Medicinal Uses of Ageratum conyzoides
The plant has been used in traditional folk medicine in Africa and South America to treat a variety of ailments. A common custom is to treat burns and wounds. Laboratory studies have verified the antibacterial activity of Ageratum conyzoides extracts against the human pathogens Staphylococcus aureus, Pseudomonas aeruginosa, and Escherichia coli. Studies have been conducted on the efficacy of this plant in treating human systemic illnesses; however, the plants contain toxic pyr- rolizidine alkaloids that can cause liver damage in humans.
Effects of Ageratum on Agriculture
The breadth of secondary metabolites found in this plant causes it to affect a number of other species, both invertebrate and botanical. Ageratum conyzoides produces the compound ageratochromene, which inhibits the growth of insects by interfering with their juvenile growth hormone. The leaves of this species of ageratum have been used as moth repellants, and extracts of the leaves have been shown to interfere with the development of a number of types of insects, including the domestic fly, a type of moth, and several types of mosquitoes. It is considered a promising candidate for the development of natural insecticides.
Another class of invertebrates affected by ageratum is nematodes. In this case, Ageratum houstonianum was found to suppress the growth of nematodes just by using the plants as a mulch. The research was inspired by the knowledge that Crotalaria species were used to inhibit nematodes because of their production of pyrrolizidine alkaloids. The production of these toxic compounds is a common trait of ageratum.
Ageratum conyzoides is one of three species of plants listed as the most economically destructive weeds in the world. It is able to invade cultivated fields and is extremely difficult to eradicate. Part of the reason is that it spreads by stolons and has wind-borne seeds, but the major factor is that the plant is allelopathic. It produces chemicals that inhibit the growth of plants around it, including crops. One strategy of weed control is to identify the chemicals responsible for this effect and incorporate them into herbicides. Another is to utilize the plant as a mulch, or even a counter crop, to help control other weeds. Such approaches fit nicely into sustainable agricultural programs. For instance, Ageratum conyzoides has been grown on the floor of citrus orchards in southern China for a long period. It crowds out other weeds and inhibits the growth of pathogenic fungi. Unexpectedly, there was also an increase in predatory mites when ageratum was planted. These mites cause a decrease in the levels of parasitic citrus red mites in the orchards. This effect was found to be due to volatile chemicals released from the essential oil of Ageratum conyzoides.
The data so far suggest that the prolific production of secondary metabolites by ageratum species will facilitate their incorporation into new and alternative methods of pest, pathogen, and weed control.
Helga George
Further Reading
Kong, C. H. “Ecological Pest Management and Control by Using Allelopathic Weeds (Ageratum conyzoides, Ambrosia trifida, and Lantana camara) and Their Allelochemicals in China.” Weed Biology and Management 10 (2010): 73-80.
Mahr, S. 2009. “Ageratum.” University of Wisconsin Extension. http://pddc.wisc.edu/ factsheets/Low%20Color%20PDF%20Format/Ageratum.pdf (accessed 13 September 2011).
Ming, L. C. “Ageratum conyzoides: A Tropical Source of Medicinal and Agricultural Products.” In Perspectives on New Crops and New Uses, edited by Jules Janick, 469-73. Alexandria, VA: ASHS Press, 1999.
Thoden, T. C., and M. Boppre. “Plants Producing Pyrrolizidine Alkaloids: Sustainable Tools for Nematode Management?” Nematology 12 (2010): 1-24.
