Jian Guo*
Brontispa longissima is the most serious pest of coconut palms in Hainan where 11 species of palms have been attacked. When the beetle was first found, many quarantine measures were taken. The affected palms were cut down and burned, broad spectrum insecticides were screened and insecticide bags were inserted at the bases of the unopened leaves. A 3-kilometre buffer zone was established to prevent further spread. Traditional chemical control measures were used against the pest from 2002 to 2005. The treatments depressed the population of the beetle in the field and reduced the dispersal rate. About one million palms have since recovered. Two biological agents, a parasitoid (Tetrastichus brontispae) and an entomo-pathogenic fungus (Metarhizium anisopliae) have been tested successfully. The search for a synthetic mixture with the smallest possible number of constituents that possess the same attractant as fresh coconut leaves for Brontispa longissima and the development of this synthetic multicomponent is desirable and could play a role in biological plant protection.
Hainan Island is the second largest island in China; it lies between latitudes 18°10′ to 20°10′ N and longitudes108°37′ to 111°03′ E, with a land area of 33 900 square kilometres. The mean temperature ranges from 23.4 to 26.5ºC; the highest temperature has been recorded at 40ºC and the lowest at 6ºC. Mean annual precipitation is 2 412.3 mm and relative humidity ranges from 50 to 95 percent.
Coconut palms play an important role in the economy of Hainan Province, directly by providing food and income from coconut products, and indirectly as important features of the landscape, where tourism is a major component in the economy. Coconut palms are planted across the island both as economic and ornamental plants. The palms are traditionally planted around villages, along roadsides and in cities; large plantations can be found in some locations.
The coconut beetle, B. longissima Gestro is the most serious pest of coconut palms in Hainan. The larvae and the adults feed on the tender leaf tissues of the developing and unopened leaves of the coconut palm. If the young palms are attacked by the beetle, any prolonged attack will usually damage the palms severely, and in most cases the palms die.
The coconut leaf beetle is native to Indonesia and has invaded many countries in the Asia-Pacific region. It is believed that the beetle was introduced into Hainan Island around 2001 or 2002 through shipments of ornamental palms.
In June 2002, coconut leaf beetles were found in Haikou and Sanya cities for the first time. Surveys estimated that the beetles affected about 31 000 coconut palms over about 6 700 hectares. Since then, the beetle has spread to many nearby counties, although natural barriers such as mountain ranges, wide expanses of croplands and large lakes may have slowed down its natural dispersal; however the beetle can travel long distances by various means of transportation. By June 2003 the beetles were found in 11 counties; the epidemic covered about 30 000 hectares and 604 000 coconut palms were infected. In December 2004 the number of infected counties increased to 16, encompassing 390 000 hectares and 1820 000 coconut palms were infected. Reports showed that the beetles were also found in Taiwan, Hongkong and Guangdong.
Both adults and larvae live and feed on young unfolded coconut leaflets. They remove strips of tissue from both sides of the leaves, and the resulting small brown patches of varying sizes and light brown streaks can be found in the damaged leaf. The streaks are typically parallel to the mid-rib. Later, these feeding lines mingle with each other so that the remaining tissues dry up and rot; the brown leaves then shrivel and curl and this gives the attacked palm a scorched appearance. If the attack lasts a little longer, the palm may die. In Hainan Province, 11 species of palms have been attacked. The preferred host plant is C. nucifera. Currently, thousands of coconut palms have been destroyed by the beetles.
When the coconut leaf beetle was first detected in Hainan, the State Forestry Administration of China organized a joint effort to deal with the outbreak. Subsequently, the Chinese Academy of Tropical Agricultural Sciences, the Chinese Academy of Agricultural Sciences, the Chinese Academy of Forestry Sciences and the Chinese Academy of Sciences joined the battle. A series of studies were carried out on B. longissima, including biological characteristics, chemical control, biological control and monitoring of development.
The developmental threshold temperature and effective cumulative temperature of coconut leaf beetle are 11.08 and 96.2ºC respectively. Temperatures between 24 and 28ºC are favourable for the growth of the beetle. Royal palm (Roystonea regia) and coconut were its preferred hosts, followed by Livistona chinensis and oil palm. In Hainan, there are ants and parasitic acarids, but no other parasitic natural enemies. Beetles killed by Green Muscardine Fungus have been found in the field.
