How easy is it to predict the potential of a plant to become invasive in a new habitat?
When an alien species, be it a pest or a weed gets into another habitat, there is generally a time lag before it becomes invasive. Its introduction, as such is sudden and people don't even realize that it has been introduced into a habitat. For example, in case of the parthenium hysterophorus, even realized that an alien species had entered the country. At that time, in the early 1950s, India was going through a big famine and was importing wheat, which contained parthenium seeds. The weed was first sighted in Pune in 1955 and after that it began spreading. Looking at the history, one will notice that the weed spread virtually along the railway tracks because the wheat was being transported by rail. The same thing happened on the roadways.
Soon, it spreads beyond control. Then the best thing to do is understand the kind of situation the weed thrives in its place of origin and do a GPS mapping of the habitat and its likely spread. Other data sources such as climexa programme that uses climatological information from throughout the world, can also be used. In my opinion, we require both biological and climatological information, including maybe even physiographic data, and then predict the areas where these species are likely to become pests.
This has generally not been done in India except once, for predicting the occurrence of the sugarcane woolly aphid when the genetic algorithm and the GIS were used. They were able to predict the kind of areas that the aphid is likely to become a pest. The thing with invasive species is that it is best for you to look for natural enemies from the country of origin of the pest or the invasive weed. So when we start looking for the natural enemy in the country of origin, we need to see whether that natural enemy will do well in our own environment.
Which weeds and invasives in India would you find the most difficult to handle?
The worst weeds would be the siam weed (Chromolaena odorata) lantana (Lantana camera) water hyacinth (Eichhormia crassipes) water fern (Salvinia molesta) parthenium (Parthenium hysterophorus) and mile-a-minute (Mikania micrantha).
Would the impact of invasive species on complex ecosystems be very different from those in monocultures, such as plantations?
There will be a difference. Several other biological entities, such as other weeds and indigenous plants are already thriving in a complex eco-system. When the invasives take over, they have the tendency to displace the indigenous plants or pests. When that happens, the effect is much more pronounced in a complex eco-system. There could be a large-scale displacement of indigenous plants. Sometimes this could be beneficial. But more often than not, it is harmful because once the indigenous species are gone, the entire ecosystem is upset.
In a monoculture situationan invasive plant is also likely to displace the indigenous weeds that are already there. So whether in a monoculture or a complex ecosystem, the difference is not much. But the difference will be more pronounced in a complex ecosystem because of the landslide effect.
Which of the three general models of invasions - the fluctuating resource availability modeland enemy release hypothesis and the niche opportunity model - would explain the principles of invasion best?
In the niche opportunity modelan invasive tries to occupy either a vacant area or displaces several other species. I would say this model is the best.
In the enemy release hypothesis, for instance in a crop situationa person looking for good traits in a crop goes to its centre of origin. With rice, for instance, the centre of origin is south-east Asia, where one will find numerous variants. In the natural enemy modea balance has been set up between the pest or weed and its natural enemies, which have co-evolved over a period allowing both to thrive. This balance helps the natural enemy have a upper hand over the weed or pest. One can identify this natural enemy and introduce it in a new area to set up a similar balance. This works if the climactic conditions are suitable.
Have any such introductions been made in India?
Yes, a lot of introductions have happened in India with weeds and pests. One of the earliest was in 1898 when the Australian ladybird beetle was introduced by a planter in the Palanis to control a citrus pest. There are a lot of commercial insectories producing natural enemies in large numbers. It is being used for the control of mealy bugs, which is a pest in grape plantations.
Can India have a specialised and central group of experts for research and management of invasives as in the US? Is it practicable for India to implement strict quarantine rules like New Zealand does?
In the US, virtually every state has its own independent way of looking at pests. They have strict quarantine regulations there. Quite often, if you land in the US with a plant material, it is burnt. But despite all this, maximum invasion is takes place in areas like Florida because of its huge tourist inflow.
Australia is one good example of quarantine functions. The Australian Quarantine Inspection Service keeps track of what is happening in and around Australia, as in Papua New Guinea, New Zealand and the surrounding islands. Recently on a trip to New Zealand, Indian cricketer Harbhajan Singh was asked to throw away his shoes as it contained a lot of soil, which may have contained invasives.
In India, all international airports have a quarantine inspector, but they are never present. Invasions are very difficult to control. The coffee berry borer pest came to India in 1991 not because of lack of quarantine but due to a humanitarian crisis. This was basically the crisis, which happened in Sri Lanka. This pest could be found in almost all coffee-growing countries, except India, at that time. During the troubles in Sri Lanka, however, Tamil migrants, especially plantation workers, coming in boatloads to India brought the pest with them.
The Siam weed, for instance, came during World War II from Indonesia and Burma, through Assam right down to the south of India. Today it is found all through the Western Ghats, where it is fast replacing the lantana. During the war, when soldiers came from Assam towards Kerala, brought the weed.
The mile-a-minute weed grows 20-30 cm a day and most of these weeds are South American in origin. This particular weed was used in Assam during World War II to quickly cover and hide runways. Today it has spread and is a major problem in the tea gardens in Assam as well as in the Western Ghats in Kerala.
