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Plantwise Technical Factsheet

coconut beetle (Oryctes monoceros)

Host plants / species affected
Agave sisalana (sisal hemp)
Areca
Cocos nucifera (coconut)
Dracaena
Elaeis guineensis (African oil palm)
Musa (banana)
Phoenix dactylifera (date-palm)
Raphia
Saccharum officinarum (sugarcane)
List of symptoms/signs
Fruit  -  internal feeding
Growing point  -  dead heart
Growing point  -  internal feeding; boring
Leaves  -  external feeding
Stems  -  internal feeding
Whole plant  -  dead heart
Whole plant  -  external feeding
Whole plant  -  internal feeding
Whole plant  -  plant dead; dieback
Symptoms
The adults bore through the basal parts of the leaves into the tops of the growing point of the palm and enter the heart of the unfolded leaves, or centre spike, working downwards. The youngest expanded leaves die off, and the leaves in the central bud generally unfold, showing triangular cuts on each side of the central rib. Eggs are laid in the soft growing-point. The larvae feed on the younger tissues and top part of the palm, the whole of the centre of the top being usually eaten away (Anon., 1921).

In Benin, the adults bore into the bases of the petioles of oil palm (Elaeis guineensis), towards the terminal bud. Growth is retarded, young leaves are small or deformed, and if attack is severe, the whole shoot may die (Alibert, 1936).

The adults bore into the heart of the coconut palm, damaging the still folded fronds while feeding on the sap. If the insect reaches the growing point, as it often does on seedlings and younger palms, the plant dies (Bedford, 1979).

Prevention and control
Cultural Control

Mayné (1928) recomended clean cultivation; the use of trap heaps of debris to attract ovipositing females; and the destruction of adults in galleries by means of wire for control of O. monoceros.

In Benin, Alibert (1936) suggested the removal of waste matter on which eggs might be laid and the felling or rotten trees, which should be buried >3 ft deep and replaced by healthy ones. Borassus palms should be removed from the neighbourhood, and traps of vegetable debris and dung heaps near the oil palm plantations examined for larvae every 2-3 months. Holes made in the palms should be plugged with a mixture of salt and sand (1:2). Adults can be killed by pushing a wire into the gallery or by pouring in turpentine, carbon bisulphide or a solution of copper sulphate.

In new commercial coconut plantations created in 1967-71 in south-eastern parts of the Côte d'Ivoire, where the forest vegetation had been completely removed by burning, populations of O. monoceros were much lower than in plantations where felled trees were left on the site for seasoning. Burning or removing all wood after felling is not, however, advisable as a general or habitual practice in all plantations, since the fertility of clay soils is adversely affected and the labour involved in transportating the timber would be uneconomic.

An investigation into the degree of damage caused by Oryctes spp. (mainly O. monoceros) in young plantations where felled wood had been completely, partially, or not removed, and into various methods of control was undertaken in 1972. The mean level of infestation was low (5 adults/ha), probably owing to the covering growth of Pueraria javanica over the felled trees, but Orycyes populations were 2.8 times as high in plots containing felled trees as in those that had none, and the number of attacks on standing coconut palms was correspondingly greater in such plots. Palms less than 10-months-old that became infested always died, but these were few. The degree of defoliation from July 1971 to July 1972, which was assessed from samples according to the number of living leaves per tree, the number of leaflets on the fifth leaf and the number of leaflets destroyed by O. monoceros on each living leaf, was 2.8%. Partial deforestation, therefore, seems permissible in young plantations, but the protection measures adopted for oil palm (Elaeis guineensis) could be extended to coconut, namely to fell forest trees and to leave one row in two to season on the site, encouraging the growth of P. javanica over them (Julia and Brunin, 1974).

In order to prevent attack by O. monoceros, commercial coconut plantations sited in former forest areas in the Côte d'Ivoire have in the past been planted after the felling and complete destruction of the old trees, which are potential breeding sites for the beetle. This technique has disadvantages, and methods of preserving organic matter for fertilizing the new trees, while limiting Oryctes attacks, were investigated. Two series of experiments were undertaken in which trees were felled and stacked in windrows at intervals of 1-5 rows of new trees and a cover crop of Pueraria javanica was sown at different dates; the rates of infestation of the new trees by O. monoceros were compared with those on sites on which the old trees had been uprooted and burned. It was found that Oryctes infestation was very severe (75% of the central shoots damaged and a population density of over 50 insects/ha) on sites where Pueraria covered the old trees only after 2 years, and infestation was still quite considerable if the trees became covered after 1 year. The burned sites became re-infested from these heavily attacked neighbouring plots. It was concluded that the use of a cover crop is effective only when the wood is completely covered after a maximum of 7-9 months, depending on the nature of the wood and the initial population levels of O. monoceros (Julia and Mariau, 1976a).

