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Managing rhinoceros beetle of coconut

By Our Agriculture Correspondent



Coconut palms severely infested by the dreaded rhinoceros beetle showing the characteristic triangular cuts.

SCIENTISTS AT the Central Plantation Crops Research Institute (CPCRI), Kasaragode, have developed a package of effective eco-friendly methods of managing rhinoceros beetle, one of the dreaded pests of coconut. "In addition to phyto-sanitation and mechanically hooking out the beetles from the infected palms using beetle hook, we found that two promising pathogens of rhinoceros beetle can be effectively used for the biological control of the pest," says Dr. Chandrika Mohan, Senior Scientist at CPCRI Regional Station at Kayangulam in Kerala.

Rhinoceros beetle (Oryctes rhinoceros), in its adult stage, bores into un-opened fronds and spathes of coconut.

The attacked frond when fully opened shows characteristic triangular cuts. Infestation on spathe often destroys the inflorescence, and thus prevents production of nuts. The beetle breeds on decaying organic debris, farmyard manure, dead coconut stumps, logs and compost. Baculovirus of Oryctes, Oryctes baculovirus (OBV), and the green muscardine fungus, Metarhizium anisopliae, are the two promising bio-control agents, which can be effectively used against the dreaded pest, according to Chandrika Mohan.

The virus culture can be easily maintained by following simple procedures developed at CPCRI. An OBV infected grub of rhinoceros beetle is selected, and its midgut is removed after dissecting its body.

The infected midgut is transferred to a mortar, and after adding 3 to 5 ml of sterile water, it is ground to form a fine suspension using a pestle. This viral suspension is carefully mouth-fed to the healthy grubs using a syringe. About one ml of inoculums is used for each healthy grub, and one midgut suspension can be used to inoculate 5 to 6 healthy grubs, according to the scientist.

The inoculated grubs are then put in a plastic box of 15 cm diameter and 15 cm height, containing about 200 g of sterilised cow dung or coir dust moistened sufficiently with sterile water. Care should be taken to ventilate the box by making holes in the lid.

Then the grubs should be regularly examined for OBV infection, and the procedure is to be repeated for maintenance of the viral culture. The cadavers or virus triturate can be stored at 4 degrees Celsius indefinitely.

For releasing the OBV in the field, about 10 to 15 healthy adult rhinoceros beetles should be allowed to crawl into a baculovirus infected midgut suspension kept in a shallow glass trough for 30 minutes. Then the beetles should be transferred to plastic boxes and starved for 12 to 24 hours. They are then released in the field, preferably at dusk. Then the field should be monitored for the beetles' movements once every six months.

The entomopathogenic fungus, Metarhizium anisopliae, has also been found to be effective in controlling the emerging grubs at the breeding grounds.

The pathogen is mass-produced using coconut water method and cassava chips method, and the when the green spores are multiplied in numbers, they should be sprayed over the breeding sites of the beetle.

Alternatively, the green spores may be mixed with powdered cow dung and sprinkled over the breeding sites for effective control, according to the scientists.

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