Oryctes rhinoceros

Oryctes rhinoceros, also known as coconut rhinoceros beetle, Asiatic rhinoceros beetle, and coconut palm rhinoceros beetle, is a large species of beetle (typically 4–5 cm long), belonging to the rhinoceros beetles subfamily Dynastinae. Oryctes rhinoceros attacks coconut palms and other palms such as the economically important oil palm. It can cause serious damage to the developing fronds (leaves) up to death of the palm. The beetle breeds in decaying palm trunks or other organic matter like sawdust or compost heaps.

Distribution
The native distribution of this beetle covers most parts of tropical Asia, from India and the Maldives in the west to the Philippines and southern China in the east. Through human activity, it has been accidentally introduced into a larger number of tropical islands in the Pacific starting with Samoa where larvae arrived in 1909 with the soil of rubber seedlings from Ceylon (Sri Lanka). From Samoa, the coconut rhinoceros beetle spread to many others parts of the Pacific, most recently to Guam, Hawaii, the Solomon Islands and Vanuatu.

Description
Oryctes rhinoceros is a large dynastid beetle with a length of 35–50 mm and a dark-brown to black colour. The head has a horn which is more prominently developed in males and in larger specimens. The pronotum has a large central depression with 2 humps at the hind margin and the tibia of the fore leg has 3 large teeth. Male beetles can be reliably separated from females by the tip of the abdomen which is rounded, shiny and hairless in males. In females the tip is more pointed and densely covered with hair.

The eggs are white and about 3 mm in diameter. The larvae are the typical C-shaped white grubs of scarab beetles. The 3 larval stages can be separated by the size of their head capsule which are around 2.5–3 mm (1st instar), 5-6 mm (2nd instar) and 10-11 mm (3rd instar).

Biology and life cycle
The coconut rhinoceros beetle is nocturnal and flies only during the night. Its main host is the coconut palm, but other palms, especially the economically important African oil palm are also hosts. Attacks have been further reported from various other crops like banana plants or screw palms.

The beetle most often breeds in the decaying trunks of coconut or oil palm. These trunks become suitable for breeding after palms are felled for replanting or when the palm dies of old age. The dead standing palms are the most preferred breeding places. Other breeding sites are sawdust heaps around sawmills, compost heaps in gardens, and other decaying organic matter such as heaps of cocoa (cacao) pods. The breeding substrate must have reached a certain degree of decay for the females to accept it.

Female beetles typically arrive first at a coconut trunk suitable for breeding and start preparing the trunk for egg laying by breaking up the wood into small particles suitable for consumption by the freshly hatched first instar larvae. A male beetle often follows the female for mating and to help the female with preparation of the breeding site. Females deposit eggs, packing the wood particles tightly around the eggs. Males often stay behind in the trunk, breaking up additional wood.

The eggs hatch after 1.5–2 weeks and the total duration of the three larval stages last around 4–6 months. The fully grown larvae construct a protective cell, pupate and the young adults emerge after an additional 3 weeks.

After emerging from the pupae, the young adults remain at the breeding site for an additional 3-4 weeks until their cuticle is hardened and their flight muscles and reproductive organs are fully developed. They then leave the breeding site and fly to a nearby coconut palm, where they feed and mate. Breeding starts after the first feeding, only 2-3 weeks after the young adults have emerged from the breeding site.



Damage
The coconut rhinoceros beetle attacks a palm at the base of the fronds (leaves) where they are attached to the growing trunk. They start at one of the middle leaf axils, boring through the base of other fronds into the centre of the crown, the palm heart, where the young, still white fronds develop. The beetle feeds on the undeveloped fronds, causing major damage to the palm. While the damaged fronds still continue to grow, become green and unfold, they typically have parts cut off. Most often the damaged fronds show triangular cuts, or when the fronds unfold, parts break off and wilt. The damage reduces the production of fruits on the palm. In young, newly planted palms the growing point of the palm may be damaged by the attack and the palm dies as a result.

Outbreaks and management
Coconut rhinoceros beetle outbreaks are always associated with an abundance of breeding sites. These are usually created during replanting of coconut and oil palm plantations, when large numbers of palms are felled. The trunks decay and become available for breeding over a period of several years. At the same time, the young, replanted coconut or oil palms are particularly vulnerable to attacks by the beetle.

Apart from how abundant breeding sites are, the severity of an outbreak depends mainly on two factors, on the ground vegetation, as well as the presence/absence of the most important natural control agent, the Oryctes rhinoceros nudivirus (see below). During large scale replanting, like in oil palm plantations in Indonesia, the trunks are often piled up high in large heaps, exposing them to beetles searching for breeding sites. Attempts to burn them is often not successful and creates air pollution. On the other hand, if the trunks lie on the ground and are hidden by vegetation (e.g. overgrown by a cover crop), most beetles will not find them. In addition, dense ground vegetation can also reduce the damage on young palms.

In Indonesia, hand-picking of beetles from the axils of palms is often practiced in newly planted oil palm plantations. Pheromone traps have been also used for control, as well as for monitoring and for determining the incidence of virus infections in O. rhinoceros populations.

Natural enemies and biological control
Since chemical control of Oryctes rhinoceros is not feasible, research on effective biological control agents has received special attention for many years. This included searching within the native distribution of the beetle for suitable predators, parasitoids and diseases. Many species of predators and parasitoids were identified over the years and a variety of species were introduced into the invaded Pacific islands.

However, there is little evidence that native or introduced predators and parasitoids cause a significant impact on the populations of the coconut rhinoceros beetle. For example, during a field survey of 600 breeding sites in the Philippines, <5% contained elaterid predators and only ~1% a predatory shrew. Parasitoids like Scolia ruficornis can be common in sawdust heaps where the adult wasps can find and reach the larvae relatively easily. However, Scolia wasps rarely reach larvae in coconut trunks.

A virus disease of O.rhinoceros, the Oryctes rhinoceros nudivirus, turned out to be the most effective natural control agent. This virus was discovered by Alois Huger in Malaysia in 1963. It was later found to occur naturally in other countries within the native range of the beetle like the Philippines, Indonesia and India. However, the introduced beetle populations in the Pacific and those in the Maldives were found to be virus-free.

Introduction of the virus into virus-free populations caused a significant decline of the O. rhinoceros population. For example, virus releases in the islands of the Maldives typically reduced the beetle populations to less than one quarter of the level before the release. The virus effects mainly the population of adult O. rhinoceros beetles. It spreads easily among adults, reducing their life span and the fecundity of females.

However, during replanting of palms, when many breeding sites become available, transmission of the virus is often interrupted and outbreaks still occur, even with the presence of the virus. Under such conditions it is recommended to grow a cover crop over the breeding sites and under the palms, as well as to use a few dead standing palms to promote the spread of the virus. There is also evidence of resistance to the virus in some O.rhinoceros populations.

A second disease agent the fungus Metarhizium majus is also effective in controlling coconut rhinoceros beetles in breeding sites. However, unlike the virus disease, it does not spread very well by itself within an O. rhinoceros population. In the Philippines, only about 10% of the breeding sites naturally contained this fungus, and the effect of the fungus on the population of adult beetles is negligible.

Still, once established in a breeding site, this fungus provides long-term control of larvae in this particular site. Unlike the virus, the fungus survives for some time outside its host (as conidia). It can be an effective control agent if applied manually to each breeding site.