To win the battle against pests and weeds
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Members of native flora can also become weedy due to the activities of man
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Initial attempts made to control weeds and pests were based on mechanical approach supplemented by chemical control
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Though biological control sound novel, Sri Lanka has been using this control from the past
Sri Lanka is a country with a population of over 21 million. Being an agricultural country, crop yields have a huge impact on the economy, social well-being and the
livelihoods of people. Spread of pests and weeds has become a major threat to the native fauna and flora in Sri Lanka. Several sectors, such as agriculture, irrigation and the native bio-diversity have been affected by them.
Aquatic weed menace
Majority of the farming community relies on irrigation water for agricultural activities. Most of the cultivation is carried out in the dry zone experiencing periodic droughts during the cropping season. Reservoirs and integrated irrigation networks have been developed over the centuries to serve the need. Excessive growth of aquatic weeds is one of the reasons that restrict the potential benefits of these inland water bodies.
Invasive Alien Species
Typically, a species becomes a pest/weed due to an imbalance in the ecosystem whereby they develop an increased fitness, which makes them more competitive in exploiting an ecosystem. Most pests and weeds are alien species, which become invasive to a natural environment. Alien pests can accompany any live material that the public usually bring in their luggage from different regions or countries. Water hyacinth (‘Japan Jabara’) has been brought intentionally for ornamental purposes, while Salvinia for experimental purposes, which have now turned into Invasive Alien Species (AIS).
Members of native flora can also become weedy due to the activities of man. Land disturbances favour plant species with rapid growth rates and high seed production. Thus, although natural enemies (organisms that naturally control other organisms) are present, they survive poorly and operate slowly and inefficiently within the ecosystem, favouring the rapid population increase of the host plant. Some local examples are ‘Lotus’ and ‘Pan’ species. Plant species, such as ‘Gandapana’ and ‘Yodha Nidikumba’ have also become invasive in Sri Lanka, posing a threat to both agricultural and natural ecosystems (e.g. national parks).
Switching from mechanical control to biological control
Initial attempts made to control weeds and pests were based on mechanical approach supplemented by chemical control. It resulted in a high yield with better quality. However, the use of chemicals has many deleterious effects, such as resistance to pesticides/weedicides, high cost, safety issues, short-term control requiring repeated applications and the threat to favourable organisms and aquatic systems. Due to rapid multiplication of the weeds and pests and the relatively low efficacy of control methods, subsequently, control attempts were diverted towards biological control.
Advantages of biological control
Biological control is a human activity of manipulating natural enemies to reduce or suppress the pest/weed population below the economic threshold level. The agents involved in biological control are termed as Biological Control Agents (BCA). Bacteria, viruses, fungi, nematodes and insects are utilized as agents in biological control. They infect the pest or weed through direct penetration or by spores. Their mode of action has a vast variation; some act as organisms themselves through competition, predation, parasitism, and induction of plant resistance, while others produce chemical substances (antibiotics, toxins) harmful for the host organism. Though they have some drawbacks like slow action, low persistence when exposed to solar UV, high cost of production and lack of awareness among growers, they are still eco friendly. Moreover, biological control has many advantages over mechanical or chemical control; safety, limited host range (very low risk to non-target organisms), low cost, less frequent applications, sustainability and the ability to control viral diseases for which no chemical control is available.
Though biological control sound novel, Sri Lanka has been using this control from the past. The coconut leaf miner which devastated coconut cultivation, the beet leaf minor which was a threat to the vegetable cultivation in the hill country and the Guava white fly which killed many guava trees even in-home gardens were completely eradicated or kept under control by imported parasitoids.
The Salvinia weevil has helped successful control of Salvinia in most of the aquatic habitats at low elevations. The mottled water hyacinth weevil and the chevroned water hyacinth weevil brought from Thailand have been introduced to control water hyacinth.
Damping off disease caused by fungal pathogens at the nursery stage of big onion is a major constraint on its production. Scientific studies have been carried out to assess the ability of selected Trichoderma sp. isolated from local onion fields in suppressing the damping off pathogen Fusarium sp., which is known to be an effective method worldwide.
The Papaya Mealybug (‘Piti Makuna’) is an alien invasive insect species feeding on many plant species. Due to the nature of the pest and the variety of host plants it attacks, insecticidal control is not practicable. Therefore, a parasitic insect named Acrorhaguspapaya,already proved to be successful elsewhere, is to be introduced to our country.
Minimizing negative impacts of biological control
The main mission in developing a BCA is to have a minimal disruptive influence upon the environment and its inhabitants for sustainable and environmentally compatible management. However, there is always a doubt when introducing another species to control a pest/ weed, since it can also impact native flora and fauna, alternative vegetation and compete to establish itself in that environment. Some BCAs have failed in the controlling process due to the use of general predators (e.g. Lady bird beetles). Therefore, during the introduction of a BCA it is very important to carry out host specific studies. Today, under the International Plant Protection Convention there are international guidelines for safe importation and release of biological control agents. Those with strict and properly functioning quarantine regulations will minimize the negative impacts of biological control.
Importance of discovering new Biological Control Agents
Since Sri Lanka consists of a great diversity and endemism in flora and fauna, our ecosystems might be potential sources of BCAs. Indigenous knowledge is a valuable source in finding out new BCA. Financial and technological support from the governmental departments, universities and private organizations is very important in the process of invention and commercialization of new BCAs.
Making headway the biological control way!
Strengthening the commercial natural enemy industry, promoting research and educating the usage of BCAs, developing quality certification programmes for BCAs, implementing necessary legal procedures (Water Hyacinth Act, Plant Protection Act) and awareness programmes on environmental concern in our society augur well for the way foreward in controlling pests and weeds.
(The writer is an undergraduate of the Faculty of Science, University of Colombo)
