Biocontrol of Parthenium for regenerating lost biodiversity
- Part 1 -

Prof N Irabanta Singh *


Alien plant invasions into natural ecosystems are one of the major threats to the conservation of biological diversity across nearly all biogeographical regions on the Earth. An invasive alien species is a species that is introduced from outside its range of distribution by either intentional or unintentional human activity, has established self-reproducing population in the wild and has caused obvious changes in local, artificial or natural ecosystems.

Invasion is considered as the second most important threat to biodiversity after habitat destruction (Akter and Zuberi, 2009). Invasion of exotic species has become a global problem causing adverse impact on the ecosystem, economy, and human health (Sharma et al., 2005; Xie et al, 2010).

Exotic invasive plants threaten the integrity of natural ecosystems throughout the world by displacing crops or native plant communities (Jung et al., 2010). It is likely that the invasion of weeds into natural areas has been associated with human movements throughout our evolutionary development, but as is now widely recognized, the rate of this process has accelerated considerably.

A combination of factors, including developments in transport technology, changes in life-style patterns (particularly in the “western” world), and a seemingly cosmopolitan interest in the introduction and utilization of foreign plant species, have all been powerful forces in shaping the changing flora of natural areas world-wide.

The general trend of an increase in introducing components of natural vegetation accompanied by a decrease in native components has also been greatly assisted by the increasingly extensive exploitation of natural areas, which can alter natural disturbance regimes and thereby provide enhanced opportunities for the colonisation and establishment of introduced plants.

The invasive plants are one of the major threats to the natural environment. They are destroying native habitats, threatening native plants and animals and choking our natural systems including rivers and forests. These weeds reduce farm and forest productivity, invade crops, smother pastures and some can also harm livestock and human health.

Land and water managers incur material and labour costs to control these weeds-these costs are passed on to the Indian public through higher prices for produce. The exotic weeds are the major biological constraints to crop plants. They compete with crops for growth resources (water, light, nutrients and space) leading to reduction in crop yield and quality.

There are no common characteristics of a poison or harmful weed that would help distinguish them. But as a general rule, plants with a bitter taste, unusual smell, milky sap or red berries may be poisonous with some plants having poisonous roots and bulbs.

These weed plants may out-compete native plants because:
* They may not be affected by the pests or diseases that would normally control them in their natural habitats
* The disturbed environment provides different conditions that better suit the invading weed.
As a result the weed may: *grow faster than native plants and successfully compete for available nutrients, water, space and sunlight
* reduce natural diversity by smothering native plants or preventing them from growing back after clearing, fire or other disturbance
* replace the native plants that animals use for shelter, food and nesting.

These exotic weeds are often excellent at surviving and reproducing in disturbed environments and are often the first species to colonize and dominate in these conditions. At present, one of the major invasive weeds that are threatening our natural ecosystem in Manipur (India) is Parthenium (Parthenium hysterophorus L.)

It is commonly known as Congress grass or carrot grass, Gajarghas, White top, etc. and in Indian language like chatak chadani, nakshathra gida, broom brush, etc. In India it was first reported from Pune (Rao, 1956). It is native to South and Central America and has been accidentally introduced into India in 1955 along with imported PL480 food grains (Tower et al. 1977).

Today Parthenium or congress grass is a household name in the country and becomes a ‘national weed’. It has become a serious weed of pastures, agricultural fields and wasteland in most parts of India. It is capable of establishing four successive generations at the same site in a year under favourable temperature and moisture conditions (Pandey and Dubey, 1988).

Parthenium weed has been reported from all States of India. In general, it has been reported that the overall spread in terms of density and infestation level is that it is highest in Andhra Pradesh, Bihar, Chhattisgarh, Delhi, Haryana, Karnataka, Maharashtra, Madhya Pradesh, Punjab, Tamil Nadu and Uttar Pradesh.

The spread and infestation level is medium in Assam, Gujarat Himachal Pradesh, Jharkhand, Jammu Kashmir, Uttarakhand, Orissa, West Bengal and Rajasthan State while spread is low in Andaman & Nicobar, Arunachal Pradesh, Daman Diu, Goa, Kerala, Manipur, Mizoram, Meghalaya, Nagaland, Pondicherry agricultural problems such as loss of crop productivity, fodder scarcity, biodiversity depletion and health problem as for livestock (Kohli and Rani, 1994; Evans, 1997).

