The Impact of Exotic Pests on Asian Agriculture
New weeds, pests and crop diseases are entering the Asian and Pacific region
Other exotic pests have been introduced intentionally, for some expected benefit, and have then become a problem. Asia's foremost aquatic weed, water hyacinth, was brought to Asia intentionally as an ornamental plant. Tilapia, a tropical freshwater fish, was originally introduced into Asia as a food fish, particularly for aquaculture. In some areas, such as Plampang Lake in Indonesia, tilapia have spread aggressively. As a result, their introduction has caused a major decline in the populations and range of indigeneous fish species. The golden snail, also known as the apple snail, is an another example of a species which was intentionally introduced into Asia as a beneficial food species and became a major pest.
Many exotic species, whether brought in accidentally or introduced on purpose, spread rapidly in a new environment. This is because they lack the natural enemies and competitors which limit their numbers in their original habitat. In November, FFTC held an international seminar on these species and their impact in Asia.
Impact of Exotic Species
Nearly all plant pathogens introduced into Asia from overseas are accidental introductions. An example is coffee rust, a fungal disease which has destroyed nearly all the Arabica coffee plantations in the coastal regions of Indonesia.
In Thailand, two important bacterial plant pathogens are fruit blotch of watermelon, caused by Acidovorax avenae, and bacterial wilt, caused by Ralstonia solanacearum. Fruit blotch was probably brought into Thailand in infected seeds as part of the international seed trade. The source of bacterial wilt, which infects many important crops such as potato, tomato and ginger, was probably infected potato tubers which were imported as planting materials.
Some Exotic Weeds
Accidental introductions are also the source of most, though not all, weeds. For example, more than three hundred species of exotic weeds have become established in Korea. Of these, more than a third are recent introductions, reported since 1980. A study of the distribution of exotic weeds in Korea showed that they are generally found near ports, along roadsides and in reclaimed land. This suggests that the weeds were probably accidentally introduced in imported grain which was contaminated with weed seeds.
Plants which are likely to become a weed problem or invade natural habitats are those which show rapid growth, produce a lot of seeds which are widely dispersed, are able to out-compete native species, and are difficult to remove or control.
However, many of Asia's most serious weeds were introduced on purpose, as plants used in conservation programs. Plant species used for conservation must be vigorous and spread quickly. The same features that make them successful in soil conservation also make them successful weeds!
Leucaena
It will surprise many people to see that the leguminous tree Leucaena leucocephala has become a weed in some areas. A native of Central America, leucaena was introduced into Asian countries as a fodder crop and as a means of reclaiming degraded soils.
In Taiwan, the tree was introduced several hundred years ago. Since the 1980s, many marginal agricultural areas in Taiwan have been abandoned.
Leucaena trees have spread rapidly in the abandoned fields, often forming pure stands. Leucaena is difficult to remove, since it is a prolific producer of highly fertile seeds. It also has the ability to produce several dozen new sprouts when the trunk is cut. Furthermore, leucaena has strong allelopathic potential, and is able to kill competing plant species growing nearby.
Current control programs in Taiwan are based on the high light demand of leucaena trees. Small areas are cleared by cutting the trees and treating the cut trunks with herbicide. Fast-growing native trees are then planted which alter the light environment as they grow. It is hoped that they will eventually shade out the leucaena.
Mikania Micrantha
One of the common names for M. micrantha is "mile-a-minute". It is a good name for this perennial vine with its rampant growth! A native of tropical Central America, it can spread both by its windblown seeds and by creeping stems which root at the nodes on contact with soil. As with leucaena, M. micrantha is a prolific seed producer. Studies in Taiwan showed that at the peak flowering season it produced 0.17 million seeds per square meter of coverage!!
It has caused severe economic losses all over Asia. M. micrantha is a serious weed of plantation crops such as tea and rubber, and fruit orchards. It is also a major threat to forests, and is sometimes called "the cancer of the forest". When the vines of M. micrantha cover the crown of a tree, they prevent light from reaching the leaves and interfere with photosynthesis. The vines may eventually kill the tree. They also make the tree less favorable as a habitat for birds, and can have a disastrous effect on native bird populations. In agricultural areas, the vine is sometimes controlled by chemicals, but these cannot be used over large areas of forest without posing a public health risk and a danger to the environment. In Taiwan, a procedure has been developed of controlling the weed by cutting the vines close to the ground once a month for three successive months. This is most effective when done in late summer or autumn, and can eliminate 92-98% of the vines.
