Invasive alien species (IAS) have become an environmental concern in India. The issues of invasive weeds and insects and their management in the two sectors of agriculture and forestry are highlighted. Some aquatic and terrestrial weeds are described, the former generate more problems. Three categories for invasive insects in forest tree crops are described. Many pest issues, earlier considered to be minor, have attained pest status in India owing to intensive management practices. Also, new and emerging pests have been attributed to agrosilvicultural practices. The main management option for weeds and insects is biological control. There is an urgent need to address the various issues connected with alien weeds and insects in a focused and coordinated manner at the national level warranting intersectoral approaches.
It is maintained that invasive alien species (IAS) are the greatest threat to biodiversity around the globe. The introduction of IAS can be intentional or accidental. There are several examples of international introductions of exotics in the agricultural, forestry and fishery sectors. Accidental introduction mainly occurs through travel or imports such as food grains and wood. Some invasive species can affect the structure and function of ecosystems. India has a vast range of biodiversity. There are nearly 500 wildlife sanctuaries, 90 national parks and 13 biosphere reserves. Invasive species, especially weeds, have been a serious problem for forestry and agriculture. It is estimated that out of about 45 000 species of plants recorded from India, nearly 1 800 are alien and out of the known 54 430 arthropods (including insects), nearly 1 100 are alien. Thus IAS have become an environmental issue of concern.
In the agricultural sector more attention to the management of alien invasive weeds is given because of the social needs attached to the farming community. No serious attempt has been made in the past to look at the intersectoral problems related to alien invasive species. This paper highlights the issues of invasive weeds and insects and their management in the two sectors of agriculture and forestry.
Both aquatic and terrestrial weeds are of concern, the former causing more problems for farming, fishing and navigation. Only aquatic weeds of relevance to agriculture are dealt with here.
Salvinia molesta Mitchell (Family: Salviniaceae) and Eichornia crassipes (Martius) (Family: Pontederiaceae) are problematical for farmers in many places and are of serious concern among paddy cultivators in the state of Kerala. Salvinia made its entry in India before 1900 and by now more than 0.2 million hectares of waterbodies are affected. In addition to the problems caused to the farming community, the weed can choke waterbodies and serve as ideal breeding grounds for mosquitoes. Eichornia, popularly known as water hyacinth, was introduced as an ornamental pond plant from the Amazon Basin; now it has become a menace in the backwaters.
For the two aforesaid weeds, conventional control measures have proved less effective. As a sustainable management strategy, biological control methods have been adopted in many areas nationwide. Studies by Joy et al. (1985) and Singh (2001) proved the efficiency of managing Salvinia and Eichornia with biocontrol agents. The curculionid weevil, Neochetina eichhorniae Warner and N. bruchi, natives of South America were introduced to India in 1982 as control agents against Eichornia and established in many places.
Lantana camara is a terrestrial weed of South and Central American origin introduced as an ornamental plant in 1809 to India. Usually this weed invades disturbed natural ecosystems and adversely affects biodiversity. The weed is distributed throughout India.
In forests, Lantana is considered as a potential fire hazard in deciduous forests and it is combustible even when green. Thus this weed can be dangerous in national parks and sanctuaries. Lantana also competes with agricultural crops and has an allelopathic effect – inhibiting the growth of other plants. This weed is reported to be of concern in teak, eucalypt and coffee plantations in India.
Various mechanical, cultural, chemical and biological methods have been tried to minimize the spread of Lantana in forests and pastoral lands. Serious attempts were made by the Forest Research Institute, Dehra Dun to identify insects that feed on Lantana (Beeson and Chatterjee 1939). In 1941, a tingid bug, Teleonema scrupulosa, was imported from Australia as a biocontrol agent. However, this insect fed on Tectona grandis (teak) and hence the insect culture was destroyed (Khan 1944). In spite of this, this insect has been reported from many states in India (Sushil Kumar 2001).
Mikania micrantha has been a serious problem in the southern states and also in the northeastern states. This perennial climber has been known since 1918 and has been reported as a menace in many parts of Asia and Oceania. Many forests and agricultural crops are being suppressed owing to the prolific spread of Mikania. Numerous field crops (sugarcane, maize, rice, pineapple, cotton, coffee), forestry crops (teak, eucalypts) and agroforestry systems are under the grip of this invasive weed.
Many insects have been reported to feed on Mikania. However, most of them are polyphagous and are of no use as biocontrol agents. Similarly indigenous pathogens were also found to be ineffective in controlling this weed, mainly due to their non-virulence. In India, attempts are being made to assess the potential of an exotic rust fungus, Puccinia spegazzinii against Mikania (Sankaran pers. comm.). This fungus is undergoing strict quarantine watch and several economically important agricultural and forestry crops are being screened. There have been many advocates of 2,4-D compounds against Mikania but its toxic action on biota and long persistence (Wang et al. 1994), make it unsuitable as a control agent.
Mimosa pudica, an introduced weed, has become a serious problem in coconut and cashew plantations. Similarly it has been reported in glades of natural forests and also in teak plantations. Manual removal of this weed is difficult, due to its spiny nature.
