Parthenium hysterophorus

The use of insect and fungal pathogens and the exploitation of allelopathic plants is considered by Kaur et al. (2014) as the most economical and practical way to manage the infestations of the species. Biological control has been, and continues to be, considered the best long-term or sustainable solution to the parthenium weed problem in Australia (Haseler, 1976, McFadyen, 1992) and because of the vast areas and the socio-economics involved, this approach has also been proposed for India (Singh, 1997). South Africa was the first country in Africa to implement a biological control program against the species in 2003 (Rubaba et al., 2017). Four host-specific biocontrol agents have been released sequentially since 2010 after evaluation of their suitability, with variable establishment and spread (Strathie et al., 2016).;The use of insects as biocontrol agents had been tried in various countries (Kaur et al., 2014). Searches for, and evaluation of, coevolved natural enemies have been conducted in the neotropics since 1977. So far, nine insect species and two fungal pathogens have been introduced into Australia as classical biological control agents (Julien, 1992, McClay et al., 1995, Navie et al., 1996, Dhileepan and McFadyen, 1997, Evans, 1997a). Callander and Dhileepan (2016) report that most of these agents have become established and have proven effective in central Queensland, but that the weed is now spreading further into southern Queensland where the biocontrol agents are not present. Several of the agents are therefore now being redistributed into south and southeast Queensland.;The rust fungus, Puccinia abrupta var. partheniicola, is a prominent natural enemy in the semi-arid uplands of Mexico (Evans, 1987a, b), but since its release in Queensland in 1992, climatic conditions have been largely unfavourable (Evans, 1997a, b). It was accidentally introduced into Kenya (Evans, 1987a) and Ethiopia in mid-altitudes (1400-2500 masl) with disease incidence up to 100% in some locations (Taye et al., 2002a). Screening of another rust species (Puccinia melampodii) from Mexico was carried out (Evans, 1997b, Seier et al., 1997) and released in Australia in the summer of 1999/2000 (PAG, 2000). This fungus was later renamed Puccinia xanthii Schwein. var. parthenii-hysterophorae Seier, H.C.Evans & ç.Romero (Seier et al., 2009). Retief et al. (2013) report on specificity testing carried out in quarantine facilities in South Africa, and conclude that the fungus is suitable for release as a biological control agent of P. hysterophorus in South Africa. The authors suggest that this species has more potential for biocontrol in South Africa than Puccinia abrupta, which may have little impact in the low-altitude, high-temperature areas of the country where the weed is spreading.;In India, the mycoherbicide potential of plurivorous fungal pathogens belonging to the genera Fusarium, Colletotrichum, Curvularia,Myrothecium and Sclerotium, has and is being evaluated (Mishra et al., 1995, Evans, 1997a). Parthenium phyllody disease caused by the phytoplasma of faba bean phyllody group (FBP) was reported to reduce seed production by 85% (Taye et al., 2002b) and is being evaluated for use as a biological control agent in Ethiopia. Kaur and Aggarwal (2017) have tested an Alternaria isolate found on the weed, and report that it is worth investigating as a mycoherbicide for control of parthenium. Metabolites of Alternaria japonica and filtrates of Alternaria macrospora have caused significant damage to Parthenium (Kaur et al., 2015, Javaid et al., 2017).;Among the established insect biocontrol agents, the leaf-feeding beetle, Zygogramma bicolorata, the stem-galling moth, Epiblema strenuana, the stem-boring beetle, Listronotus setosipennis, and the seed-feeding weevil, Smicronyx lutulentus, are proving to be the most successful when climatic factors are favourable (McFadyen, 1992, Dhileepan and McFadyen, 1997, Evans, 1997a). Some control of parthenium weed has also been achieved in India with Z. bicolorata (Jayanth and Visalakshy, 1994, Singh, 1997, Sarkate and Pawar, 2006), although there has been controversy concerning its taxonomy and host specificity (Jayanth et al., 1993, Singh, 1997). Shabbir et al. (2016) reported that Z. bicolorata was most effective when applied in higher densities and at early growth stages of the weed. The distribution of this leaf beetle in South Asia was investigated by Dhileepan and Senaratne (2009), when it was present in many states in India, and in the Punjab region of Pakistan. Shrestha et al. (2011) reported that Z. bicolorata arrived in the Kathmandu Valley of Nepal in August 2010, and that by September it had spread over half of the valley areas where P. hysterophorus was present, although damage to the weed was only appreciable at one site.;Z. bicolorata has been seen attacking sunflowers in India and the use of Epiblema strenuata has not been effective, as it was found affecting Guizotia abyssinica crops (Kaur et al., 2014). More recently, Z. bicolorata and L. setosipennis have been released in South Africa and S. lutulentus is being evaluated under quarantine. Before approval as a biocontrol agent in South Africa in 2013, extensive testing suggested that Z. bicolorata would not become a pest of sunflowers in the country (McConnachie, 2015).;The use of antagonistic, competitor plants, such as Cassia spp. and Tagetes spp., has been recommended to control and replace P. hysterophorus in India (Mahadevappa and Ramaiah, 1988, Evans, 1997a, Mahadevappa, 1997, Singh, 1997). In Australia, Bowen et al. (2007) tested a number of grass and legume species against the growth of parthenium weed plants and identified further species that could suppress weed growth. Recently, Khan et al. (2013) tested a number of native and introduced pasture species and identified several of them to be suppressive against parthenium weed in both glasshouse and field conditions. The sowing of selected pasture plants in infested areas can suppress the growth of parthenium weed by as much as 80% and also provide improved fodder for stock (Adkins et al., 2012). Shabbir et al. (2013) showed that the suppressive plants and biological control agents can act synergistically to significantly reduce both the biomass and seed production of parthenium weed under field conditions. The suppressive plants strategy is easy to apply, sustainable over time, profitable under a wide range of environmental conditions and promotes native plant biodiversity. Species reported as effectively outcompeting P. hysterophorus are Cassia sericea, C. tora, C. auriculata, Croton bonplandianum, Amaranthus spinosus, Tephrosia purpurea, Hyptis suaveolens, Sida spinosa, and Mirabilis jalapa. Extracts of Imperata cylindrica, Desmostachya bipinnata, Otcantium annulatum, Sorghum halepense Dicanthium annulatum, Cenchrus pennisetiformis, Azadirachta indica, Aegle marmelos and Eucalyptus tereticornis are reported as inhibiting the germination and/or growth of P. hysterophorus (Kaur et al., 2014).