Risk Assessment
Risk assessment is the process of seeing how much damage a new pest is likely to cause. Two different models of risk assessment are needed, one for intentional releases and one for accidental ones.
A risk assessment model for an intentional release should focus on its history, and look at whether it has caused problems in other areas in the past. Other points to consider are whether the species has a wide distribution and a high rate of increase. Species are more likely to be a problem if they have a large natural range, a high rate of increase, and a large founding population.
While these principles might help identify high-risk species, it is not so easy to apply them in practice. Most countries do not wish to be invaded by any alien species at all. Furthermore, risk assessment may not seem to justify its cost. It is difficult to prove that advance action has in fact prevented invasions of exotic species. The only proof is pest species which aren't there!
Detection and Monitoring
All countries have quarantine and monitoring systems which attempt to keep out accidental invasions of exotic species, and strictly monitor intentional releases. However, even the best systems cannot hope to be completely successful. Some alien species will get through. Early detection and development of control measures are the key to preventing a new exotic pest from becoming established.
In all countries, the increased flow of imported agricultural products and international tourism mean that quarantine inspectors are finding it very difficult to monitor imports. Hundreds of thousands of shipping containers may enter a country each year. Even if interception rates are perfect, it is impossible to detect all the contaminated containers which are harboring pests.
As well as the problem of checking a massive tonnage of imported goods, there is the problem of identifying invasive species. They include a wide range of plants, insects and microorganisms. By their nature they are new pests, and therefore many of them are unfamiliar to quarantine staff and farmers. Fortunately electronic identification keys are now available on the Internet and on CD-ROM.
Molecular Studies of Genetic Relationships
New molecular techniques of genetic analysis are a valuable aid in identifying unfamiliar species. Techniques such as PCR are also able to characterize different strains of bacterial and fungal pathogens. In this way, it is possible to define the relationships between different types in different countries, thus showing the pathway of invasion.
For example, a study of the genetic sequences of Papaya Ringspot Virus (PRSV) has clarified how the virus became established in Asia. PRSV has two biotypes. Only one of these (PRSV-P) infects papaya. The other (PRSV-W) infects watermelon and other cucurbits. PRSV-W probably traveled to Asia in curcurbits, and once in Asia mutated into a virus which can infect papaya (PRSV-P). This originated from a single amino acid mutation, and probably happened not once but a number of times, independently in different Asian countries.
Conclusions
Not all exotic species are harmful. Nearly all agricultural crops are exotic species i.e. they originated in some other area. Furthermore, many exotic species which enter Asia do not become established. Of those which do become established, not all are harmful. The "Tens Rule" suggests that only about 10% of the species that arrive will establish, and of these, only about 10% will have an ecological effect.
Unfortunately, it is very difficult to identify which species will belong to this harmful group. We can only partially predict the impact of invasive species. Many of Asia's worst exotic pests were introduced intentionally because they were expected to bring some benefit. In the case of intentional introductions, it can be very difficult to balance the risks and benefits, especially since the people who benefit may be different from those who bear the risk. A nurseryman and an ecologist may view a new exotic shrub with quite different eyes. One may see it as an opportunity, the other as a risk!
In the case of accidental introductions, we do not have any chance to assess the risk in advance. We can only respond to an exotic species after it has arrived. Again, it is very difficult to measure its potential impact. However, we do know that once an invasive species has become established, the prospects for complete eradication are poor. Good import regulations and quarantine, and early detection and control, are all vital in minimizing the damage from exotic pest species.
Index of Images
Figure 1 Mikania Micrantha, Sometimes Called "the Cancer of the Forest". This Perennial Vine Covers Trees, Preventing Light from Reaching the Leaves. a Native of Tropical Central America, It Has Become a Major Weed in Asia.
Figure 2 Close-up View of M. Micrantha