When the coconut leaf beetle was first found in Haikou in 2002, many quarantine measures were taken. The affected palms were cut down and burned, broad spectrum insecticides were screened and insecticide bags were inserted at the bases of the unopened leaves. A 3-kilometre buffer zone was established to prevent further spread. Transporting palms from other provinces to Hainan, or between counties in Hainan, was strictly forbidden. Check points were established to enforce this regulation.
An epidemic survey was started immediately and plots were established where highly susceptible hosts were planted. Regular surveys were carried out. A reporting mechanism was established for immediate action and its telephone number was made public; a manual on how to control the beetle was developed and distributed to the public. Any beetle sighting could be reported immediately. A DVD about the coconut leaf beetle was made; it was broadcast by television stations and played on long-distance buses.
Initially, insecticides were used to control the pest. Traditional chemical control measures were adopted from June 2002 till the end of 2003. Broad spectrum insecticides, such as imidacloprid, cypermethrin, deltamethrin and matridine, were applied by spraying at intervals of three to four weeks. All of these treatments depressed the population of the beetle in the field and reduced the dispersal rate.
From the end of 2003 to January 2005 a new powdered pesticide, developed by South China Agriculture University, was introduced. The powder was put into bags which released slowly with rainfall. These “tea bags” were inserted at the base of the unopened leaf. If the bag was placed in the right spot, it would be effective for five to six months. Currently, two million “tea bags” have been applied and about one million palms have since recovered from the damage.
Biological control via parasitoids, predators and entomo-pathogenic fungus cannot eliminate the pest totally but maintain the natural balance by keeping the pest population below the economic threshold level. In China, two biological agents, a parasitoid and entomo-pathogenic fungus, have been tested against the pest – promising results have been obtained and used in the field on Hainan Island.
Parasitoid – Asecodes hispinarum
China organized experts to visit Viet Nam in December 2003 to study coconut beetle control with A. hispinarum. Subsequently, A. hispinarum was released in the north (Haikou), the south (Sanya) and the east (Qionghai) of the island from August 2004 onwards after a safety evaluation indicated that it was not a threat to other species. A primary survey showed that the population of the coconut leaf beetle had decreased; six months later the infested palms in Haikou had recovered noticeably, but at another release site, the expected results were not manifested. Recently increasing evidence indicates that the successful release of the parasitoid depends on many factors, inter alia temperature, humidity, the environment, pesticide used by the farmers and wind.
The techniques for the mass-rearing of parasitoids have been mastered by Chinese scientists. Factories now produce 50 000 to 80 000 insects per day.
Parasitoid – Tetrastichus brontispae
Another parasitoid was introduced to Hainan from Taiwan Province of China in October and November 2004. This parasitoid was bred under isolated conditions in the Chinese Academy of Tropical Agricultural Sciences. The safety evaluation will be completed by early 2005. If the parasitoid passes the safety evaluation, it will be released into the field immediately.
Entomo-pathogenic Green Muscardine Fungus (Metarhizium anisopliae)
A total of 11 strains of Green Muscardine Fungus were screened, of which ten strains were used for field trails in Haikou. Field trials revealed that two strains infected the beetle quickly with high mortality rates. Palms recovered when many beetles died in the field after formulations containing these two strains were sprayed. However, microbial control faced two problems: (1) the new formulations need to be screened to suit the climatic conditions in South China, especially in the south of Hainan Province; (2) to develop low-cost spraying equipment that can deliver the bio-insecticide to the tall palms.
The preference of B. longissima for certain palms suggests that some of the emitted plant odours are attracting the coconut leaf beetles. Investigations on the chemical composition of the volatiles emitted by fresh, tender coconut leaves and the responses of the beetle have been conducted. Volatiles emitted by coconut leaves were collected by a mend distillation-concentrate method and analysed by gas chromatography–mass spectrometry. The laboratory bioassays proved that B. longissima can be attracted by coconut leaf odours. No significant differences in response were evident between males and females both in electro-anetrogrogic and laboratory bioassays.
Identification of the host volatiles of the fresh coconut young leaves and the bioassays are the first major breakthrough in understanding the host-finding behaviour of B. longissima. The results of this work show that the beetle has high olfactory sensitivity to the host plant. Therefore host volatiles are of paramount interest as potential attractants.
The next step is to search for a synthetic mixture with the smallest possible number of constituents that possesses the same attractant as fresh coconut leaves for B. longissima in laboratory bioassays and field traps. The development of this synthetic multicomponent is desirable and could play a role in biological plant protection.
In the battle against coconut leaf beetles, a number of technical issues need to be addressed:
* Vice Director General, Haikou Forestry Administration, Hainan Province, PR China 570125.