When establishing a coconut plantation on forest land in Côte d'Ivoire, preventive control of O. monoceros was achieved by complete destruction of timber by felling, stumping and burning; windrowing the felled trunks every other inter-row or windrowing one inter-row in five, and covering the windrows rapidly with a vigorous cover plant. Harvesting records over 3 years showed that trees adjoining windrows were the most precocious and productive, and treatment by windrowing the felled trunks every other inter-row gave a more homogenous yield than the other two treatments (Ouvrier, 1980).

Preliminary studies of the effects of exposure to gamma radiation from a 60Co source showed that the adults of O. monoceros can be sterilized by exposure to 4000 and >6000 rad, but the vigour and competitiveness of the irradiated males are reduced (Hurpin, 1968).

Biological Control

O. monoceros larvae all died after 3 weeks at 28°C in a breeding medium infected with mild doses of Metarhizium anisopliae conidiospores (10³/g of medium). Temperature greatly influenced the development of the disease, which reached a maximum intensity between 25 and 30°C. Only strains belonging to the major type (condiospores measuring more than 9 µm in length), isolated from five Oryctes spp., were pathogenic to O. monoceros (Diomandé, 1969; Ferron and Diomandé, 1969).

Several specialized predators of O. monoceros have been introduced to the Seychelles, but only one was found to survive: a large elaterid beetle, Lanelater sp., whose larvae readily consume young Oryctes larvae. Scolia ruficornis, introduced in the 1950s, was observed, although no parasitized larvae were found (Lomer, 1985).

The establishment of the virus Baculovirus oryctes, isolated from the Oryctes rhinoceros in Malaysia and introduced into populations of O. monoceros infesting coconut palms, was assessed on three islands of the Seychelles archipelago in 1981-83. On two of the islands, Ste Anne and Mahe, which were previously free of the virus, the percentage of infected adults of O. monoceros fluctuated between 20 and 50, and a modest population reduction (approximately 30%) was recorded. On a separate group of islands, Praslin group, the virus was found to have survived from a previous release made in 1973 and the rate of infection was between 70 and 90% (Lomer, 1986).

Introductions of Baculovirus oryctes into populations of O. monoceros in Tanzania were made from November 1983 to June 1987. Six sites, characterized by a high pest density, were chosen for the field release trial of the virus. Nearly 2000 infected adults were released in four periods during the period 6 June to 23 July, 1985, in Vuo-Gezani, and 9 October, 1985 to 3 March, 1986, in Magawa. The infection rates in the release sites (1.5 years after the last release in Vuo-Gezani and 1 year after in Magawa) ranged from 40 to 60%, indicating that Baculovirus oryctes was already established in the wild population of the beetle. Results of damage surveys for the period from 1983 to 1987 showed a considerable decrease in the damage of palms in Vuo-Gezani, where damage fell from 50% (before release) to 10-20% (1.5 years after virus release), while surveys in Magawa indicated a small decrease of leaf damage from 70% (before release) to only 60% (1 year after virus release). It is known, in the case of Magawa, that even in the presence of virus, the beetle attacks in young or mature palms can be severe, if old coconut logs are present as breeding places for the pest. The decrease in damage at both release sites was followed by an equivalent decrease in the population density of O. monoceros as indicated by the last traps and field collection. In control (non-release) areas, the leaf damage followed by a high density of beetle population remained nearly at the same level during the investigation period (Purrini, 1989).

Chemical Control

Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:

IPM Programmes

Integrated control in which several compatible methods are used simultaneously in an effort to reduce pest numbers to a subeconomic level, has been attempted for decades. However, results have frequently been unsatisfactory, and control was often affected by the economics of, and local attitudes towards, the crop. The vigour of application of the methods often fluctuated with the changing market value of the crop, and it was difficult to have the control measures carried out uniformly by all sectors of the communities concerned. Only in recent years has the development of new measures, such as the use of Baculovirus oryctes and cover crops, added highly effective and practical components to integrated control programmes (Bedford, 1980).

