of rice are significantly reduced by 18 to 30% when grown in the rhizosphere soil of A. conyzoides (Batish et al. 2009a). Leaf debris of A. conyzoides deleteriously affects the early growth of rice (Batish et al. 2009b). Lastly, A. conyzoides causes reduction in chickpea (Cicer arietinum) growth and nodulation and (Batish et al. 2004). Phytoinhibition/allelopathy increases when plants are grown in nutrient-deficient conditions and decreases under fungal infection or aphid feeding (Kong Hu & Xu 2002).
Plant pathogen transmission: Kashina, Mabagala and Mpunami (2003) found that A. conyzoides is a weed host and disease reservoir of the Tomato yellow leaf curl Tanzania virus near tomato farms in Tanzania (Kashina, Mabagala & Mpunami 2003). A. conyzoides is a potentially important TYLCTZV reservoir because it occurs in abundant num¬bers within the tomato farms, it is associated with whitefly vectors, and it is found naturally infected with the virus. Elsewhere, A. conyzoides has been identified as host of Ageratum yellow vein virus.
Human health: A. conyzoides causes allergic reactions in some humans (Negi & Hajra 2007) and may pose a hazard to human health (Kohli & Batish 1996).
Case Study: Himalayan plant communities: The Shivilak Ranges form part of the North Indian Himalayas, well known for their rich floral diversity. However, A. conyzoides is poses a threat to the structure of natural plant grassland and forest communities and the dynamics of natural ecosystem processes (Singh et al. Undated A; Dogra et al. 2009). The weed replaces native grasses and medicinally important herbs and studies suggest it lowers biomass, biodiversity and creates homogenous monospecific stands (Dogra et al. 2009). It also negatively interferes with crop plants (wheat, chickpea, rice, maize, and sugarcane).
Location Specific Impacts:
Disease transmission: Yellow vein disease of Ageratum conyzoides, a weed species that is widely distributed throughout Asia, has been attributed to infection by the geminivirus Ageratum yellow vein virus (AYVV). Members of the Geminiviridae genus Begomovirus (Briddon and Markham 1995, in Saunders & Stanley 1999) are transmitted by the whitefly Bemisia tabaci (Gennadius) to a wide range of vegetable and fibre crops worldwide in which they cause serious diseases (Brown 1994, in Saunders & Stanley 1999).
A. conyzoides plants coinfected with AYVV DNA A and DNA 1 remain asymptomatic, indicating that additional factors are required to elicit yellow vein disease. The author's results provide direct evidence for recombination between distinct families of plant single-stranded DNA viruses and suggest that coinfection by geminivirus and nanovirus-like pathogens may be a widespread phenomenon. The ability of plant DNA viruses to recombine in this way may greatly increase their scope for diversification.
South East Asia (Asia)
Agricultural: A. conyzoides is one of the most dominant weeds in upland crop areas of South East Asia (Kato-Noguchi 2001).
Disease transmission: Weed species are believed to act as reservoir hosts for many economically important plant viral diseases. A. conyzoides, a weed that occurs throughout South East Asia, frequently exhibits yellow vein symptoms that are associated with infection by Ageratum Yellow Vein Virus (Saunders et al. 2000).
South China (China)
Agricultural: The fresh leaves and volatile oil of A. conyzoides in south China were found to be highly inhibitory to seedling growth of various cultivated crops, especially in an adverse habitat.
Agricultural: A. conyzoides interferes with crops and causes yield reductions of major staple crops of India (Kohli et al. 2006). It forms thick monospecific stands at the expense of commonly grown crop species such as wheat, chickpea, rice and maize (Kohli et al. 2006, in Batish et al. 2009a). When it invades rangeland areas, it out competes native grasses causing scarcity of fodder (Kohli et al. 2006), thereby putting livestock under starvation conditions. Severe infestations by A. conyzoides reduces crop yields significantly and to such a low level that some farmers in the lower Shivalik ranges of the Himalayas have had to abandon their fields (Batish et al. 2004; Kohli & Batish 1996).
Other: A. conyzoides poses a human health hazard also (Kohli & Batish 1996). A number of health problems in human beings and toxicity to livestock have been reported. These include contact dermatitis, skin irritation, nausea, giddiness (due to their pungent smell), and respiratory problems such as bronchitis and asthma.
Reduction in native biodiversity: Rich medicinal plants, an important resource, are being out-competed by A. conyzoides (Kohli & Batish 1996).
Shivalik Ranges (India)
Agricultural: The weed is a problem for farmers, ecologists, locals, environmentalists and animals (Batish 2008). It is a troublesome weed of cultivated lands. A. conyzoides is responsible for replacing grasses in pastures and causing fodder loss (as cattle do not feed on the weed), deteriorating soil quality and reducing crop quality and yield (wheat, chickpea, rice, maize, and sugarcane) (Singh Undated). A. conyzoides also adversely affects animal health; and causes hindrances to ploughing (Batish 2008).
Competition: A. conyzoides replaces other vegetation including native grasses (Batish 2008; Singh Undated).