Chromolaena odorata is a perennial shrub, native to South and Central America, and capable of establishing in a wide variety of agro-ecological conditions. Chromolaena odorata is a serious problem in pastures, forests, orchards and commercial plantations in South and Northeast India (Singh 1998). It is widespread in coconut, rubber, oil palm, tea, teak, coffee and cardamom plantations and also in natural forests. Several insects and pathogens have been reported to be useful as biocontrol agents, but none has been effective in a practical way. Of late, this weed is also being used as a source of green manure by farmers in many parts of the country.
There are three categories for invasive insects in forest tree crops: (i) exotic insects on exotic plants, (ii) exotic insects on native plants and (iii) native insects on native plants.
The spread of Heteropsylla cubana (Homoptera: Psyllidae) is a good example of exotic insects on exotic tree species. However, H. cubana is now not regarded as a serious problem in India.
The occurrence of the spiraling whitefly, Aleyrodicus disperses, is an example of an exotic insect invading native crops. It is a native of Central America and spread westward across the Pacific, Southeast Asia and entered India through Sri Lanka in 1994. This insect has been reported as feeding on more than 150 species including fruit plants, vegetables and avenue trees. This insect is also reported to feed on the leaves of intensively managed teak plantations (Varma et al. 2001).
Native insects becoming invasive on native plants are also an issue. For example, the Rhinoceros beetle, Oryctes rhinoceros is an established pest of coconut palms in India. Very recently it has become a problem for oil palm in southern states of India. The incidence of the pest in oil palm plantations closer to natural forest areas has been noted in many places. Beetles were also collected from natural forest areas – decaying logs served as breeding grounds and the adult beetles later migrated to susceptible crops like oil palms and coconut. This is also an example of a pest connection between the agriculture and forestry sectors.
There are several cases of invasion of indigenous insect pests on indigenous tree crops in forest plantations, for example Hyblaea puera, a major defoliator pest of teak. There are periodic outbreaks of this pest in all age classes of teak plantations.
Though teak is indigenous only to India, Myanmar, Thailand and Lao PDR, this pest has been reported in other countries where it is an exotic. The level and extent of H. puera invasion in these countries need to be monitored.
Many pest issues, which earlier were considered to be minor, have attained pest status in India owing to intensive management practices, especially for teak (Varma et al. 2001). Also, new and emerging pests have been attributed to agrosilvicultural practices. For example, Helicoverpa armigera, which is known to be a notorious agricultural pest, was found to feed on the terminal shoots of young teak in Tamil Nadu. Here, groundnut was introduced as an intercrop in a young teak plantation with a view to increasing the nutrient content of the soil and obtaining extra revenue. But the agroforestry practice generated a new pest problem (Varma et al. 2001).
There has been inertia among agricultural, forestry and other stakeholders in tackling invasive species. The ecosystems concerned are so sensitive that the control strategy to be adopted must be as safe as possible. Eradication is usually not feasible but strategies can be evolved to reduce the density and abundance of invasives below the threshold level.
As a short-term strategy, insecticides and weedicides have been suggested, but they have far-reaching consequences in the long term. The main thrust is on biological control of weeds and insects. However, care should be taken when employing exotic pathogens/parasites for biological control – they should undergo strict quarantine procedures at recognized national institutions before introduction as a control agent.
Some alien weeds have been naturalized in India and provide income for the rural community. One example is the Lantana basket weavers in some districts of Tamil Nadu, who have been in this business for more than 50 years. Thus there is also a need to address related socio-economic aspects.
The agencies that control the activities of the agricultural sector on IAS include various agricultural universities at the state level, state agricultural departments and the Central Agricultural Ministry with the Indian Council of Agricultural Research as the core centre. For the forestry sector there are state forest research institutes, state forest departments and the Ministry of Environment and Forests with the Indian Council of Forest Research as the core centre. Coordination and cooperation between forestry and agriculture and also among other related sectors is not taking place at the required level.
The knowledge and expertise among the agriculture and forestry sectors have to be tapped and how to best integrate the skills of these two groups is crucial.
Habitat degradation and loss of biodiversity are two major consequences of IAS. Networking among experts in the fields of agriculture and forestry, both at the state level and at the centres is very important. Areas in which experts from the two sectors can work together have to be explored.
There is an urgent need to address the various issues connected with alien weeds and insects in a focused and coordinated manner at the national level. Some of the examples cited clearly warrant the need for and scope of intersectoral approaches. A number of possibilities still exist for the introduction and multiplication of invasive species. Thus quarantine and monitoring aspects have to be strengthened with vigilance and caution. A more sensible approach would be to prevent the introduction of IAS and hence prediction methods and risk analysis have to be perfected to suit national needs. India needs to study the approaches of other countries for managing invasive species and the technologies that have been adopted.
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Joy, P.J, Satheesan, N.V, Lyla, K.R & Joseph, D. 1985. Successful biological control of the floating weed Salvinia molesta, using the weevil, Cyrtobagous salviniae. Paper presented at the Asian–Pacific Weed Science Society, Chiang Mai, Thailand.
Khan, A.H. 1944. On the Lantana bug (Teleonema scrupulosa Stal.). Indian J. Entomol., 6: 149 - 161
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* Kerala Forest Research Institute, Peechi-680653, Kerala, India.