Integrated pest management involving the possible use of Baculovirus oryctes is discussed by Paul (1985).
Impact
The rhinoceros beetles, Oryctes spp. are important pests of palms. Attacks by adults may reduce yield, kill seedlings and young and old palms, and discourage replanting. They may provide entry points for lethal secondary attacks by the palm weevil, Rhynchophorus, in some countries, or by pathogens (Bedford, 1980).

On some plantations of 2½- to 3-year-old coconut palms in Zanzibar, O. monoceros was found to have killed more than 50% of the palms and retarded the growth of others. Mature palms were also attacked, but the damage was not serious. Isolated palms and those on poor soil were attacked more often than those in dense groves (Mansfield-Aders, 1920).

In Kenya, attacks occurred more frequently in palms in outer plantation rows (Dry, 1922). At one locality, deaths of bearing palms averaged 2% per year. Assuming replanting took place immediately, this represents about a 15% loss of bearing capacity, because the replants require 7-8 years to come into bearing (Wheatley, 1961).

In Uganda, Hargreaves (1924) included O. monoceros amongst the pests of shade and ornamental trees. In tropical Africa, O. monoceros is among the most common and most injurious pests of coconut and other palms (Mayné, 1928). In the Congo Democratic Republic, O. monoceros is an economically important pest of Elaeis (Ghesquière, 1935). In Benin, it is included amongst the most important pests of oil palm (Elaeis guineensis) (Alibert, 1936).

According to Alibert (1936), the females of the oil palm pest weevil, Rhynchophorus phoenicis, deposit eggs in the soft tissues of wounds, often in those caused by Oryctes. Attack by Oryctes allows the palm weevils, Rhynchophorus and Temnoischoita to enter (Hartley, 1967).

In the Seychelles, O. monoceros is the most important pest of coconut, and causes serious injury to the heart leaves of young trees, but rarely damages old ones. Seedling coconuts were killed, or the palms were seriously damaged during their first 5-7 years of life. O. monoceros is abundant near distilleries, where it breeds in heaps of waste (Vesey-Fitzgerald, 1941).

In the Côte d'Ivoire, O. monoceros appears to be more economically damaging than O. boas, because of the greater variety of larval habitats available to it (decayed wood, compost, manure), but it is also more susceptible to natural or artificial changes in the environment (Mariau, 1967).

The main damage caused by adult O. monoceros results from tunnelling through the central whorl of unexpanded leaves towards the terminal bud, in which process the fronds are cut into jagged 'fishbone' shapes that become apparent as the leaves grow and unfurl. Nursery seedlings are attacked at the level of the collar and both the terminal bud and the nut are penetrated, which kills the plant. One-year-old trees become infested at the axils of the later-developing fronds, while trees that were at least 2-years old are attacked at the level of the first fronds, nearer the base. O. monoceros attacks higher in the crown than O. boas. The average rate of infestation in the plantations studied was 75%. The length of tunnels averaged about 18 inches, which was long enough to enable the beetle to reach the terminal bud of trees less than 2-years old. On 4-year-old trees, the whorl of leaves grew faster during the rainy season than the insect could tunnel, and the terminal bud remained intact. Clean-cultivated plantations with no potential oviposition sites become infested from the outside by flying adults; if the trees are large, infestation is restricted to the edges, whereas small trees permit penetration further into a plantation. Observations in three adjacent plantations of different ages, where the oldest trees (although the furthest from the source of infestation) were the most extensively invaded, indicated that trees that were 6-10 feet tall were more attractive and more easily recognized than smaller ones. The same tree could be attacked more than once, and previous wounds appeared to attract ovipositing females. Over 80% of O. monoceros adults made feeding flights within the first 2 hours of darkness. The yield of nut-bearing trees is hardly affected by the feeding damage of Oryctes on the leaves, but younger trees may be killed or their development to the nut-bearing stage greatly retarded. Large plantations, in which attack is limited to the edges, are recommended as they are less vulnerable than small ones (Mariau, 1968).

In the Seychelles, the loss of leaf area in coconuts leads to a reduction in the nut production and in the rate of growth; in seedling palms the beetle sometimes burrows its way to the growing point of the palm, killing the entire tree (Lomer 1985).

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