Ageratum conyzoides (herb)
Insecticidal: A. conyzoides has natural biocide activity that may have agricultural insecticidal use, as shown by several research investigations in different countries. The leaves of the plant reportedly have moth-repellent properties (Pereira in 1929, in Ming 1999). The plant's terpenic compounds, mainly precocenes, with their antijuvenile hormonal activity are probably responsible for the insecticide effects. The affect of A. conyzoides on insect larva is to arrest juvenile development; this effect has been seen in Musca domestica (fly) larvae, Chilo partellus (Lepidoptera, Pyralidae), a sorghum pest, mosquitoes (Culex quinquefasciatus, Aedes aegypti, and Anopheles stephensi)
Essential oil extracts from the leaves of A. conyzoides caused significant morality of the maize grain weevil, Sitophilus zeamais. Mortality increased with the concentration of essential oil and the duration of exposure (Bouda et al. 2001).
Herbicidal: A. conyzoides showed strong inhibition of Raphanus sativus (radish) germination and growth in a bioassay. The leaves exhibited a greater suppression than the stem and root. The leaves of A. conyzoides reduced about 70% of the growth of Echinochloa crus-galli var. formosensis and completely inhibited emergence of Monochoria vaginalis var. plantaginea) and Aeschynomene indica in calcareous soil condition. Application of A. conyzoides leaves caused about 75% paddy weed reduction and increased yield by 14% compared with a herbicide treatment. Three phenolic compounds were identified in the leaves, stem and root including gallic acid, coumalic acid, and protocatechuic acid, and catechin was found only in the stem. P-hydroxybenzoic acid was detected in both A. conyzoides leaves and stem. Three additional putative allelochemicals were found in the leaves consisting of p-coumaric acid, sinapic acid, and benzoic acid. A. conyzoides might be a natural herbicide for weed control in paddy fields to reduce the dependence on synthetic herbicides (Xuan et al. 2004).
Chemical: Pre-emergence application of simazine, atrazine, diuron, oxadiazon, oxyfluorfen, methazole or metribuzin provides excellent control of this weed. Post-emergence application of 2.4-D controls established infestations (Rao 2000).
On the other hand eco-based, environment-friendly strategies for the effective control of A. conyzoides are suggested. Plant extracts of parthenin and eucalyptus (volatile essential oils) may hold promise in controlling A. conyzoides (Batish et al. 1997, Singh et al. 2002, in Batish et al. 2004) and some success has already been achieved using these and other natural plant extracts. For example, a study on the allelopathic effect of two volatile monoterpenes (cineole and citronellol) on A. conyzoides has revealed their potential for future weed management. Both the monoterpenes severely affected the germination, speed of germination, seedling growth, chlorophyll content and respiratory activity of A. conyzoides and after two weeks of exposure, the weed plants wilted. Cineole was the more toxic of the two monoterpenes (Singh Batish and Ravinder 2002).
The addition of activated charcoal, an inert material with high affinity for organic biomolecules, partly ameliorates the negative effects of A. conyzoides phenolic allelopathic root residues on rice (Oryza sativa) growth (Batish et al. 2009).
Field and crop management: Increased fallow length in slash-and-burn rice (Oryza sativa L.) production systems of northern Laos decreases weed pressure (Roder et al. 1998). Compared with continuous rice treatments treatments with fallow or cowpea (Vigna unguiculata) in the previous year had 32% less herbaceous weed biomass and 90% fewer A. conyzoides.
The timing of weed seedling emergence relative to the crop is important in planning and optimizing the time of weed control. Ekeleme and colleagues (2005) have predicted seedling emergence of tropical weed species, specifically in low-input and small-scale farms. To control weeds adequately, especially with limited use of herbicides, farmers need to know the timing and extent of weed seedling emergence before and during the growing season. Knowledge of when weeds emerge is equally applicable and beneficial to all forms of weed-management technologies (see: Ekeleme et al. 2005).
Research: To respond effectively to invasive species problems quantitative measurements of the impact of invasion on biodiversity are required (Schooler et al. 2006, in Dogra et al. 2009).