NEW DELHI: The National Research Centre for Weed Science, an institute of the Indian Council of Agriculture Research, has launched a campaign for biological control of the Mexican parthenium weed through beetles imported from Mexico. The weed is popularly known as Congress grass. It is also called carrot weed because it resembles a carrot plant and is now found in all parts of the country.

Parthenium hysterophorus is a weed that was imported into India along with the PL 480 Mexican wheat seeds in the 50s. The weed has since grown into uncontrollable proportions invading million of hectares of uncultivated wastelands, roadsides, railway tracks, etc. The fast growing weed is a nuisance in public parks, residential colonies and orchards.

Not only that, it causes health hazards such as skin allergy, hay fever and asthma in human beings and is toxic to livestock. It squeezes grasslands and pastures, reducing the fodder supply. Scientists describe it as a "poisonous, allergic and aggressive weed posing a serious threat to human beings and livestock."

The presence of parthenium in cropped lands results in yield reduction up to 40 per cent. The pollen grains inhibit fruit set in tomato, brinjal, beans, etc. It is also responsible for bitter milk disease in livestock fed on grass mixed with parthenium.

Parthenium is a fast maturing plant with a deep tap roof that can grow to a height of 1.5 to 2 metres having branched leaves covered with fine hairs. It grows a large number of small white flowers and seeds of light weight that are easily dispersed to distant places causing allergy in human beings. Each plant can produce up to 10,000 seeds. It has the capacity to re-grow from the cut or broken parts. It has no natural enemies such as insects and diseases because of which it spreads rapidly in India.

According to N.T. Yaduraja, Director of the Jabalpur-based Centre, one of the ways to control the weed is to uproot the weed before flowering. Any newly emerging seedlings should be removed. Herbicides such as glyphosate (one to 1.5 per cent) for total vegetation control or metribuzin (0.3 to 0.5 per cent) could be used if the grasses are to be saved.

Another said an effective way of controlling the weed is to spread seeds of self-perpetuating competitive plant species such as cassia sericea, cassia tora, tagetus erecta and tephrosia purpurea.

After much research, Mexican beetles (zygogramma bicolorata) was considered a "safe" biocontrol agent. The institute is now campaigning for using this beetle for parthenium suppression.

The larvae feeds for 10 to 15 on the leaves and on maturity enters the soil and pupates below up to 15 cm depth. The beetles emerge after eight to 12 days and completes its life-cycle in 27 to 32 days. Both the adults and larvae are capable of feeding on the parthenium leaves thus checking the plant growth and flower production.

Adults defoliate the plant. Immature flowers are cut by the beetles in an effort to chew the soft tissues beneath the flowers. Completely defoliated plants start to show die-back symptoms and gradually get killed.

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