Ecosystem change: Dogra and colleagues (2009) evaluated the impact of A. conyzoides on the diversity and floristic compositions of native species. It was found that as compared to control, in the Ageratum invaded area:
It was concluded that invasion by A. conyzoides drastically affects the productivity, composition and diversity of the invaded areas in Shivalik hills of Hamirpur district.
- The average number of plant species was reduced by 32% (81 species in the control area as compared to 55 in Ageratum invaded areas);
- Margalef's index of species richness, alpha species diversity and evenness index were reduced by 37%, 41% and 15%, respectively;
- The fresh and dry biomass of plants was significantly reduced by 52% and 48%, respectively ;
- The number of abundant species (N1) and very abundant species (N2) were also significantly reduced; and
- The value of index of dominance was higher, indicating that communities were homogenous in nature and dominated by a single species (compared with un-invaded communities which showed more heterogeneity).
Habitat alteration: The invasion of these three invasive plant species (A. conyzoides, L. camara, P. hysterophorus) highly reduces the available habitats or niches for the growth of other useful plant species. They are responsible for the loss of productivity and diversity of species in the invaded areas. They drastically alter the structure, function and dynamics of invaded habitats (Dogra, Kohli & Sood 2009).
Human health: A. conyzoides presents a health hazard to humans (Batish 2008).
Modification of hydrology: A. conyzoides chokes water channels (Singh Undated).
Modification of nutrient regime: A. conyzoides deteriorates soil quality (Batish 2008). The weed is strongly allelopathic and adversely affects crop productivity through its allelochemicals. Almost all parts of the weed (stem, roots, leaves, inflorescence), are known to cause allelopathic effects. Volatile allelochemicals include Precocene I, Precocene II and Caryophyllene etc. Non-volatile phenolic acids include gallic acid, coumalic acid, and protocatechuic acid, p-hydroxybenzoic acid, p-coumaric acid, sinapic acid and benzoic acid.
Dogra and colleagues (2009) found that the amount of phenolics in the soil was 41% higher in A. conyzoides-invaded areas than in control areas. The pH in the A. conyzoides-invaded areas was slightly higher than in the control areas. The conductivity of the ions was 31% more in the A. conyzoides invaded area as compared to control. Percent organic carbon and organic matter also increased in the invaded area by nearly 49%. The increase in the available nitrogen content was highest among all other nutrients. It increased by 57% in the invaded area. The amount of available phosphorus, potassium and sodium were more by 48%, 38% and 25% respectively in the A. conyzoides-invaded soil as compared to the control soil. The amount of available calcium, magnesium and chloride were more by 37%, 32% and 33% respectively in the A. conyzoides-invaded soil as compared to the control soil (Dogra et al 2009).
Reduction in native biodiversity: Dogra and colleagues (2009) found that A. conyzoides, L. camara and P. hysterophorus reduce species diversity and composition in the Shivaliks. The decrease in diversity indices and biomass in the invaded sites show that plant communities become less productive in response to these invasive plant species in the lower Shivaliks of Himachal Pradesh, India (Dogra et al. 2009). Batish (2008) found that A. conyzoides adversely affects biodiversity and checks the growth of grasses and other weeds. A. conyzoides can result in an up to 50% decrease in average dry biomass (grams per meter squared) and can cause an over 30% reduction in total species number (Batish 2008). Singh and colleagues (Undated) found that, compared with controls, native vegetation in A. conyzoides-infested habitats was significantly reduced (including numbers of species, plant density and biomass) especially in wastelands and grassland, indicating that invasion by A. conyzoides reduces plant biodiversity.
Dogra and colleagues (2009) found that the number of herbs decreased sharply (as compared to other life forms) in the A. conyzoides-invaded areas. Results showed that herbal vegetation was affected maximally as compared to other vegetation types. Major medicinal plants affected by the invasion of A. conyzoides were Murraya koenigii, Dichanthium annulatum, Cynodon dactylon, Trifolium repens, Achyranthes aspera, Adhatoda vasica and Carissa carandas (Dogra et al. 2009).
West Bengal (India)
Agricultural: A. conyzoides is a crop weed in the plains of Eastern Himalayan region of West Bengal. It is an unwanted plant in the crop land that competes for nutrients, water and space.
Doon Valley (India)
Modification of fire regime: A. conyzoides introduced from tropical America has expanded at an alarming rate, especially in agricultural fields, along footpaths, road sides and in gardens of heavy peat content. Its dominance in fire-burn areas makes it appear a permanent denizen of India.
Other: It creates allergic problems.
Reduction in native biodiversity: It is harmful to native floristic composition.
Dodoma (Tanzania, United Republic of)
Disease transmission: Kashina, Mabagala and Mpunami (2003) found that A. conyzoides is a new weed host and disease reservoir of the Tomato yellow leaf curl Tanzania virus near tomato farms in Dodoma, Tanzania. A. conyzoides and Sida acuta are potentially important TYLCTZV reservoirs because they occur in abundant numbers within the tomato farms, they are associated with whitefly vectors, and were found naturally infected with TYLCTZV.