Parthenium weed is not palatable to livestock due to its irritating odour, taste. Although overall spread of Parthenium weed has been reported to be low in certain areas such as Kerala and some States of North East but is also true that in some part of these States is present in high abundance which might reflect its future potential for spread in these States (Sushil Kumar, 2012).

The fast invasion of this weed is attributed to its wide adaptability to extreme soil and climatic conditions, ability to grow throughout the year, shorter life cycle, and easy dispersal by wind, water and movement of vehicle and animal. The invasive nature of this weed is evident from its ability to form huge monocultural stands with no other plant in the vicinity.

It has the capacity to grow again from the root cut or broken parts. Its allelopathic effects coupled with the absence of natural enemies like insects and diseases are the two important factors responsible for its rapid spread in India. The current spread of Parthenium weed has been estimated to be around 35 million ha. of land which includes wastelands, crop lands and forest lands.

Until 1980, Parthenium weed infestation was restricted mostly to uncultivated land. At that time, it was not considered to be a problem in Rajasthan, Gujarat, Jammu & Kashmir, Kerala, Orissa, West Bengal and North East States but in a span of 30 years, many arid areas in Rajasthan and hilly areas in Uttarakhand and Himachal Pradesh are now heavily infested with the weed (Sushil Kumar, 2012).

Parthenium is a threat to the biodiversity in the country. It is known to exert significant impact on the natural communities as they caused their displacement and hence exert imbalance in the natural and agricultural ecosystem (Sakai et al., 2001). It has been reported to be causing a total habitat change in native Australian grasslands, open woodlands, river banks and flood plains (McFadyen, 1992; Chippendale and Panetta, 1994).

Similar invasions of National parks have been observed recently in Southern India and Madhya Pradesh (Evans, 1997 Sushil Kumar, 2012). It is known to cause a number of environmental and presence of trichome hairs. However hungry cattle were found to eat the Parthenium which causes clinical signs such as mouth ulcers with excessive salivation, anorexia, pruritus, alopecia and dermatitis. It has been reported that it can taint sheep meat and make dairy milk unpalatable due to its irritating odour.

Consumption of milk from the livestock grazed around the Parthenium invaded grazing land could be hazardous to man (Tudor et al., 1982.) Almost every part of the plant except the root is a cause of a frightening reaction in humans which is almost all over the sun exposed area of the skin.

In India, this weed has been considered as one of the greatest sources of airborne contact dermatitis, rhinitis, asthma, nasal-dermal and nasobronchial types of diseases etc. in susceptible humans (McFadden, 1995; Cheney, 1998). The pollen of the Parthenium is also reported to have allelopathic activity and has inhibited seed germination and development of fruit in brinjal, tomato, chilli etc. (Jarvis et al., 1985).

Positive and negative allelopathic effects have been reported of Parthenium hysterophorus on many agricultural crops and plants species (Oudhia et al., 1997). So, there is an urgent need for the management of the Parthenium hysterophorus.

Till now biological control has proved to be the most effective measure over the several physical and chemical methods as it is the most cost effective, environmentally safe and ecologically viable providing long-term and sustainable control.

Manual and mechanical control methods though effective, have their own disadvantages like time consuming and expensive and provide only a short-term control requiring repeated application. Selectivity, residual toxicity and cost of herbicides restrict the chemical control also.

Under these circumstances, the concept of biological control means of controlling Parthenium offers a potential alternative. In Manipur also, it is now seen growing gregariously in wasteland, National and State highways, and is spreading like a wildfire. Due to its prolific cover and hazardous properties to the environment, adequate measures are required for the proper management of this weed.

Increasing public concern on environmental issues requires alternative weed management systems which are less herbicide dependent or based on naturally occurring compounds. In light of the above facts, it is proposed to undertake certain aspects of management of Parthenium hysterophorus L for checking its proliferation and thus control its harmful effects using selected botanical agents.

To be continued.....

* Prof N Irabanta Singh wrote this article for The Sangai Express
This article was webcasted on January 05 2021.

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