Maize is one of Asia's most important crops, second only to rice. . However, average grain yields in the region are low, at only 1.2-1.4 mt/ha - less than half the average world yield of 3.9 mt/ha. These low yields are partly due to unfavorable growing conditions, the high humidity and high temperatures. The other reason is attack by Asian cornborer. Asian cornborer is widespread, and severe outbreaks can cause maize losses of 20-80%.
Another major pest in the region is fruitfly. Fruitfly is economically the most important insect pest of fruit in Southeast Asia, and can cause losses as high as 90-100%. It can also prevent the development of export markets. Countries which do not have fruitfly are anxious to remain free of the pest. They usually impose strong quarantine restrictions on fruit imported from countries where the pest is endemic, or ban imports altogether.
Both fruitfly and Asian cornborer can be controlled by chemical applications. However, chemical pesticides are too expensive for many Asian smallholders, and are also a major cause of environmental pollution. There is thus an urgent need for economical, effective and environmentally friendly measures to protect maize from cornborer, and the fruit industry from fruitfly.
In 1998, FFTC carried out a survey of fruitfly and Asian cornborer. in four Asian countries: Malaysia, Thailand, the Philippines and Vietnam. The survey had three objectives. The first was to evaluate the occurrence of Asian cornborer and fruit fly, and the economic damage they cause. The second was to observe current control measures used against both pests. The third objective was to discuss with specialists in Southeast Asian countries the possibility of cooperative projects to control Asian cornborer and fruitfly, based on the use of natural enemies and attractants.
In North Vietnam, weather is strongly seasonal and winters are cold, with temperatures as low as 10
oC. Only one crop of maize can be grown each year, so there is a break in the population build-up of the cornborer. As a result, losses are relatively small (10-20%). The situation is quite different in the south of Vietnam, and also in the Philippines and Thailand, where mild weather allows year-round production of corn. In these warmer areas, cornborer damage is often severe. Up to 100% of stems may be damaged, and 70% of ears.
Even minor cornborer damage to the ears can affect the market value, especially if the corn is to be eaten fresh. Many farmers in Vietnam, Thailand and the Philippines are trying to grow sweet corn, which fetches a higher price than feed corn. They are finding that cornborer damage is reducing both their yields and the market value of their crops. While cornborer in warm climates is present all year round and damage levels are high, the same factor does mean a good environment for control by natural enemies.
An important natural enemy of cornborer is
Trichogramma, a parasitoid wasp which attacks the eggs of cornborer. Tests of
Trichogramma in the Philippines showed that up to 80% of cornborer eggs had been parasitized. As a result, cornborer damage was found in less than 9% of corn stalks, while damage to the ears was insignificant.
Trichogramma egg cards are being mass produced for distribution to farmers in Thailand and the Philippines, but not on a large enough scale to cover corn-growing areas. Another problem is that the weevil commonly used in the Philippines as an alternate host for mass rearing,
Sitotroga cerealella, is thought by some to be a potential pest in its own right. FFTC is carrying out a pilot project to help solve this problem ( see FFTC Newsletter No. 124 on this database).
As with cornborer, the warm temperatures in the south of Vietnam mean that the pest is present throughout the year, and populations levels are fairly high. Fruitfly in Vietnam is mainly distributed in the south, in the fruit-growing areas of the Mekong Delta. Fruit consumption in Vietnam is mainly for domestic use, so although fruitfly is causing serious damage to crops, it is not causing quarantine problems. The attractant methyl eugenol was used by the survey team in the northern part of Vietnam, but failed to trap any fruitfly.
Methyl eugenol is widely used in fruitfly control programs in Thailand, for example in guava plantations. It is effective, but rather expensive for many growers. Many fruits grown in Thailand, including guava and litchi, have great economic value and are important exports. However, the quarantine restrictions imposed by the presence of fruitfly hinder the development of export markets. Thailand's Department of Agriculture has recently completed a survey of fruitfly species in Thailand, and is field testing integrated control based on the use of methyl eugenol. There is also a Sterile Insect Technique (SIT) project supported by the International Atomic Energy Committee. Since there is no natural boundary around control areas, the success of SIT is seriously jeopardized by the re-invasion of fertile flies. The major fruitfly species in Thailand are
Bactrocera correcta and
B. dorsalis. Both are strongly attracted to methyl eugenol.
Malaysia has also completed a thorough nation-wide survey of fruitfly. Control programs in this country are based on the use of methyl eugenol, insecticide sprays, poisoned protein hydrolysate bait, and the bagging of fruit. Malaysia is a major production area for papaya and starfruit. The two most important fruitfly species which attack these crops,
Bactrocera papayae and B. carambolae, are both strongly attracted to methyl eugenol.
Fruitfly are a major concern in the Philippines, particularly in mango production. The Philippines produces 26 thousand mt of mango every year, many of them for processing and export. There are three major mango production areas, one of which, Guimaras Island, has good potential to become completely free of fruitfly. This would mean that the fruit produced on the island could be shipped to Japan or United States, adding to the value of the harvest. Eradicating fruitfly from Guimaras Island will be done in two stages. Firstly, numbers will be reduced to a minimum by mass trapping, using methyl eugenol. Finally, the residual fruitflies will be eradicated using the SIT technique. The survey team estimated that the cost of such a control program would be only 2% of the value of mango production on the island, so it would be a worthwhile investment.
During the project the team gathered samples of fruitfly from each survey area, using methyl eugenol bait. The specimens collected have been brought back for morphological study and classification. This may provide interesting information on regional distribution and intra-specific variation.
For lepidopterous insects, pheromones are the most common attractant. These are not yet a feasible control measure for cornborer, but the FFTC team provided the pheromones for four other common pests:
Spodoptera litura (tobacco cutworm)
Spodoptera exigua (armyworm),
Heliothis armigera (corn ear worm) and
Plutella xylostella (diamondback moth). Vietnam, Thailand, the Philippines and Malaysia are all interested in developing control programs with pheromones, and tried the pheromone strips brought by the survey team in a number of field tests. If these are successful, the Center hopes to offer further technical assistance, including formulations for mass production, field application techniques and quality control.
Location: Malaysia, Philippines, Thailand, Vietnam
Date: August-November 1998
Figure 1 Oriental Fruitfly (Female on Left, Male on Right)
Figure 2 Life Cycle of Oriental Fruitfly
Figure 3 Fruitfly Laying Egg in Guava
Figure 4 Adult Cornborer Female (a Brown Moth)
Figure 5 It Is the Cornborer Larvae That Do the Damage