Parthenium hysterophorus L. (family Asteraceae), a noxious, ubiquitous and annual weed, native to North and South America and the West Indies, but now spreading wildly in Asia and Africa.
The weed is considered to be the worst weed in the world and is responsible for loss in agricultural production as well as its other harmful effects on animal and human health. In humans it causes dermatitis, asthma and bronchitis. Parthenium weed, carrot weed, congress weed, false ragweed and bitter weed are some of its common names.
Hysterophorus L. is a fast growing, branched and erect herb. The life cycle of parthenium consists of two distinct stages: vegetative stage and reproductive stage. In vegetative stage, the leaves are small and lack flowering. The lower large leaves cover the ground, without allowing any vegetation to grow underneath it. In reproductive stage, the roots are deep reaching to 2m in length.
Stem is erect with much branches and becomes woody and tough with the passage of time. How this plant is reproduced is much important. The plant contains creamy flowers, each flower produces 4-5 wedge shaped seeds. It is very productive seed producer, producing some 25,000 seeds per plant, adding to the seed bank in the soil.
Although the weed can grow in all types of soil, however, alkaline soils are best for its establishment. It can grow anywhere in lawns, roadsides, railway tracks, play grounds, forest lands, pasture areas and agricultural areas. Can grow under drought and saline conditions. The spread of the seeds and its ability to germinate vigorously pose serious problems to its control.
Parthenium is well known due to its harmful effects and difficult control.
Effects on Agricultural Crops:
A yield loss of 40% in agricultural crops is observed due to parthenium weed infestation. The weed poses allelopathic effects on agricultural crops and compete with them for nutrients water, space and sunshine leading to reduced production.
The allelopathic effects of this weed is mainly due to secretion parthenin, hymenin, ambrosin and hysterin. Parthenin is reported as inhibitor of germination and radicle growth. Parthenium also impacts on nitrogen fixing bacteria like azotobacter, azospirillum and actinomycetes reducing their activity in root nodulation and nitrogen fixation. The pollens of parthenium is very harmful for crops like tomato, brinjal, capsicum and beans inhibiting fruit setting in them. The weed is also a host for certain viruses which cause serious diseases in crop plants.
Effects on Animals:
In horses and cattle, parthenium causes dermatitis with lesions on skin, ulcers with excessive salivation when eaten and even cause death. In dogs it can cause severe irritation to eyes, alopecia, pruritus, and diarrhea. The meat and milk of buffaloes, goat and cow fed on parthenium can cause acute illness in humans. It also reported to weakening immunity in some animals by reducing white blood cell count.
Effects on Human Beings:
Parthenium secretes some allergens like ambrosin, tetraneuris, coronopilin and parthenin which causes allergies in human beings like dermatitis, hay fever, bronchitis and asthma. The pollen, dried leaves and even roots of this weeds can cause allergies in children playing outdoor or in adults. Contact of this weed to any body part can cause dermatitis and the allergy spread to other parts of the body with contact.
Control of Parthenium:
Control of this weed below threshold level is not an easy task. Many practices around the world are implicated to control and complete eradication. Some of them are discussed here.
It involves uprooting of parthenium manually by hand, sickle or by ploughing with tractor. But due to its ill effects the parthenium cannot eradicate and it’s a very difficult task to completely eradicate it. Some farmers plough the field at its rosette stage before it seeds and achieved success, but it must be followed by direct seeding of main crop. Some farmers burn the parthenium which is also not a good technique because it causes the main crop to be damaged also it require a huge quantity of fuel to burn all the weed.
Parthenium can be controlled by using different herbicides such as glyphosate, atrazine, chlorimuron, metsulfuron, and ametryn. Complete eradication can be obtained by application of 2,4-D EE and metribuzin, however glyphosate can also be used as a replacement of 2-4-D EE.
The time of herbicide application is also very important in controlling this weed. In no arable land like railway tracks or road sides, the control of parthenium can be obtained by spraying 15-20% solution of common salt (sodium chloride). Overall, the efficiency of herbicides application on parthenium is more at rosette plant as compare to bolted plants.
Chemical control is effective in parthenium but it should be kept on last because of the harmful effects of herbicide application. Some herbicides are toxic to crop plants. Some herbicides like glyphosate and 2-4-D are known to increase resistant among weeds. Regular use of herbicides also affects soil health and groundwater quality.
Control of weeds through natural enemies or by using biological agents like insects, bacteria, fungi or allelopathic plants is called biological control. It is very important due to its less impact on overall environment. Parthenium can be controlled by certain insects such as Leaf feeding beetle (Zygogramma bicolorata) and the stem galling moth (Epiblema strenuana). Both these insects are native to Mexico and have potential to control this weed. The adult and larvae of this insect feed on leaves of parthenium.
There are certain fungi which have ability to control this weed. Puccinia abrupta var. partheniicola. A rust pathogen, have reported to decrease the seed producing ability of parthenium by 42% and could be an effective control strategy. However, there is also some chances of developing rust on the main crop.
Integrated Weed Management
It is the combine use of all control practices physical, biological and chemical to control weeds. This technique is gaining importance in recent years. The biological, physical and chemical strategies when applied alone cannot be effective to control this weed. There is an example of controlling parthenium with IWM strategy.
In Australia, two plants Mitchell Grass and Butterfly pea along with two biological control agents a leaf and seed feeding beetle (Zygogramma bicolorata) and a stem galling moth (Epiblema strenuana), have been used to control this weed. The biomass of the weed was 6 to 23% more when the biological agents were not present. It shows that combine effects of different control techniques are useful in controlling this weed.
Parthenium hysterophorus can grow in a wide range of habitats and can cause not only economic but also serious health problems. There should be an appropriate method for the management of this weed to avoid potential threat to environment. The efficient and environment friendly method is the use of allelopathic insects and fungal pathogens, as compare to time consuming, costly, toxic and less efficient methods. Nine insect species and two rusts have been released in Australia to check this weed. Of these, two insects Z. bicolorata and E. stenuana, and two rust fungi, Puccinia abrupta var. partheniicola and Puccinia xanthii var. parthenii-hysterophorae, have shown potential and are being used to control this weed.