In the past 14-15 years, India has had at least five or six invasions. With globalisation, it is now even more to control what pests are coming in and going out.
Australia has a very good method. When parthenium hysterophorus, which is a serious problem in India, was cited in Western Australia and Queensland, the authorities put a strong check on traffic and cattle movement in the ranches of Western Australia. They set up several check points where they would stop vehicles and wash the under-chassis.
In India we could have a very good quarantine system that should be linked with others who are working on invasive weeds or pests. There is, in fact, a regulation guided by the FAO where carries out pest-risk analysis for any import of biological organisms. Every country that wants to export material to India has to make this document of pest risk analysis.
Today, with a lot of genetically modified (GM) organisms coming inthe checking is also done by the National Bureau of Plant Genetic Resources.
How should the systematic study of invasive species continue in India? Should the money be put in research or in management?
In both. One needs to be proactive and understand the invasives. One should know what plants are coming into a country and the ones hat are likely to carry pests. We should have a list of those invasives that are endemic and those that are likely to come into our country.
So far, we have not been so proactive, but we do have list of pests that are likely to come in from other parts of the world and the quarantine officers check for the presence of these pests. However, sometimes we fail to implement the rules because of the volume of traffic.
There is a need to link all the universities, research organizations and the plant quarantine storage department. Most often, one organisation does not know what the other is doing. This problem could be solved only if all organisations sat together.
Several pests and invasives have been quite common. The water hyacinth which, for instance, is a common invasive weed found throughout the world. So a lot of research has been made on it. We can use that knowledge.
You mentioned that 69 bio-control agents have been established, leading to recovery. How exactly would you define recovery?
One-thirty-three agents were introduced, of which at least 69 agents have resulted in recovery. This is not a situation where we are talking about success, just recovery, even if in small numbers. We have very little data on the situation before and after the introduction of the agent. If the pests' natural enemies that are introduced are able to successfully parasitise the pest, we say recovery has taken place. Whether we get a good percentage of parasitism or whether it has really been able to bring down the population of the pest that is not really looked at.
Are the introduced agents usually animals, or is it possible to introduce plants with allelopathic effects to counter other weeds?
We generally introduce insects. There is a strong co-evolution between the pest and the plant. For example, if you take a weed control agent, this strong co-evolution has established that balance and so we try to introduce that particular natural enemy so the balance is established. Introducing a plant with allelopathic effect would be counter-productive because if we introduce a plant which is likely to displace this plantthe new one is again going to become a weed. For example in India we are talking about introducing a leguminous plant called cassia to displace parthenium. It is an indigenous plant and has an allelopathic effect, which means it is able to displace other plants.
Once you have a biological organism you really have no control over it. So it is very important to first study the possible consequences and only then go in for field releases. There is a system which is known as the centrifugal method, by which we keep making circles beyond the close relative of a particular plant and keep testing all the host plants in that circle and make sure that even the closest relative of that particular plant is not affected by the agent we are introducing. After doing this battery of tests, we test them in a no-choice situation as well as a choice situation, and make sure that the agent does not feed or breed on any other plant other than the plant against which it has been targeted. Only then do we introduce that agent.
There have been some exceptions. When we introduced the Mexican beetle against parthenium, it was able to bring down the parthenium population to some extent. But unfortunately, parthenium is a close relative of the sunflower and once the beetle population exploded and all the parthenium was destroyed, the beetles started feeding on sunflower. Later of course it was proved that this beetle was not able to complete its lifecycle on sunflower. There are some risks in biological control.
We haven't had a very good control over the weed. Generally what happens is, if we are controlling a particular weed, such as parthenium, we need to target the reproductive parts. For example, in Australia, 16 agents have been introduced against parthenium, while we have introduced only one. Since we had a problem with the Mexican beetles feeding on sunflowers, if we apply for a permit to bring in an agent for parthenium, it will probably get rejected. But in the case of Australia they have gone about bringing in 16 agents to control the weed. They have not had a very good success but they still have been able to check its spread.
What are the agents that have been introduced successfully in India?
The Australian ladybird beetle has given substantial benefits and is today being commercially produced by several insectories. At least three agents have been introduced for water hyacinth. But one of the most phenomenal successes as far as weed control is concerned is the water fern. In the canals of Kerala, a particular weevil has been able to keep the water fern under check. It is estimated that up to around Rs. 68 lakh have been saved in one year in Kerala because of the introduction of this weevil.
Has the Indian government given too much stress on pesticides? Has the trend changed perceptibly in the last few decades?
Of course, in India at least, we have to be largely dependent on pesticides. I don't think we can do away with pesticides because we don't have any alternatives in place. Pesticide production and usage can be slowly reduced but today we don't have a mechanism. For example, in the US you can buy some of these bio-control agents across the counter, sometimes even in supermarkets. We don't have viable mass production methodologies. Of course, today the emphasis has shifted from pesticides to botanical and bio-control agents and the integrated pest control strategy (IPM) emphasizes on a mixture of methods in an economical fashion. Some problems only have pesticide solutions, you have no other alternatives. Methods like bio-control are slow and don't produce dramatic results. The farmer is not easily convinced.